Risk in Structural Engineering PDF

 

Risk in structural engineering October 2013

 

Membership of the Task Group C J Bolton  BSc CEng FIStructE MICE  (Sellafield Ltd) Chairman Ltd)  Chairman R A Davis  MSc CEng FIStructE  (TPS) J K Kenward  BEng(Tech) CEng FIStructE MICE MCIHT  (Hyder Consulting Limited) J Lane*  MSc CEng FICE  (RSSB) Dr A P Mann  FREng BSc(Eng) PhD CEng FIStructE MICE  (Jacobs) B S Neale  CEng FIStructE FICE Hon FIDE  (Consultant and Hazards Forum) D A B Thomas  BSc(Eng) MSc AKC CEng FICE CFIOSH  (The heightec Group Ltd) Corresponding members Corresponding D H Bardsley  BSc ACGI CEng FICE  (Consultant) Dr W G Corley**  PhD CEng FIStructE  (CTL Group)

 A M Cormie BSc CEng FIStructE FICE FIES MWeldI  (J&D Pierce (Contracts) Ltd) K K Kwan  CEng FIStructE MICE FHKIE  (Arup) Secretary to the Task Group Dr J D Littler  PhD  (The Institution of Structural Engineers) (until June 2011) B Chan  BSc(Hons) AMIMechE  (The Institution of Structural Engineers) (from June 2011)  A Rahman  MEng(Hons)  (The Institution of Structural Engineers) (from January 2013)

*  representing ICE  **  deceased March 2013  Acknowledgements  Acknowledgements Figure 3.1: Chris Bolton Figure 4.1: Contains public sector information published by the Health and Safety Executive and licensed under the Open Government Licence v1.0 Figure 5.1: TPS Figure 7.1: Mandy Reynolds (ss Great Britain Trust) Figure 7.2: Peter Smyly Figure 8.1: Guy Gorton Figure 8.2: Chris N Illingworth Figure 9.1: Minnesota Department of Transport Figure A.1: Nicholas Smale Figure A.2: Lee L Lowery, Jr., PE PhD Figure A.3: CA Group Figure A.4: U.S. Department of Defense

Published by the Institution of Structural Engineers International HQ, 11 Upper Belgrave Street, London SW1X 8BH  Telephone:  T elephone: þ44(0)20 7235 4535 Fax: þ44(0)20 7235 4294 Email: [email protected], Website: www.istructe.org First published 2013 ISBN 978-1-906335-08-3 #2013 The Institution of Structural Engineers

 The Institution of Structural Engineers and those individuals who contributed to this this   Report  have   have endeavoured to ensure the accuracy of its contents. However, the guidance and recommendations given in the  the   Report  should   should always be reviewed by those using the  the   Report  in   in the light of the facts of  their particular case and specialist advice obtained as necessary. No liability for negligence or otherwise in relation to this  Report  and   and its contents is accepted by the Institution, the members of  the Task Group, their servants or agents. Any agents. Any person using this  Report  should  should pay particular  attention to the provisions of this Condition. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without prior permission of the Institution of Structural Engineers, who may be contacted at 11 Upper Belgrave Street, London SW1X 8BH.

 

Glossary and abbreviations

 These definitions are provided to explain how the terms listed are used in this  Report , and are not necessarily as used in other documents. Note that ‘risk ‘risk’’ and ‘hazard’ are defined in the singular; singular; in practice there will usually be many hazards and even more risks.

Term Haza Ha zard rd

Definition Thee po Th pote tent ntia iall fo forr ha harm rm ar aris isin ingg fr from om an in intr trin insi sicc pr prop oper erty ty or di disp spos osit itio ionn of so some meth thin ingg to ca caus usee de detr trim imen ent. t.

Risk Ri sk

A co comb mbin inat atio ionn of th thee se seve veri rity ty of th thee ha harm rm re resu sult ltin ingg fr from om a ha haza zard rd an andd th thee pr prob obab abililit ityy th that at th thee ha harm rm is realised.

Constr Con struct uctor or

The per person son or org organi anisat sation ion res respon ponsib sible le for exe execut cution ion of the wor work. k. Som Someti etimes mes ref referr erred ed to as the contractor.

Desi De sign gner er

Anyy pe An pers rson on or or orga gani nisa sati tion on de desi sign gnin ingg pe perm rman anen entt or te temp mpor orar aryy wo work rks. s.

Elimin Eli minati ation on

Avoidance Avoida nce of ris riskk by cha changi nging ng the des design ign (or (or,, pos possib sibly ly,, the con constr struct uction ion met method hod)) so tha thatt the haz hazard ard ceases to exist.

Structural Struc tural engi engineer neer

Any quali qualified fied struc structural tural engi engineer neer,, such such as a Charte Chartered, red, Asso Associate ciate-Memb -Member er or or Techni Technician cian Membe Memberr of of the the Institution of Structural Engineers, or the equivalent in another body worldwide, whether working on behalf of a designer, constructor or client.

Harm Ha rm

Anyy un An unwa want nted ed co cons nseq eque uenc nce, e, in incl clud udin ingg de deat ath, h, in inju jury ry,, da dama mage ge to he heal alth th an andd se seri riou ouss ec econ onom omic ic de detr trim imen ent. t.

Mini Mi nimi misa sati tion on

Meas Me asur ures es th that at re redu duce ce th thee pr prob obab abililit ityy th that at th thee ri risk sk oc occu curs rs..

Miti Mi tiga gati tion on

Meas Me asur ures es th that at re redu duce ce th thee se seve veri rity ty of th thee ha harm rm,, gi give venn th that at th thee ri risk sk oc occu curs rs..

Residu Res idual al ris riskk

Residual Residu al ris riskk is the lev level el of ris riskk rem remain aining ing whe whenn the cur curren rentt ris riskk con contro troll mea measur sures es and the their ir deg degree ree of effectiveness are taken into account.

Risk assessment document

 A document recording the conclusions of a risk assessment.

Riskk asse Ris assessm ssment ent

A proc process ess tha thatt iden identif tifies ies haz hazard ards, s, eva evalua luates tes ris risks ks and inf inform ormss its its use users rs abo about ut risk risks, s, the their ir pos possib sible le eliminati elim ination, on, the contr controls ols that might be put in place to reduce them and the resid residual ual risks that might remain.

Riskk manag Ris manageme ement nt

The who whole le proc process ess of iden identif tifyin yingg risks risks and res respon pondin dingg to the them m (not (not just just the con contro troll measu measures res). ).

Safety culture (of an organisation)

The combination of individual and group values, attitudes, perceptions, competencies, and patterns of behaviour that determine the commitment to, and the style and proficiency of, an organisation’s health and safety management.

 Abbreviation

Definition

 ACoP

(UK) Approved Code of Practice

 ALARP

As Low as Reasonably Practicable

ERIC

Eliminate, Reduce, Inform, Control

CDM CD M

(UK UK)) Co Cons nstr truc ucti tion o n (Des esig ignn and Man anaage gem men ent) t) Re Reggul ulat atio ions ns

HAC

High Alumina Cement

HAZOP

HAZard and OPerability review

HSE

(UK) Health and Safety Executive

SFAI SF AIRP RP

So Fa Farr As Is Re Reas ason onab ably ly Pr Prac acti tica cabl blee (m (may ay al also so be wr writ itte tenn SF SFAR ARP) P)

QA

Quality Assurance

QRA

Quantitative Risk Assessment

The Institution of Structural Engineers  Risk in structural engineering

  v

 

Contents

Glossary and abbreviations   v

5.4.3 5.4.3 5.4.4 5.4 .4

1

Scop Sc ope e an and d ob obje ject ctiv ives es   1

5.5 5.5 5.6

Documenting Documenti ng the the risk risk asse assessme ssment nt   21 Whatt to do and not Wha not to do in risk  risk  assessment   23 HAZ AZO OP   23 Codes Cod es of pra practi ctice ce   23

1.1 1.2 1. 2 1.3 1.4 1.5 1.6 1. 6

Introduct Introd uction ion   1 Typ ypes es of ri risk  sk    1 Intend Int ended ed rea reader derss   1 Releva Rel evance nce to cli client entss   1 Summar Sum maryy of obj object ective ivess   1 Refe Re fere renc nces es   2

5.7 5.8 5.9 5.10 5.11 5.12

Risk mana managemen gementt fram framework  ework    23 The impor importance tance of compe competence tence   24 QA and cha change nge con contro troll   24 Independe Inde pendent nt review review   24 Conclusio Conc lusions ns and recommenda recommendations tions   24 Reference Refe rencess   25

2 2.1 2.2 2.3 2.4 2.5 2. 5 2.6 2. 6

Hazard Haza rdss an and d ri risk skss   3 Introd Int roduct uction ion   3 The mea meanin ningg of ris riskk and and hazar hazardd   3 Structural Struc tural engin engineeri eering ng haza hazards rds   3 Structural Struc tural engin engineeri eering ng risk    3 Summ Su mmar aryy   5 Refe Re fere renc nces es   5

3 3.1 3.2

Principle Princi pless of ris risk k man manage agemen mentt   6 Introd Int roduct uction ion   6 The impor importanc tancee of of risk mana managemen gementt   6

6 6.1 6.2 6. 2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6. 9 6.10

Statistic Statis tical al and pro probab babil ilist istic ic met method hodss   26 Introd Int roduct uction ion   26 Back Ba ckgr grou ound nd   26 Quantifyi Quan tifying ng proba probabilit bilityy   26 Safety Saf ety fac factor torss   27 Low pro probab babili ility ty eve events nts   27 Applic App licati ation on   28 Assessme Asse ssment nt of exist existing ing struc structures tures   28 Conclusio Conc lusions ns and recom recommenda mendations tions   29 Refe Re fere renc nces es   29 Bibliogra Bibli ography phy   30

3.3 3.4 3. 4 3.5 3.6 3.7 3.8 3.9 3. 9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19

How big iscethe  7ris risk? k?   6 Comp Co mpet eten ence Commun Com munica icatio tionn   7 Whatt is an acc Wha accept eptabl ablee level level of of risk? risk?   7 Proportio Prop ortionalit nalityy and ALARP   8 Risk mana managemen gementt hiera hierarchy rchy   8 Risk Ri sk av aver ersi sion on   8 Resilienc Resi liencee   9 Ownership Owne rship and and control control of risks   9 Societal Soci etal risks   9 Humann failure Huma failure and accide accidents nts   9 Safety Safe ty cultu culture re   10 Prescription versus engineering judgement   10 The safe safety ty case case   11 Conclusions usions and recommendati recommendations ons   11 Reference Refe rencess   11 Bibliogra Bibli ography phy   12

4 4.1 4.2 4.3 4.4 4.5 4.6 4. 6 4.7 4.8 4.9 4.10 4.11 4.1 1 4.12 4.1 2 4.13 4.14 4.15 4.16

Legall ba Lega back ckgr grou ound nd   13 Introd Int roduct uction ion   13 Law as itit affects affects struc structural tural engin engineerin eeringg   13 Acts, regulations, ations, guida guidance nce and ACoPs   13 Reasonabl Reas onably pract practicabl icablee   13 Practi Pra cticab cable le   14 Burd Bu rden en of pr proo ooff   14 Liability Liab ility under civil law and duty of care care   14 Law enf enforc orceme ement nt   15 Desig De signer ner’s ’s rol rolee   15 Building Build ing Regul Regulation ationss   15 Europe Eur ope   15 Hongg Kong Hon Kong   16 Unitedd State Unite Statess   16 Summary Summa ry and conclu conclusion sionss   16 Reference Refe rencess   17 Bibliogra Bibli ography phy   17

5

How Ho w to ma mana nage ge ri risk  sk    19

8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

5.1 5.2 5.3 5.4

Introduct Introd uction ion   19 First Fir st ide identi ntify fy the the haz hazard ardss   19 Apply the risk manag managemen ementt hiera hierarchy rchy   19 How to do do ‘ris ‘riskk asses assessme sment’ nt’   20 5.4.1 5.4 .1 Gen Genera erall   20 5.4.2 5.4. 2 Proce Process ss for risk mana managemen gementt   21

8.10 Access Access and work areas areas including including working working at height   42 8.11 8.1 1 Lif Liftin tingg   42 8.12 Conc Conclusio lusions ns and recommenda recommendations tions   43 8.13 Refe Reference rencess   43 8.14 Bibli Bibliogra ography phy   44

Foreword   vi

7 7.1

Risk in de dessig ign n   31 Introd Int roduct uction ion   31 7.1.1 7.1 .1 Ove Overvi rview ew   31 7.1.2 7.1. 2 What does the desig designn stage stage include include??   31 7.1.3 7.1. 3 The consequ consequence encess of design design stage stage errors errors   31 7.1.4 7.1 .4 Wha Whatt can can go go wron wrong? g?   31 7.1.5 7.1. 5 Manag Managing ing uncer uncertaint taintyy   31 7.1.6 7.1. 6 Proje Project ct risk aware awareness ness   32 7.2 Man Managi aging ng the the desi design gn proc process ess   32 7.3 Clar Clarity ity of respo responsibi nsibility lity   33 7.4 De Desig signn cha change ngess   33 7.5 Clar Clarity ity of desig designn requi requiremen rements ts   33 7.6 Designing gning robus robustt struc structures tures   33 7.6.1 7.6. 1 Princ Principles iples of robus robustnes tnesss   33 7.6.2 7.6. 2 Desi Designing gning for accid accidenta entall loads loads   34 7.7 Designing gning for cons construct truction ion   34 7.7.1 7.7 .1 Bui Builda ldabil bility ity   34 7.7.2 7.7. 2 Desi Designing gning out const constructi ruction on hazards hazards   35 7.7.3 7.7. 3 Commu Communica nication tion of risk    36 7.7.4 7.7. 4 Desi Designing gning for unfami unfamiliar liar enviro environment nmentss   36 7.8 Design gn for for the whole build building ing life ife cycle   36 7.9 Designing gning for futur futuree demo demolitio litionn   36 7.10 Proc Procureme urement nt and planning anning   37 7.11 Conc Conclusio lusions ns and recommenda recommendations tions   37 7.12 Refe Reference rencess   37 Risk man Risk manage agemen mentt dur during ing con constr struct uction ion   39 Introd Int roduct uction ion   39 Causes Cau ses of inc incide idents nts   39 Resour Res ourcin cingg and and pla planni nning ng   39 Competenc Comp etence, e, manag managemen ementt and and welf welfare are   40 Commun Com munica icatio tionn   40 Loadin Loa dingg con condit dition ionss   40 Sequence Sequ ence of const constructi ruction on   41 Temp empora orary ry wor works ks   42 Protec Pro tectiv tivee equ equipm ipment ent   42

The Institution of Structural Engineers  Risk in structural engineering

  iii

 

9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9. 8 9.9

Risk man Risk manage agemen mentt duri during ng the lif life e of of a str struct ucture ure   45 Introd Int roduct uction ion   45 The lif lifee cycle cycle of a str struct ucture ure   45 Risks Ris ks durin duringg the life life of of a struc structur turee   45 Risk mana managemen gementt stra strategy tegy   46 Maintenan ntenance ce reg regime ime   46 Appraisal Appra isal and asse assessme ssment nt   47 Conclusio Conc lusions ns and recom recommenda mendations tions   47 Refe Re fere renc nces es   48 Biblio Bib liogra graphy phy   49

10

Risk mana manageme gement nt for demo demoliti lition on and

refurbishment   50 Introducti Intro duction on   50 The potential potential for for unplanned unplanned events events   50 The need to know know the existing existing structur structuree   50 Structural Struc tural refur refurbishm bishment ent   50 10.4.1 10.4 .1 Overview ew   50 10.4.2 Structural stability   51 10.4.3 10.4 .3 Fire precauti precautions ons   51 10.4.4 10.4 .4 Under Underpinni pinning ng works   51 10.5 Managing (deliberate) structural instability   51 10.6 Reduc Reducing ing uncer uncertaint taintyy   51 10.7 Guid Guidance ance   51 10.8 Conc Conclusio lusions ns and recommenda recommendations tions   52 10.9 Refe Reference rencess   52 10.10 Bibliography   52 10.1 10.2 10.3 10.4

 Appendix A Case studies   53  A.1 Introduction   53  A.2  A.3  A.4  A.5  A.6  A.7  A.8  A.9  A.10  A.11  A.12  A.13  A.14  A.15  A.16  A.17  A.18  A.19  A.20  A.21  A.22  A.23  A.24  A.25  A.26

iv

Uncertainty loading   to 53an invalid extent   53 Extension ofintechnology Fatigue loading   54 Uncertainty in extreme loading   54 Failure to understand materials   54 Failure to identify the hazard   54 Errors in dynamics   55 Errors in stability   55 Errors in design or detailing   55 Deterioration and lack of maintenance   55 Identifying significant risks   56 Demolition   56 Human factors   56 Design change   56 Temporary Te mporary works and construction failures   56 Inadequate procedures   57 Systems failures   57 Robustness   57 Mobile structures   58 Failure ure to lear learnn from previous ous incidents incidents   58 Safety culture   58 Competence and quality   59 Failure to understand the structure   59 Novel design   59 References   60

  The Institution Institution of Structural Structural Engineers Engineers  Risk in structural engineering

 

1

Scop Sc ope e an and d ob obje ject ctiv ives es

1.1

Intr In trod oduc ucti tion on

 This chapter of the  Report  defines   defines the scope. This includes includ es what kinds of risk are covered, covered, who the intended readers are and what, in broad terms, the Report  seeks   seeks to achieve.

1.2 1. 2

to comply with the law. The exceptions to this are the chapter on legal requirements and where reference to laws is used to illustrate a principle. So far as the authors are aware, none of the guidance conflicts with any legal requirements.

Typ ypes es of ri risk  sk 

 This  Report  covers   covers all risks which fall to structural engineers to manage. Primarily, this means risks to the health or safety of workers and the public, and serious environmental damage, where there are both legal and moral obligations to keep risks low. Significant cost and programme risk arising from structural engineering decisions is also in the scope. Business risk in engineering projects is not unique to structural engineering and is covered elsewhere 1.1, 1.2, so is not included. Communication and co-ordination is an essential part of risk management. The structural engineer is in a good position to take a lead role on suitable projects and should develop the competences required to do this.  The scope is not limited to design of new works; risks will occur throughout the life cycle of a building or structure; from concept through detailed design, construction, commissioning, operation, maintenance, dismantling, demolition and ultimately to disposal.

1.3

 This  Report  has   has been compiled by a group of  practising engineers and defines the Institution of  Structural Engineers’ view of good practice in risk  management. As legal requirements vary between countries the  Report  does  does not, in general, define how

 The  Report  is   is divided into chapters but important concepts are illustrated throughout. Anyone concerned with risk in structural engineering, at any stage in the life of a structure, is encouraged to read the whole  Report .

1.4

Rele Re leva vanc nce e to cl clie ient ntss

While structural engineers, as designers or constructors, should have the experience and competence to understand and manage risks, clients also have a significant role. They are responsible for providing sufficient information, time and resources to ensure that risks can be properly managed, and they often have essential knowledge on the use and lifetime management of the structure. The engineer should ensure that the client understands these responsibilities. Clients should take care as to whom they allocate risk, as some risks are best managed by the client. Engineers are encouraged to point out to clients the value of sound risk management, underpinned by a culture of safe design, and to support them in achieving this.

Inte In tende nded d re read ader erss 1.5

 The objective of this  Report  is   is to help structural engineers engine ers in all parts of the industry to identify fy risks and deal with them. ‘Structural engineers’ includes not only designers and consultants but also engineers engaged in site work, existing structures and demolition. An understanding of the whole picture is important for engineers in all sectors and will assist exchange of useful information to reduce risk. The  Report  is   is relevant to all structural engineers, from students and graduates learning how to manage risks to experienced engineers and technicians involved with any scale of project from domestic improvements to unusual or high risk  projects.

Summ Su mmar aryy of ob obje ject ctiv ives es

 The  Report  is  is mainly based on UK experience but the principles of good practice are applicable worldwide. Local laws, cultures and economic circumstances

 The  Report  promotes   promotes a proportionate and open approach to risk assessment and management as a process for preventing harm, not for producing documents. It aims to put more meaning behind the words ‘risk in structural engineering’ by describing the types of risks that may affect structural engineering work and by providing a decision making framework for risk, taking account of legal principles, available availa ble guidance and research. research. As set out by the UK’s Engineering Council1.3, good risk management requires engineers to make judgements, avoid risk  aversion and hence facilitate innovation. The  Report  describes the legal background, in the UK and elsewhere, and provides examples of risks that were managed badly and that resulted in serious accidents.  Tools,  T ools, techniques and selected references to assist

may require a different approach in detail. Although the recommendations deal with structural engineering, examples of risk issues from related industries are used to illustrate common themes and to show the potential to apply learning from any source.

engineers are described, although once the principles are understood, managing risk is mostly a matter of  attitude, culture and common sense. The  Report  cannot say everything about risk in structural engineering, but it allows the reader to make further studies. The Institution of Structural Engineers  Risk in structural engineering

  1

 

1.6

Scope and objectives

1.6 6 1.

Refe Re fere renc nces es

1.1

 

Actuarial Profession and Institution of Civil Engineers. Risk analysis and management for projects  [RAMP].   [RAMP]. 2nd ed. London: Thomas Telford, 2005

1.2

 

Actuarial Profession and Institution of Civil Engineers. Handling uncertainty – the key to truly effective  enterprise risk management . Available at: http://www.actuaries.org.uk/sites/a http://www.actua ries.org.uk/sites/all/files/documents/  ll/files/documents/  pdf/ermguidehandlinguncertainty_0.pdf [Accessed: 18 February 2013]

1.3

2

 

Engineering Council. Council. Guidance  Guidance on risk for the  engineering profession . Available at: http://www.engc. org.uk/risk [Accessed: 18 February 2013]

  The Institution Institution of Structural Engineers Engineers Risk  Risk in structural engineering

 

2

Haza Ha zard rdss and ri risk skss

2.1 2. 1

Intr In trod oduc ucti tion on

 This chapter introduces the th e topics of hazard and risk, together with their relevance to structural engineering.

damage to health, pollution, delay to the project, or the completed structure being unserviceable or unstable. If a hazard can be removed entirely, the risk will

It discusses the type of hazards and risks that structural engineers should address.

disappear. It is never too early to consider elimination of hazards; if a hazard is inherent in the concept design, it is often not possible to remove it, only to manage the risk.

2.2 2. 2

Some hazards are outside the control of engineers or cannot be eliminated. These include frequent events like ‘wind loading’, rare events such as ‘earthquake’ or ‘out of control vehicle’ vehicle’ and less well defined hazards such as ‘terrorists’. A higher probability of  failure against risks from less frequent hazards may be considered tolerable, but good engineering can still reduce that risk.

The Th e mea meanin ning g of of ris risk k and and haz hazar ard d

Risk is an integral part of everyday life and one of the most widely considered topics in modern society.  Almost every activity includes some risk. This may be obvious or unrevealed, well or poorly understood and may then be accepted or ignored. Written ‘risk  assessments’ are now produced for many activities but these often miss the point, which is not to produce documents but to remove hazards and to understand, communicate and manage the remaining risks.  The practice of structural engineering acknowledges risk as an inescapable reality. If the structure is not designed or executed properly it may fail with significant consequences, including a liability on those at fault. In everyday language, ‘risk’ refers to danger, peril, and exposure to loss, injury or destruction. In engineering terms, it is useful to distinguish hazard from risk. In this  Report , a ‘hazard’ is something with the potential to cause harm. For example, fire is a hazard; hazar d; death, injury and loss of property as a resu result lt of fire are risks. Risk is a combination of the likelihood of the harmful event occurring and the consequences if it did occur. Depending on the type of risk, the likelihood might be expressed as a probability (e.g. 2%, 1 in 1000) or in words (e.g. barely credible, likely, unlikely, possible). Similarly the consequences could be identified by descriptions such as injury, fatality, cost or delay.  These might then be quantified, if information is available, using details such as ‘loss of injured person’s sight’, ‘cost of £2 million’ or ‘up to six weeks delay’. As a method of combination, risk ¼ likelihood  consequence captures the broad concept, but it is often not possible to be mathematically precise. While 10%  £2 million is clear, it becomes meaningless to multiply descriptive terms, such as ‘unlikely’  ‘up to 6 weeks delay’. In addition, for high consequence risks, the consequence is often given greater weight than the likelihood.

2.3

Struct Str uctura urall eng enginee ineerin ring g haz hazard ardss

 A hazard is anything that may cause harm. In the context of structural engineering, that means anything that can go wrong. Examples include death, injury,

Within the construction process itself there are well known hazards such as ‘work at height’ and hazards to health such as ‘asbe ‘asbestos’ stos’ or ‘wet concr concrete’ ete’ (risk of  dermatitis). There are accepted ways to deal with these; can be eliminate eliminated d and in design, others choice choic e some of construction construc tion method the risk from by the remainder can be minimised. Other hazards, such as instability of the permanent or temporary works, are specific to the project and require individual consideration. Particularly on complex projects, these may not be obvious unless a struct structured ured approach approach is taken to identifying them. A list of common hazards that structural engineers might influence is included in Figure 2.1. Useful additional information is given in CIRIA reports C6622.1 and C6632.2; although written to address UK legislation these are applicable elsewhere. In some cases, hazards that led to serious structural failures may not, and in some cases could not, have been anticipated by the designer. To take two unconnected examples, the causes of the failure of  the terminal building at Charles de Gaulle Airport 2.3 and the collapse of the World Trade Center 2.4 may not have appeared on any list of hazards.

2.4

Struct Str uctura urall eng enginee ineerin ring g ris risk  k 

Risk is not a new subject for structural engineers. engineers. Ways to manage the risk of structural failure have been evolved over hundreds of years. The profession has developed codes of practice and ways of  working that will, most of the time, ensure that structures are adequate for the loads they are called on to resis resist. t. With knowl knowledge edge of the oper operating ating conditions, loads, environment, etc., it is possible to design structures with high confidence of satisfactory performance. Each structure is unique, however, with the result that structural engineering is characterised by the production of ‘prototypes’, none of which will be tested until put into servi service, ce, and will probably probably never be tested against accidental loads unless an accident happens. In practice, knowledge of the operating conditions is usually imprecise. Although The Institution of Structural Engineers   Risk in structural engineering

  3

 

2.4

Hazards and risks

(a) At any time

(b) In design and construction

(c) In use, between uses, after use

 At any time:

In design:

In use:

To structure and people 

To structure and people 

To people in building 

– Inappropriate concept (risks that arise arise later cannot be managed) – Inadequate resources, short-cuts, short-cuts, inexperienced/incompetent inexperienced /incompetent staff – Poor communication, communication, co-ordination co-ordination – Inappropriate procurement – Pressure of time and money – Inappropriate tolerances – Novel materials materials and design design concepts – Defective, unsuitable, unsuitable, undersized or badly specified materials – Foundations: settlement, chemical chemical attack  – Unexpected ground ground conditions (soil (soil strength, contamination, chemical effects) – Unexpected or accidental loads: overload, overload, misuse, weather, terrorism, explosion, impact, temperature – Natural events, events, e.g. floods – Instability – Unidentified critical element(s) element(s) – Lack of redundancy redundancy or other robustness – Fire – Corrosion and and ageing, dynamic dynamic effects, progressive/disproportiona progressive/ disproportionate te collapse – Risks to, or from, from, adjacent buildings, structures and other facilities

– Misunderstandings Misunderstandings in design (including thoughtless use of computers) – Omission of load cases; cases; neglect of dynamic dynamic effects; thermal movements; deflection; inadequate consideration of structure-soil interaction – Neglect of constructability, constructability, maintainability maintainability and demolition

– Stairs, floor finishes, glazing To structure and people 

– Inadequate maintenance – Change of use use – Inadequate access In maintenance: To people doing maintenance 

– Access, confined confined spaces spaces – Hot materials, materials, toxic materials – Falls from height, fragile roofs

In extension, refurbishment and repair: – – – –

In construction:

Misunderstanding the original Misunderstanding original structure Faults in the original structure structure Earlier inappropriate inappropriate modifications Fire-fighting and and emergency emergency services

To people doing construction 

– Falling materials, materials, excavations, excavations, falls from height – Manual handling, handling, toxic materials, vibration, vibration, noise – Vehicular movements – Confined spaces – Putting people close close to hazards (e.g. site office below bridge construction)

In assessment: – Incorrect assumptions assumptions (materials, (materials, structural form, loads) – Inadequate inspection – Structural behaviour – Incorrect analysis

To structure and people 

– Failure of of critical elements elements – Instability of part part completed structure – Temporary works failure due to instability, capacity, foundations or concept

In demolition: – – – – –

Misunderstanding structure Misunderstanding structure Defects in structure structure Inappropriate approach Premature collapse, flying debris High risk elements; elements; cantilevers, cantilevers, flat slabs, prestressed structures, retaining structures

Note  This list is NOT exclusive and is provided as an example of hazard identification identification

Figure 2.1   Some hazards that may be encountered in structural engineering engineering

much has been learned over the years, not every technique is tried and tested, with new methods and products being constantly introduced against a background of pressure to reduce costs and deliver faster.  The emphasis on managing risk to people working on construction is more recent. Construction generally involves several different companies, requiring communication across the interfaces. The site changes as construction proceeds, resulting in risk changing daily. It is exposed to weather and controlling access can be difficult. The structure itself  and the materials used to build it are heavy, meaning that substantial forces are involved and powerful 4

  The Institution Institution of Structural Enginee Engineers rs   Risk in structural engineering

machinery is used. There are few work environments with as many different risks.  The type of risks to be considered at each stage of  the design and construction process will differ. It is important to recognise the general impossibility, for both technical and cost reasons, of being able to eliminate all hazards or reduce risk to zero. Hence, through an experience based decision making process, engineers may choose to ‘accept’ a certain degree of risk, but that has to be done with proper understanding, logic and justification. While the risks of bad weather and poor ground conditions may appear, initially, to be outside the

 

Hazards and risks

control of the structural engineer, they are nevertheless risks which can be reduced if they are given proper consideration. Similarly, structural engineers engine ers can make a benef beneficial icial input in reduc reducing ing the risks in proc procureme urement, nt, such as a less than competent constructor or an inappropriate contract form for the type of structure.

2.4

 

National Commission on Terrorist Terrorist Attacks on the the United ted States. The States.  The 9/11 Commission Report . Available at: http://govinfo.library.unt.edu/911/report/911Report.pdf [Accessed: 18 February 2013]

2.5

 

 ˜ e´ ´ car:  ˜ e ‘Almun car: seeds of a falsework failure’.  New Civil  Engineer , 24 November 2005, p12

Structural failures can also occur during construction and there have been a number of dramatic examples such as the falsework failure of the Almun Almu n˜e´ ´ car car bridge2.5 in Spain (2005). Failures rarely have a single cause, but a signi cause, significan ficantt contr contributo ibutorr to many of these failures was poor communication between temporary and permanent works designers or failure of designers to understand the loads applied during construction. Some of these failures occurred on large projects involving major companies. This highlights the existence of risk and the fact that no design team is immune. Failures have occurred throughout the history of  structural engineering, often as a result of hazards that were not identified or risks that were underestimated in terms of probability or consequence. Each generation of engineers is faced with new challenges. Unfortunately, many of the old challenges remain and, in some cases, the same mistakes are made. There are many useful lessons in what could go wrong and how it might be prevented, but ‘corporate memory’ is hard of to ‘risk’ retain. starting point for promoting knowledge is The to study incidents and their causes. Appendix A includes a number of case studies.

2.5 2. 5

Sum Su mmary

 The hazards and risks in structural engineering can be considered in terms of structural stability, health and safety of workers and commercial consequences. Construction is at the centre of  structural engineering and involves multiple interfaces, a constantly changing site environment and handling of heavy materials. This results in many hazards and risks which can be severe in both probability and consequence. Structural engineers have a legal and professional duty to eliminate hazards and minimise the probability and consequences of any remaining risks. Some are easy to identify and manage; others require detailed consideration and a structured approach. Chapter 3 will discuss the broad principles and issues in this task, Chapter 4 will define the legal const constraint raints s and Chapter 5 will give specific recommendations.

2.6 2. 6

Refe Re fere renc nces es

2.1

 

Gilbertson, A. A. CDM  –  Construction work sector   CDM 2007  – guidance for designers. C662 . London: CIRIA, 2007

2.2

 

Gilbertson, A. A. CDM  CDM 2007  –  –  Workplace ‘‘in-use’’  guidance for designers. C663 . London: CIRIA, 2007

2.3

 

Wood, J.G.M. ‘Paris airport terminal collapse: lessons for the future’. The future’.  The Structural Engineer , 83(5), 1 March 2005, pp13-14 The Institution of Structural Engineers   Risk in structural engineering

  5

2 .5

 

3

Princi Pri ncipl ples es of ri risk sk ma mana nage geme ment nt

3.1 3. 1

Intr In trod oduc ucti tion on

 This chapter discusses structural engineering risk in general terms; how and why it may arise and some of  the principles that influence risk management. Specific guidance on how to apply risk management tools is given in Chapter 5.

3.2

The imp import ortanc ance e of ris risk k mana managem gement ent

In most countries, countries, the accid accident ent rate in construction construction is well above the average for industry in general. Even without accidents, many common construction practices can affect workers’ health. Unforeseen events can cause projects to run seriously late or over budget. Frequently, the lack of foresight occurs during the structural design phase or in the detailed planning for construction. Every risk is a learning opportunity. Only a small percentage of structures fail in harmful ways but minor mishaps are more common. The evidence from individual tragedies and ‘near misses’ is crucial to education in assessing risks and producing safer designs, particularly for low probability, high consequence events.  All parties have an obligation to assess and minimise risks that might cause personal harm and, in some  jurisdictions, ctions, specific specific roles and recor records ds are requir required. ed. Many engineers will also perceive a profound self  interest in minimising risks that might damage their professional lives. There is also good economic  justification cation for taking the trouble to manage risks properly proper ly.. Failur Failures es and accide accidents nts can be very expensive. ve.  The cost of structural failure invariably invariably exceeds the costs of preventing the incident by a significant margin. In addition to the human cost, a single fatality can result in millions of pounds in direct and indirect costs, such as stopping work, internal inquiries and contract penalties, with the possibility of prosecution.  The true cost of accidents and ill-health resulting from work is often underestimated. Research in the UK  has reported that various employers estimated their costs at £30000 to £2 million per year 3.1. Large organisations tend to have a better understanding, due to the number of accidents; in a small company, accidents are infrequent and the costs are not appreciated. The cost to employers of workplace injuries, work-related ill-health and accidental damage events in the UK has been estimated to be between £3.5 billion and £7.3 billion a year 3.2. The economic cost and disruption to society from infrastructure failures such as loss of a bridge is almost incalculable.

at risk minimisation may be best applied, it is necessary to understand how these factors affect the size of each risk, so that they can be given consideration in proportion to their seriousness. Neither structural engineering nor risk management is an exact science. While the probability of a particular load, such as wind, can be estimated, it is generally difficult to quantify structural risk in numerical terms.  Although many techniques exist to attempt this, some of which are discussed in Chapter 6, their main use is to make comparisons between risks.  Assessment of residual risk for different options can help to select the best approach. Most risks can be minimised by following good practice and using engineering judgement which, in turn, requires the kind of background information this  Report  provides.  provides. Even within the profession, there will be different attitudes to risk; engineers are required to produce safe structures against a background of uncertainty in loading, using materials that have variable properties, all supported off uncertain ground conditions.  To understand a risk it is necessary,  To necessary, among other things, to have a realistic or bounding estimate of the probability of the initiating event. Perceptions of risk  vary, and are not always an accurate pointer to areas for risk reduction. Some risks may have high probability but limited consequences, while others may have low probability probab ility with seriou serious s consequ consequences. ences. If there are risks with high probability and high consequence they are unlikely to be tolerable; the hazard should be eliminated or the project would not be viable. Engineers often work with unknowns, and have developed methods to manage this. While the actual strength of a given section of concrete is unknown, as is the highest wind speed next winter, statistical methods and factors of safety enable engineers to have appropriate confidence in their designs. Wind speed and concrete strength may be defined as ‘known ‘know n unknow unknowns’, ns’, because the events they relat relate e to have been identified.  There are also ‘unknown unknowns’, where even the possibility has not occurred to the engineers involved. If the designers of the World Trade Center had anticipated that terrorists might deliberately crash a large fully fuelled aircraft into their structure, they might have been able to estimate the range of  possible impact parameters, or ‘known unknowns’.  As it was, the event was outside anyone’ anyone’s s imaginatio imagi nation n at the time of desig design n3.3. Completely unknow unk nown n eve events nts are som someti etimes mes kno known wn as ‘bl ‘black ack swa swan’ n’ events3.4. Until the first black swans were were discovered in  Australia, ornithologists never considered the possibility that swans could be anything other than white. whi te. A fur furthe therr exa exampl mple e is the fai failur lure e of the Nis Nishin hinomi omiya ya Harbour Bridge during the ‘Kobe’ Earthquake 3.5

3.3 3. 3

How Ho w bi big g is th the e ri risk sk? ?

 The combination of the type of harm, its severity and its probability defines the risk. To decide where effort 6

  The Institution Institution of Structural Engineers Engineers Risk  Risk in structural engineering

(1995) . The made ground slumped slumped towar towards ds the harbour, taking the bridge foundation with it and causing the side span to lose its bearing (Figure 3.1); it appears appea rs that this failure mode was not antic anticipate ipated d by the des design igner er.. Rob Robust ustnes ness s and sen sensit sitivi ivity ty stu studie dies s arethe best approach to managing such unknown risks.

 

Principles of risk management

Figure 3.1   Nishinomiya Harbour Bridge

3.4 3.4

Comp Co mpet eten ence ce

Competence is crucial to risk management; decisions should only be made by people who have appropriate skills, knowledge and experience. Sometimes, this requires involvement of more than one person. A group of experienced people, using a brainstorming approach, will often identify risks and ways to reduce them which would not occur to an individual working alone. Ensuring adequate competence and resources is a fundamental part of procurement for any contract, be it for design, construction, maintenance or demolition. Passing risk along the supply chain may appear to save money, but it will not do so if the risk becomes the responsibility of somebody who is unable to manage it.

and safety safet matbest matters, ters,people which intomany cases means they should bey the co-ordinate the design to minimise risk. Communication is not only necessary from engineer to engineer, but between engineers and the public. It is very easy to find that public opinion is opposed to a development because the risks (both from building it and from not building it) have not been fully explained. There was controversy in 2012 when a number of Italian earthquake scientists were convicted for apparently giving inappropriate reassurance that a severe earthquake in L’Aquila 3.8 was not imminent. The error was said to be not in their scientific predictions, but in the way these were expressed to the public.

3.6 3.5

Comm Co mmuni unica cati tion on

Management of any risk requires good communication, co-ordination and co-operation. Many environmental and human disasters are caused not by deliberate omission but by oversight and lack of communication. Many accidents occur because of changes made by somebody who didn’t understand why it was done the way it was. In many countries there is now a legal requirement for co-ordination of both design and construction work. In the European Union, this stems from the  Temporary  T emporary or Mobile Construction Sites Directive3.6, implemented in the UK as the Construction (Design and Management) Regulations3.7. This has resulted in an unfortunate trend for co-ordination of health and safety to be seen as a separate responsibility from co-ordination of the design itself. Structural engineers should, in order to do their job, be competent in relevant health

Whatt is Wha is an an acce accepta ptable ble lev level el of ris risk? k?

 The acceptability of the risk partly depends on the type of harm. In the case of harm to people, then the tolerability of that risk should be lower. If the consequence is only commercial, such as delay or lack of performance, the client and design team are entitled entitl ed to consider der how much they are prepared prepared to spend to control the risk. In some cases there are legal or regulatory requirements for particular types of  risk, but the difficulty of quantifying risk results in many laws being targeted at processes and behaviour rather than directly at risk. Consequence is an important part of risk. Structural failure in one situation may have relatively low consequences; in another situation a similar amount of structural damage could result in much greater harm to people, financial cost or disruption to society.  As an example, consider a bridge leading to a farm; the potential consequences of collapse into a river, onto a main road or onto a high speed railway could be very different. The Institution of Structural Engineers  Risk in structural engineering

  7

3.4

 

3.7

Principles of risk management

While probability and consequence are separate aspects of risk, a subjective assessment of probability might subconsciously take the consequences into account. The relative probability of the same event affecting different structures may also be relevant.  Terrorist  T errorist attack would not normally be a valid consideration for a private housing development but might well be for government offices.  Account needs to be taken of people’ people’s s risk  perception, as well as the reality of the risk. Some risks occur naturally, while others are artificial. Gravity

construction industry has the acronym ERIC – Eliminate, Reduce, Inform, Control. Further detail is given in Section 5.3. In practice, it will often be possible to combine actions from several levels to give the optimum overall reduction in risk. Designers have more opportunity to eliminate hazards than constructors do. Once the design is fixed, the scope for hazard elimination is much reduced and the constructor may only be able to reduce the risk. Other aspects of the hierarchy include use of engineered measures in preference to management systems, and controls that protect everyone rather than those that protect

loading is very predictable (the initiating event for failure under normal loads is usually an error of some kind) but the publi public c expec expectatio tation n is that all structures structures will be ‘safe’ under gravity loads so the acceptable risk level is very low. In contrast, there is some tolerance to limited structural failures in UK from severe storms and, for events such as earthquakes, there is a gene general ral feeling that some failu failures res are more or less inevitable even though the reality is that many are preventable at affordable cost.

individuals. That said, flexibility and proportionality are necessary. For a one-off or infrequent activity, it may be  justifiable and sometimes safer to use a different approach. For example, when using a crane in a restricted space, it may be better to rely on skilled operators and good planning than to try to set up slewing limits and interlocks. A second example is the use of trained roped access engineers on inspections where this avoids building a large scaffold for a day’s work.

No society can be ‘risk free’ and there are sometimes demands for more stringent regulation to control the risk created by others. Some types of harm are dreaded drea ded more than other others, s, making the risk even less acceptable; for example, the UK’s Health and Safety Executive considers that the risk of work related cancer should be weighted more highly because

In this  Report , the term ‘mitigation’ is used to refer to actions which reduce the consequence of the event but do not prevent it. Mitigation therefore sits at the bottom of the hierarchy. The use of a full body harness and energy absorbing lanyard for work at height is an example of mitigation. By allowing a fall to occur, however, control of the situation is lost, and

3.9

people this more thanisother causesless of death In mostfear countries, society becoming willing to. accept risk, but risk appetite in any situation is influe inf luence nced d by the ove overal ralll bal balanc ance e of cos cost, t, re resou source rces s and risk from other sources.

3.7

Propor Pro portio tional nality ity and ALA ALARP RP

 The general principle of proportionality is that most effort should be applied where the probability of harm and/or the magnitude of harm are greatest.  The idea that risks should be reduced to ‘as low as is reasonably practicable’ is known by its initials,  ALARP3.10. Although a principle in UK legislation (see Section 4.4) and a useful concept, not all legal systems adopt it and it does not apply where there is a more specific legal duty, e.g. in relation to work at height. The wording in UK law is ‘so far as is reasonably practicable’ (SFARP or SFAIRP) but is effectively the same.  ALARP is a useful principle, as it means that common sense can be applied. If there is a serious hazard with a high risk of occurrence, the engineer has the support of the law in making a case to do something about it, even if it is expensive. On the other hand, if  the only way to reduce the risk is expensive, and the cost is completely disproportionate to the benefit,  ALARP is a justification for not doing it. Alternatives to  ALARP are typically less flexible so can result in wasted effort.

3.8

Risk Ris k ma manag nageme ement nt hie hierar rarchy chy

Every activity activity shoul should d take account of the risk  management hierarchy3.7. The usual form in the 8

  The Institution Institution of Structural Engineers Engineers Risk  Risk in structural engineering

unexpected occur; suchtrauma. as breakage of consequences a lanyard, injurycan or suspension For some years, this individual ‘protection’ measure was standard for work at height in the UK; it has now been recognised as mitigation. If possible, work  should be carried out from ground level or from working platforms. Collective mitigation measures such as safety nets should be considered if work at height cannot be eliminated or the initial fall prevented.

3.9 3. 9

Risk Ri sk av aver ersi sion on

Risk aversion simply means an unwillingness to take risk. In the context of risk management it has developed a number of meanings.  The first is that public perception of risk may not be what a strictly ‘statistical’ approach would suggest. Some risks are less acceptable to the public than others; other s; for examp example, le, in the UK fatal accidents accidents to rail passengers attract more public concern than road accidents. This means that engineers need to ensure clear communication of risk and to take account of  perceptions.  The second is that people can become unable to accept any risk, even where necessary to avoid a more serious risk. This can lead to a project stalling, or to a preoccupation preoccupation with trivial risks at the expense of more serious issues being overlooked 3.11. For example, examp le, on a project to refu refurbish rbish a rail railway way bridge, a ‘risk assessment’ identified ‘noise from trains’ as a hazard and proposed, in mitigation, that all personnel should wear ear protection, which would actually have placed them at greater risk. It is important to ‘step back’ occasionally, particularly if working from checklists or procedures, and look at the ‘big picture’ by asking ‘where are the significant

 

Principles of risk management

risks?’ In some cases ‘do nothing’ will carry less risk  than ‘do something’. Aversion to risk should not be allowed to stifle innovation and the freedom of  designers to create the right structure for the task 3.12. By studying the risks, it is often possible to find a ‘win-win’ ‘winwin’ solu solution tion that reduces risk and at the same time improves the structure, making it safer, more useful and more economical over its planned lifetime3.13.

good idea. In some cases, the constructor will be best placed to manage a risk; for example, adverse weather conditions. If, however, the constructor has no effective means of controlling the risk, this will be reflected in the tendered price. The client may believe the price is fixed, but that price may be much higher than if the client had accepted the risk or perhaps shared the burden. If the risk materialises, the history of contract disputes suggests the expected fixed price may not be realised. There may be circumstances where a client is prepared to pay extra in exchange for confidence in the price, but this

3.10 3.1 0 Res Resili ilienc ence e

should not be done by defau should default lt or withou withoutt knowl knowledge edge and generally it is wiser to be completely open about the risks.

Severe but unusual or infrequent hazards require a cautious approach to risk. While a natural hazard such as a tsunami cannot be prevented, its effects can be reduced by building structures that are resistant to flooding. Such events are so rare that there is often insufficient information to make a proper assessment of the probability of occurrence. An unstructured estimate may show a low probability of  failure, providing a false reassurance. For low probability, high consequence events it is better to start from the consequences, and consider whether, under any circumstances, failure would be acceptable acce ptable.. If not, somet something hing should be done to reduce the risk. Examples might be the impact of a ship a fully loade d suspe suspensio nsion n bridg bridge e or aEast naturall natura eventon such as loaded the tsunami that struck North Japan in 2011 resul resulting ting in many thousands of deaths and putting a nuclear power station into a potentially dangerous condition3.14.  The important aspect of resilience based design is that it does not involve setting a probability for the load or initiating event. Where the consequences are unacceptable, the design should eliminate the possibility that the hazard can have a serious impact.  This may require consideration of a ‘beyond design basis’ event. This does not mean, however, that the design should aim to reduce the risk, or even the consequences, to zero. A tsunami would be expected to result in significant damage to infrastructure, but gross loss of life should be avoided and the infrastructure should be repairable.

3.11 3.1 1 Own Owners ership hip and and control control of risk riskss  An ‘owner’ (i.e. a person or organisation that is responsible for managing that risk) should be identified for every risk. Unless there is a legal requirement, ownership is typically allocated to the party best able to manage the risk, although who that is may be a matter of opinion.  The ownership of risk typically varies in different communities around the world. The responsibility and accountability for risk management may lie with the professional engineer, or the developer and their team, or with the ministries of the state, or with those bodies notionally independent of actual design and construction who check for compliance to laws, codes and regulations. Some clients choose to use contracts to place commercial risk with the constructor, giving the client the ‘benefit’ of a fixed price. This is not necessarily a

While, with sufficient awareness, commercial risk may be transferred to others, risks to health and safety should not be. In many cases, the law will prevent such delegation. For example, the 2007 revision of  the UK’s CDM Regulations3.15 places more duties on the client. This recognises that the designer’s opportunit oppo rtunityy to manage risk will depend on the availability of adequate time and funds.

3.12 Soc Societ ietal al ris risks ks

Some the work of that structural can exposeofmembers the public toengineers risk, evendo those who have not chosen to be involved with the project. Failure of an industrial structure resulting in an explosion or a release of toxic gas could have serious consequences extending beyond the site or project. The Buncefield3.16 (2005) and Flixborough3.17 (1974) explosions and the Bhopal3.18 (1984) toxic gas release did not result from structural failur fai lures es,, but sim simila ilarr eve events nts cou could ld hap happen pen if sup suppor ports ts to process vessels failed.  Very large risks may give rise to ‘societal concern’, meaning risks that could impact on society and could have adverse repercussions on those responsible for putting in place the high level provisions provisions and arrangements for protecting people, e.g. Parliament or the Government of the day. The type of risks that could provoke a socio-political response would involve widespread or large scale detriment or the occurrence of multiple fatalities in a single event. Society’s concerns are not necessarily rational; risks that are mathematically or scientifically quite small may provoke serious public concern. Typical examples relate to nuclear power generation or railway travel.

3.13 Huma Human n failure failure and acci accidents dents Many accidents that are attributed to ‘human error’ are actually the result of badly designed systems that put humans into situations where they are more likely to make mistakes.3.19 HSG483.20 gives examples of  accidents in the transport, process and nuclear power industries. Failures can occur at any level in an organisation, from the shop floor to senior management. For many of these major accidents, human failure was not the sole cause but one of a number of causes, including technical and organisational failures, that led to the final outcome. The Institution of Structural Engineers  Risk in structural engineering

  9

3.10

 

3.14

Principles of risk management

Many ‘everyday’ minor accidents and near misses also involve human failures. James Reason3.21 has defined five root causes of accid accidents, ents, at least three of which relate primarily to human error. James Reason also developed the ‘Swiss cheese’ or ‘failure window’ concept. This postulates multiple barriers against failure, but each barrier contains holes that represent errors. If all the holes are aligned, or if everybody relies on somebody else to prevent the failure, the barrier fails.

and during the final flight of the space shuttle Columbia (2003), evidence that Columbia was at risk was not taken sufficiently seriously. Engineers with safety concerns were intimidated by bureaucratic systems or managers who believed the risk was small and were more concerned about cost and schedule. The official report 3.24, 3.25 states ‘‘... engineering teams were held to the usual quantitative standard of proof. But instead of having to prove it was safe to fly, they were asked to prove that it was unsafe to fly.’’

Within the field of structural engineering, there are many situations where design decisions can set up the potential for mistakes, or even create latent errors. For example: – Steel columns, identical except for the grade of  steel, may be confused on site. – Complex setting out arrangements are both difficult to execute and to check. – Concrete sections of different size and/or reinforcement at every grid line have the potential for error.  The so called ‘Murphy’ ‘Murphy’s s law’ states that if something can go wrong, it will go wrong; the only way to avoid this is to anticipate mistakes and design them out. Simplicity is helpful; it is more difficult to identify errors in complex or novel designs. It is surprisingly easy to make apparently ludicrous mistakes such as building structures the wrong way round or a metre out of  position. a major averted UK industrial a seriousthat error wasInnarrowly when facility, it was realised the site datum for two adjoining facilities differed by a metre. In its 16th report, in 2007 3.22, the Standing Committee on Structural Safety (SCOSS) used the generic headings of people, process and product – ‘the 3Ps’ – to categorise ongoing issues. ‘People’ is the most centrall of these, since people develop and use both centra processes and products.

3.14 Saf Safety ety cul cultur ture e Risk management in structural engineering requires an understanding of how organisations behave, as well as structures. Failure to understand any element of this complexity may lead to risks being higher than expected. ‘Safety culture’ describes the way an organisation and its members respond to the need to ensure health and safety. It has been defined as ‘‘the product of individual and group values, attitudes, perceptions, competencies, and patterns of behaviour that determine the commitment to, and the style and proficiency of, an organisation’s health and safety management’’3.23.  To develop a safety culture requires leadership,  To communication, employee involvement and in particular the establishment of a ‘learning culture’, not a ‘blame culture’, within the organisation. If staff, rather than the syste system, m, are blame blamed d for accidents accidents they will tend to cover up any that occur. This will distort the picture and make it difficult to achieve a genuine reduction in risk.  A poor safety culture has been responsible for many errors in risk management. For example, prior to 10

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

Following the explosion at the Texas City oil refinery (2005)3.26, official criticism of the operator extended beyond the circumstances of the actual incident to condemnation of their safety culture and the standards at their other US refineries.  An organisation with a strong ‘safety culture’ will usually find it easier to have an open discussion about risk, and reach more ‘honest’ conclusions about which risks are significant. significant. The best way to assess risks is to feel involved with the outcome, and not to assess them just to meet legal or regulatory requirements.

3.15 Presc Prescriptio ription n versus versus engine engineering ering  judgement  The objective of design codes is to ensure that engineers are well informed about how design should be done. At the same time, they reduce the need for engineers to understand the fundamental principles and discourage initiative and innovation. Some countries make compliance with design codes a legal duty, while others treat them as guidance. There will always be situations which are not covered by the codes, and in which the structural engineer will need to use judgement. This is much more than opinion; judgement should be supported by logical argument and the engineer’s peers should be convinced by this. For design of simple structures, compliance with routine design codes should result in an adequate structure, although the safety of those constructing, maintaining and using it will still need to be considered. As codes evolve, however, it is important to keep sight of the bigge biggerr pictur picture e and consider der issues which are not included in the codes. The refurbishment or demolition of even simple structures is likely to require judgement. There are many structures where a purely prescriptive approach is insufficient and structural engineers have a professional obligation to understand the background to the notion of ‘safety’ and to take active steps to promote it. This applies particularly when designing structures of any size for hazardous industries or that may be subjected to rare but potentially catastrophic events. It is therefore therefore essential essential for engineers engineers to take the time to study the fundamentals of their art and what constitutes good practice, not merely follow rules blindly. This requires training and good understanding, partly because the solutions may not be prescriptive but instead require the exercise of  considerable judgement. This is reinforced in a report by the UK’s Engineering Council3.12.

 

Principles of risk management

3.16 The saf safety ety cas case e

3.18 Ref Refere erence ncess

 The concept of a ‘safety case’ is often applied to high risk industries. It is only likely to be directly relevant to structural engineers working in those industries, but it provides another way of looking at risk management and may stimul stimulate ate thought.

3.1

 

Haefeli, K. et al. al. Perceptions  Perceptions of the cost implications of  health and safety failures, Research Report 403 . Sudbury: HSE Books, 2005. Available at: http://www. hse.gov.uk/research/rrpdf/rr403 hse.gov .uk/research/rrpdf/rr403.pdf .pdf [Accessed: 18 February 2013]

 The traditional approach to safety regulations in the UK was prescriptive; rules were laid down about what was and was not acceptable practice. In the

3.2

 

Health & Safety Executive. Executive. The  The cost to Britain of  workplace accidents and work related ill health in  1995/96. HSG101. HSG101. 2nd ed. Available at: http://www.

1970s there was a move towards a goal-setting approach; employers became responsible for deciding how to manage their operations to meet specified standards. This aligned with the increasing complexity in industries such as nuclear power, and the concept of a safety case was developed.  The safety case is a document or set of documents recording a formal assessment that may include analysis of hazards, risks, protection and mitigation. It is produced by the operator to assist with safet safetyy management of a facility or system and may be required by, and thus submitted for approval to, a regulatory body such as the Health and Safety Executive. It allows the logic (‘why this is safe’) to be set down for review; it should always be the objective of the operator to ensure that a good safety case is produced, without relying on the regulator.

hse.gov.uk/pubns/priced/hsg101.pdf [Accessed: 26 Oct 2012] 3.3

 

National Commission on Terrorist Terrorist Attacks on the United States. The States.  The 9/11 Commission  Report.   Report. Available at: http://govinfo.library.unt.edu/911/report/911Report.pdf [Accessed: 26 February 2013]

3.4

 

Taleb, N.N. N.N. The  The Black Swan: the impact of the highly  improbable . Rev ed. London: Penguin, 2010

3.5

 

EEFIT. The EEFIT.  The Hyogo-Ken Nanbu (Kobe) earthquake of  17 January 1995: a field report . Available at: http://www.istructe.org/downloads http://www.is tructe.org/downloads/resources-centre /resources-centre/ /  technical-topic-area/eefit/eefittechnical-topicarea/eefit/eefit-reports/hyogo-kenreports/hyogo-kennanbu-kobe.aspx [Accessed; 18 February 2013]

3.6

 

Council Directive 92/57/EEC of of 24 June 1992 1992 on the  implementation minimumorsafety health  sites  requirements at of temporary mobileand construction (eighth individual Directive within the meaning of   Article 16 (1) of Directive 89/391/EEC). Available 89/391/EEC).  Available at: http://eur-lex.europa.eu/LexUriServ/LexUriSe http://eur-lex. europa.eu/LexUriServ/LexUriServ.do?uri= rv.do?uri= CELEX:31992L0057: EN:HTML [Accessed: 18 February 2013]

 The concept has expanded to include railways and offshore offs hore oil and gas. While the detailed ed content of a safety case is a matter for its authors, the format and controls on the production and use of a safety case may be subject to regulatory guidance 3.27.

3.7

 

CITB – Constru ConstructionS ctionSkills. kills. ‘Sectio ‘Sectionn 2: Hazard elimina elimination tion and risk reduction ‘. In CITB – ConstructionSkills. The  ConstructionSkills.  The  Construction (Design and Management) Regulations  2007 – Industry guidance for designers . King’s Lynn: ConstructionSkills, 2007. Available at: http://uk.sitestat. com/citb/cskills/s?search.CDM_Designers4web_07_ tcm17-4643&ns_type=pdf&ns_url=http://www.cskills. org/uploads/CDM_Designers4web_07_tcm17-4643.pdf [Accessed: 18 February 2013]

3.8

 

Hall, Stephen S. ‘Scientists on trial: at fault?’, fault?’, Nature,  Nature, 477, 264-269 (2011) [Online]. Available at: http://  www.nature.com/news/2011/1109 www.nature .com/news/2011/110913/full/477264a. 13/full/477264a. html [Accessed: 22 July 2013]

3.9

 

Health & Safety Executive. Executive. Reducing  Reducing risks, protecting  people: HSE’s decision-making process . Sudbury: HSE Books, 2001. Available at: http://www.hse.gov.uk/risk/  theory/r2p2.pdf [Accessed: 18 February 2013]

3.17 Concl Conclusion usionss and reco recommen mmendation dationss Risk management is a fundamental part of  engineering and it may be argued that it should not require separate consideration. It is, however, an essential engineering skill and needs specific consideration as projects become more complex. Structural engineering failures can have very serious consequences3.28. As codes of practice become more prescriptive, the engineer’s vision can become narrower. Engineers should understand: – When to use engineering engineering judgement. judgement. – What is an acceptable level of risk. – How probable is a hazard to be realised. – Risk tools such as hierarchy, qualitative probability and proportionality proportionality.. – Risk aversion and ownership of risk. – Organisational factors such as safety culture. Structural engineers (both designers and constructors) are in a unique position to understand the most significant risks in a project. This is not limited limite d to risks arising from the struct structure ure itself and the structural engineer’s wider view of the project can help manage risks arising from, for example, mechanical services. The structural engineer should be prepared to take a lead role in explaining the risks to others and insist that all risks receive proper consideration. In particular, a flawed concept design should not be accepted. Younger engineers may find this intimidating when dealing with project managers and clients and if necessary should ask for and receive support from their senior managers.

3.10   Health and Safety Executive. Executive. ALARP  ALARP ‘at a glance’ .  Available at: http://ww.hse.go http://ww.hse.gov.uk/risk/the v.uk/risk/theory/  ory/  alarpglance.htm [Accessed: 18 February 2013] 3.11   Hackett, J. ‘Unintended consequences’. consequences’. Judith   Judith  Hackitt’s blog . Available at: http://www.hse.gov.uk/  news/judith-risk-assessment/cons news/judith-riskassessment/consequences211112. equences211112. htm [Accessed: 18 February 2013] 3.12   Engineering Council. Council. Guidance  Guidance on risk for the  engineering profession . Available at: http://www.engc. org.uk/risk [Accessed: 18 February 2013] 3.13   Powderham, A.J. ‘Safety as a driver for innovation in design and construction of underground structures’, The Institution of Structural Engineers  Risk in structural engineering

  11

3.16

 

3.19

Principles of risk management Proc. International Conference on Deep Excavations, Singapore, 2008  3.14   Office for Nuclear Regulation. Regulation. Japanese  Japanese earthquake  and tsunami: implications for the UK nuclear industry. Final report. ONR-FR-REP-11-002 . Available at: http://  www.hse.gov.uk/nuclear/fukushima www.hse.gov .uk/nuclear/fukushima/final-report.pdf /final-report.pdf [Accessed: 18 February 2013] 3.15   The Construction Construction (Design and Management)  Regulations 2007  (SI   (SI 2007/320). Available at: http://  www.opsi.gov.uk/si/si2007/20070 www.opsi.gov .uk/si/si2007/20070320.htm 320.htm [Accessed: 18 February 2013] 3.16   Explosion on Mechanism Mechanism Advisory Group report . London: Buncefield eld Major Incident Investigation gation Board, 2007.  Available  Availab le at: http://w http://www.b ww.buncefie uncefieldinve ldinvestigati stigation.gov.uk/  on.gov.uk/  reports/buncefieldagr.pdf [Accessed: 18 February 2013] 3.17   The Flixborough Flixborough disaster: disaster: report of the Court of Inquiry . London: HMSO, 1975 3.18   Srishti. Srishti. Surviving  Surviving Bhopal 2002. Toxic present – toxic  future: a report on human and environmental chemical  contamination around the Bhopal disaster site . New Delhi: Srishti, 2002. Available at: http://www.bhopal. net/oldsite/documentlibrary/survivingbhopal2002.doc [Accessed: 18 February 2013] 3.19   Williams, R. R. This  This one will bring the house down: ICE  IStructE HSE SCOSS Prestige Lecture 28th April 2009 .  Available at: http://cms.structuralhttp://cms.structural-safety.org/a safety.org/assets/  ssets/  uploaded//documents/118_SC09.039%20%20Prestige%20Lecture%202009%20%20Richard%20Williams.pdf [Accessed: 18 February 2013] 3.20   Health & Safety Executive. Executive. Reducing  Reducing error and  influencing behaviour. HSG48 . 2nd ed. Sudbury: HSE Books, 1999. Available at: http://www.hse.gov.uk/  pubns/priced/hsg48.pdf [Accessed: 18 February 2013] 3.21   Reason, J.T J.T..  Managing the risks of organizational  accidents . Aldershot: Ashgate, 1997 3.22   Standing Committee on Structural Structural Safety. Safety. 16th  16th Biennial  report . Available at: http://www.structural-safety.org/  biennialreport [Accessed: 18 February 2013] 3.23   Advisory Committee Committee on on the Safety of Nuclear  Nuclear  Installations. ACSNI Installations.  ACSNI Study Group on Human Factors  third report: Organising for safety . Sudbury: HSE Books, 1993 3.24   Columbia Accident Investigation Board. Board. The  The Report .  Available at: http://www.nasa http://www.nasa.gov/columbia/caib/html .gov/columbia/caib/html/ /  VOL1.html [Accessed: 18 February 2013] 3.25   Mason, R.O. R.O. ‘Lessons ‘Lessons in organisational ethics from from the Columbia Disaster: can a culture be lethal?’ Organizational Dynamics , 33(2), 2004, pp128-142 3.26   Fatal Accident Investigation nvestigation Report: Isomerization Unit  Explosion Final Report . Available at: http://www.bp. com/liveassets/bp_internet/us/bp_us_english/S com/liveassets/bp_int ernet/us/bp_us_english/STAGING/  TAGING/  local_assets/downloads/t/final_report.p local_assets/dow nloads/t/final_report.pdf df [Accessed: 18 February 2013] 3.27   Guidance on on the purpose, scope scope and content of  nuclear safety cases. T/AST/051, Issue 001. 001 . Available at: http://www.hse http://www.hse.gov.uk/nuclea .gov.uk/nuclear/operational/tech_ r/operational/tech_ asst_guides/tast051.pdf [Accessed: 18 February 2013] 12

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

3.28   Gilbertson, A. et al. al. Guidance  Guidance on catastrophic events in  construction. C699 . London: CIRIA 2011

3.19 Bibl Bibliog iogra raphy phy Blockley, D. et al. ‘Infrastructure resilience for high-impact low-chance risks’. ICE risks’.  ICE Proceedings, Civil Engineering Special  Issue , 165(CE6), November 2012, pp13-19 Hudson, S. et al. ‘Engineering resilient infrastructure’.  ICE  Proceedings, Civil Engineering Special Issue , 165(CE6), November 2012, pp5-12 Lord Cullen. The Cullen.  The Ladbroke Grove Rail Inquiry. Part 2 report . Sudbury: HSE Books, 2001. Available at: http://www. railwaysarchive.co.uk/documents/HSE_Lad_Cullen002.pdf [Accessed: 13 September 2010] Montgomery, M. et al. ‘An innovative approach for improving infrastructure resilience’. ICE resilience’.  ICE Proceedings, Civil Engineering  Special Issue , 165(CE6), November 2012, pp27-32 Neale, B. ‘Introduction to infrastructural resilience’. ICE  resilience’.  ICE  Proceedings, Civil Engineering Special Issue , 165(6), November  2012, pp3-4

 

4

Leg egal al ba back ckgr grou ound nd

4.1

Intr In trod oduc ucti tion on

 This chapter discusses UK law4.1 except where stated otherwise. Many of the broad principles, however, apply in other countries, although the law itself may be written very differently. A summary of the legal background in Europe, the United States and Hong Kong is given to illustrate this. Whichever country is being worked in the engineer must be aware of the local legislative framework, particularly in terms of  personal or corporate liability.

4.2

Law as it aff affect ectss str struct uctura urall engineering

Most legislation which affects structures is about preventing people from being harmed or being made ill; the law says people must not put themselves, other workers or the public in

Regulations contain more detail, so need to be easier to change. The act, and the general duties under, for example, e, The Manag Management ement of Healt Health h and Safety at Work Regulations 19994.4 are goal setting and leave employers freedom to decide how to control risks that they identify. Regulations identify some risks specifically and set out specific action that must be taken. Sometimes, these requirements are absolute, i.e. there is a need to do something without qualification.  An approved code of practice (ACoP) offers practical examples of good practice. It gives advice on how to comply with the law by, for example, providing a guide to what is ‘reasonably practicable’. For example, if regulations use words like ‘suitable’ and ‘sufficient’, an ACoP can illustrate what this requires in particular circumstances. An ACoP has a special legal status. If employers are prosecuted for a breach of health and safety law, and it is proved that they have not followed the relevant provisions of the  ACoP,, a court can find them at fault unless they can  ACoP show that they have complied with the law in some other way.

4.2

danger . Both construction work and the stability of  completed structures are subject to legal requirements. Damage to the environment is also becoming increasingly regulated. Health and safety law (criminal legislation) applies to all businesses, however small. It covers employees (full or part-time, temporary or permanent) and the self-employed. Controlling danger at work is no different from any other task; in other words, health health and safety needs specific action to manage it. The structural engineer needs to recognise problems, know enough about them, decide what to do and act on the solutions. It is not just highly unusual or exceptional circumstances that cause accidents or ill health. Some basic thought and action beforehand can usually prevent them. Civil law is concerned with allocating responsibility for loss typica typ ically lly in relation a tion to a con contra tract ct or all allege eged d neg neglig ligenc ence. e. While criminal criminal law is based on statut statutes, es, i.e. laws

Guidance is avail Guidance available able on a range of subjects. Some is specific to the health and safety problems of an industry or of a particular process used in a number of industries. The main purpose of guidance is to interpret interp ret what the law says, to help people comply with the law and to give technical cal advice. Following ng guidance is not compulsory and employers are free to take other action. But if they do follow guidance they will normally be doing enough to comply with the law.

made by parliament, civil law is mainly based on precedent. Health and safety does not have to be expensive, time consuming or complicated, but engineers may be culpable if they do not keep themselves informed over what causes harm.

expressed farasasreasonably is reasonably practicable’ (SFAIRP) or,as, ‘as‘so low practicable’ (ALARP). SFAIRP is the term most often used in the Health and Safety at Work etc. Act 4.3 and in regulations. ALARP4.5 is the term used by risk  specialists. In the view of the Health and Safety Executive (HSE), the two terms are interchangeable (except when drafting formal legal documents, when the correct legal phrase must be used) used)..

4.3

Acts, reg Acts, regula ulatio tions, ns, guid guidanc ance e and  ACoPs

 The basis of British health and safety law is the Health and Safety at Work etc. Act 1974 4.3 (HSWA, the Act4.3 ). An act is primary law, enacted by parliament. parli ament. The Act sets out the general duties that employers have towards employees and members of  the public, and that employees have to themselves and to each other. These duties are qualified in the Act by the principle of ‘so far as is reasonably practicable’ (see Section 4.4). Regulations are secondary law, approved by a minister minist er under power powers s made under the act.

4.4

Reason Rea sonabl ablyy pra practi cticab cable le

What is meant by ‘reasonably practicable’? It may be

 The definition set out by the Court of Appeal4.6 is: ‘‘‘Reasonably practicable’ is a narrower term than ‘physically possible’... a computation must be made by the owner in which the quantum of risk is placed on one scale and the sacrifice involved in the measures necessary for averting the risk  (whether in money, time or trouble) is placed in the other, and that, if it be shown that there is a gross disproportion – the risk–being insignificant in between relation tothem the sacrifice the defendants discharge the onus on them.’’ In essence, making sure that a risk has been reduced to ‘as low as reasonably practicable’ is The Institution of Structural Engineers  Risk in structural engineering

  13

 

4 .5

Legal background

4.5 4. 5 Unacceptable

Risk must be reduced unless there are exceptional reasons why not    k    s    i    r      g    n    i    s    a    e    r    c    n    I

HSE may require evidence that risk is ALARP

 ALARP applies but evidence is not usually required

Tolerable

 The words ‘reasonably practicable’ should not be confused with ‘practicable’. In a legal context, ‘practicable’ infers a statutory obligation that has to be met if, in the light of current knowledge, it is feasible (irrespective of cost or difficulty). Put at its simplest, ‘practicable’ means ‘if it can be done, it must be done’.

4.6 4. 6 Broadly acceptable

Figure 4.1   Framework for for tolerability of risk 

about weighing the risk against the sacrifice needed to further reduce it. The decision is loaded in favour of health and safety as the presumption is that the duty-holder, i.e. the employer, should implement the risk reduction To avoid having to make this ‘sacrifice’, ‘sacrif ice’,measure. the duty-holder duty-holde r must be able to show that it would be grossly disproportionate to the benefits of risk redu reduction ction that would be achieved. Thus, the principle behind the process is not one of balancing the costs and benefits of  measures but, rather, of adopting measures except where they are ruled out because they invol involve ve grossly disproportionate sacrifices. Figure 4.1 4.7 illustrates how this principle is applied across a range of risk severity.

Prac Pr acti tica cabl ble e

Burd Bu rden en of pr proo oof  f 

Where a duty-holder is required to do what is ‘reasonably practicable’ or ‘practicable’ to achieve a safe system of work, Section 40 of the Act 4.3 provides that the burden is on the defendant to satisfy the court that it was not practicable or reasonably practicable to do more to control the risk than was in fact done. This is often referred to as a ‘reverse burden’, because it reverses the normal situation that the prosecution must prove the facts beyond reasonable doubt.

4.7

Liabili Lia bility ty und under er civ civilil law and duty of  care

 As well as criminal law, those who are responsible for harm to others may be sued for damages under civil law. Liability may arise from the terms of a contract, or may exist irrespective of contract under the ‘duty of care’ principle. Duty of care is the obligation to exercise a level of  care towards an individual, as is reasonable in all the circumstances, to avoid injury or loss of property to that individual. individual. It is therefore therefore based upon the relationship of the parties, the negligent act or omission and the ability to reasonably foresee loss to that individual. individual. A negligent gent act is an uninte unintention ntional al but unreasonably careless act that results in loss. Only a negligent act will be regarded as having breached a duty of care. Duty of care arises from the precedent set by previous judgements, rather than law enacted by parliament. Liability for breach of a duty of care can therefore depend on the public policy at the time the case is heard.

 ALARP should be applied to decide whether a proposed risk management measure is necessary.  This may require an assessment of the initial risk, and then the residual risk after applying the selected measures (e.g. elimination, substitution, mitigation, etc.) and comparing the reduction in risk with the cost of providing the measures. Such cost benefit assessments can be time consuming and can be impractical where, as is often the case in structural engineering, data is not available. As an alternative, it can be assumed that follo following wing accepted accepted and relevant ‘good practice’ will ensure that risks are as low as reasonably practicable. In effect, this relies on assessments previously carried out by others, which have resulted in the body of knowledge called ‘good practice’. It is important to ensure that the precedent is relevant; what is good practice for a farm building may not be good practice in a multistorey residential block, and good practice for the residential block may not apply to a hazardous industrial facility facility..

 The law of negligence condemns as negligent any act or omission that falls short of a standard to be expected of ‘the reasonable person’. The application of this test by the courts depends on the type of  case. In a clinical negligence action the standard was defined in the ‘Bolam test’4.10 (1957). This set out the test used when a judge is considering whether or not a doctor has been negligent, and has subsequently been extended to other professions.

 These requirements can appear very complex and daunting but, in reality, industry norms, good practice and a professional approach will guide designers

 The case held that a doctor is not in breach of the duty of care, ‘‘if he has acted in accordance with a practice accepted as proper by a responsible body of 

through the process. However, underlying this is an assumption that judgements4.8 are made by competent persons. For high hazards, complex or novel situations, good practice can be built upon using more formal decision making techniques, including cost-benefit analysis4.9.

medical men skilled in that particular art’’. The practical effect of the test is that a judge will hear evidence from experts in the appropriate speciality and must decide whether the actions of the doctor were proper. Often, there are several acceptable ways of doing something and compliance with any of 

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  The Institution Institution of Structural Structural Engineers Engineers  Risk in structural engineering

 

Legal background these will mean that there was no breach of duty of  care. Naturally, experts often disagree over these issues issue s and the judge must decide whose evidence is to be preferred. It is important that anyone reviewing a case as an expert, or giving an informal view, understands the Bolam test. The fact that the person giving an opinion would not have done things in the same way does not automatically mean that there was a breach of  duty of care. The actions taken may be acceptable to ‘a responsible body of opinion’ and research (such as a literature search) may be needed to check the position. When considering whether one owes a duty of care, up to date information is essential, as case law evolves over time.

4.8

Law La w en enfo forc rcem emen entt

In the UK, the HSE and local government are generally the enforcing authorities for health and safety regulation4.11. The HSE’s mission is to protect people’s health and safety by ensuring risks in a changing workplace are properly controlled. It looks after health and safety in factories, farms, mines, nuclear installations, ations, hospitals and schools, s, offs offshore hore gas and oil installations, the gas grid, the movement of dangerous

building design and construction must comply in the interests interests of the healt health h and safety of build building ing users, of energy conservation, and of access to and use of buildings. The requirements are often referred to as ‘functional’ and are expressed in terms of what is ‘reasonable’, ‘adequate’ or ‘appropriate’. Practical guidance on ways to comply with the functional requirements is contained in Approved Documents, or a Technical Handbook. These contain general guidance on the performance expected of materials and building work. There is a legal presumption that if the guidance has been followed, then this is evidence that the work has complied with the Building Regulations. It is, however, quite acceptable to use alternative methods of compliance provided they fully satisfy the regulations. A designer may put forward other ways of meeting the regulations, but these will require approval. This will be particularly important when assessing the level of risk in modifications or changes of use to existing buildings that do not meet the current regulations. In addition, Section 4A of the Workplace (Health, Safety and Welfare) Regulations4.16 requires that a ‘‘building shall have a stability and solidity appropriate to the nature of the use of the workplace’’ and applies to any workplace

goods andboth substances andand many aspects protection of workers theother public. Local of the authorities are responsible for enforcement in offices, shops and other parts of the services sector.

irrespective of the regulations in force during its original construction.

 The consequence of failure has increased in profile over recent years. Since the Corporate Manslaughter and Corporate Homicide Act 20074.12, companies and organisations can be found guilty of corporate manslaughter as a result of serious management failures resulting in a gross breach of a duty of care. In addition, the Health and Safety (Offences) Act 20084.13 has increased penalties and provides courts with greater sentencing powers for those who flout health and safety legislation.

4.11 4.1 1 Eu Euro rope pe

4.9

Desi De signe gner’ r’ss ro role le

In the UK, the designer’s duties are defined specifically in the Construction (Design and Management) Regulations 20074.14. A designer is anyone who carries out design work as part of a business, including temporary works, fabrication details and details by the constructor. This can include the client. The term ‘design’ is a wide term, and includes drawings, calculations, design details, specifications and bills of quantity. The designer must not only design the work to be safe, so far as is reasonably practicable, but also has an absolute duty to be competent and to ensure that the client is aware of their duties. Similar laws apply throughout the European Union (see Section 4.11).

4.10 Bui Buildi lding ng Regu Regulat lation ionss In the UK, Building Regulations4.15 set out ‘requirements’ with which the individual aspects of 

 Across Europe the work of the European Commission is improving the coherence and rigour of issues concerning the safety of people in their environment. There are, however, deeper and persistent differences in collective and individual responsibility, chain of responsibility, duty to act, the necessity of insurance and its workings, and the balance between prescription and goal-setting. Each member state is required to pass legislation that implements each directive in that state, although the details of implementation may vary. In most European countries the law requires employers to protect workers against all risks. As it is not possible to be absolutely safe (otherwise, for example, nobody could drive on the roads for a living) the courts apply these laws with a view to what is reasonable. Courts in the UK, however, interpret the law precisely according to precedent and statutes. Accordingly, the concept of ‘so far as is reasonably practicable’ has developed. European directives have prompted much of the current UK legislation on health and safety. The CDM Regulations4.14, for example, were enacted as a result of the Temporary or Mobile Construction Sites Directive (92/57/EEC)4.17, which sets out minimum safety and health requirements for construction sites within the European Union. In France, the responsibility for avoiding disasters (not necessaril neces sarilyy risk, or accidents) accidents) is varied. It lies with the state to initiate understanding of issues like natural and unnatural events, landslides, flash floods and the The Institution of Structural Engineers  Risk in structural engineering

  15

4 .8

 

4.12

Legal background acceptability of industrial processes near the population. It is with the population as a whole for issues like the protection of the water cycle, for which there is a specific law. It is with the constructor of  buildings and it is with any company director, by default. The role of the professional engineer disappeared at the French Revolution. Strangely enough, enoug h, in a country with such elegantly elegantly drafted and complete compl ete legal codes, it is necessary necessary to study the effect of the law in practice to understand what society expects from its constructors.

and to complete a kind of risk assessment. The code contai con tains ns a tab table le of cri critic tical al sta stages ges of con constr struct uction ion so tha thatt enhanced supervision is imposed. Examples include demolition of complex structures, such as flat slabs, pre-stressed concrete, transfer plates, hangers, long span beams greater than 10m, steel-framed construction and cantilevered structures over streets with spans greater than 1.2m.

4.13 Unit United ed Sta States tes In a case in France, impending at the time of writing, the courts will be discussing how a disaster occurred when there were three independent structural engineering studies during design and construction.  These were by the  Maitrise d’Oeuvre  (broadly, the project manager), the constructor and finally an independent checker for code compliance. The details of the case are still  sub judice, but it appears that there were several missed opportunities to prevent the disaster. The precursors to failure should have been visible in both the procedural and the structural systems.

4.12 4.1 2 Ho Hong ng Ko Kong ng

In the United States (US), regulation of building design desig n is delegated delegated to each state state.. Most states then delegate regulation to local government, i.e. cities, counties and townships. Cities and counties with large populations generally police design well. However, jurisdictions with smaller populations may have no requirements. In addition to local ordinances to enforce building codes, each state and the six territories licence design professionals through the police powers granted by the US Constitution. The licensing process requires that design professionals use an appropriate ‘standard of care’ when preparing construction documents. Courts generally interpret this to mean that design professionals should follow

 As in other former British colonies such as Australia, much of Hong Kong’s safety and health legislation is based on UK practice. Safety and health law is basically criminal law, administered through the magistrates’ courts. The magistrate has the power, on conviction, to impose fines on companies or employees and can, in extreme cases, impose custodial sentences. Civil law comes into safety issues through contract and tort, and the latter is the basis on which personal injury claims are brought.

For construction site safety, safety, the federal rules put together by the Occupational Health and Safety  Administration  Administ ration (OSHA) apply. apply. These ‘minimum’ requirements are intended to provide a safe work  place. In some cases, the OSHA rules supersede building codes. However, in most cases they cover areas of means and methods not addressed by codes.

Hong Kong’s Occupational Safety and Health Ordinance 19984.18 is similar to the UK’s HSWA 4.3, and includes the same concept of ‘reasonably practicable’.

Construction disputes are settled in civil courts. In some cases, construction accidents are addressed in a criminal court. However, while criminal courts in the US seldom punish design professionals, compensation awards in civil courts can be ruinous.

 The Construction Sites (Safety) Regulations 19784.19 have many similar provisions to those contained in UK  legislation. Both sets of regulations impose a duty on employers to ensure the safety of their employees, whether or not a potential hazard is created by the employer or others, and they also have a duty to any other person who may be affected by the construction works. Employees also have obligations to comply with the regulations, the most obvious being to wear personal protective equipment as specified. Private sector buildings are subject to the control of  the Buildings Department. The Technical Memorandum for Supervision Plans 20094.20 is legally enforceable and lays down specific requirements for site supervision. This is supported by the guidance in the Code of Practice for Site Supervision 2009 4.21. The required number of Technical Competent Persons, the frequency of their inspections and their qualifications and experience must be calculated using a formula based on the scale, complexity and risk of the project.  Typically  T ypically,, the most senior will be a qualified engineer who is recognised as a registered professional engineer (structural, geotechnical or civil as appropriate). In the most complex geotechnical projects, director grade staff from the consultant must visit the site on a weekly basis. The site staff are required to pay attention to particular high risk items

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  The Institution Institution of Structural Structural Engineers Engineers  Risk in structural engineering

model codes and standards for construction.

4.14 Sum Summa mary ry and and concl conclusi usions ons – The legislation affecting structural engineering risk  controls: – Healt Health h and safety at work (preventing (preventing people from being harmed or being made ill). – Stability and safe maintenance of structures, also to protect people. – Protection of the environment. – The law may not signi significan ficantly tly influence influence the decisions decisions an engineer makes, but generally requires a record of the reasons for those decisions. – Controlling danger at work is no different from any other task. The structural engineer needs to recognise problems, know enough about them, decide what to do and act on the solutions. – Whichever Whichever country is being worked worked in, the engin engineer eer must be aware of the local legislative framework. While the principles of risk management remain similar, the legal responsibility for applying them can vary substantially.

 

Legal background

4.15 Ref Refere erence ncess 4.1

  Health & Safety Executive.  Health and safety  regulation: a short guide, HSC13(rev1) . Sudbury: HSE Books, 2003. Available at: http://ww.hse.gov.uk/pubns/  hsc13.pdf [Accessed: 18 February 2013]

4.2

  Health & Safety Executive website. Available at: http://  www.hse.gov.uk [Accessed: 18 February 2013]

4.3

 

4.4

 

4.5

  Health and Safety Executive.  ALARP ‘at a glance’ .  Available at: http://www.hse. http://www.hse.gov.uk/risk/ gov.uk/risk/theory/  theory/  alarpglance.htm [Accessed: 18 February 2013]

Management of Health and Safety at Work Regulations    (SI 199/3242). Available at: http://www.opsi.gov. 1999  (SI uk/si/si1999/19993242.htm [Accessed: 18 February 2013]

4.6

  Edwards v. National Coal Board, Board, [1949] [1949] 1 All ER 743

4.7

  Health & Safety Executive.  Reducing risks, protecting  people: HSE’s decision-making process . Sudbury: HSE Books, 2001. Available at: http://www.hse.gov.uk/risk/  theory/r2p2.pdf [Accessed: 18 February 2013]

4.9

4.16   The Workplace Workplace (Health, Safety and Welfare)  fare)  Regulations 1992  (SI   (SI 1992/3004). Available at: http://  www.legislation.gov.uk/u www.legisla tion.gov.uk/uksi/1992/3004/conten ksi/1992/3004/contents/made ts/made [Accessed: 18 February 2013] as amended by  The  Health and Safety (Miscellaneous Amendments)    (SI 2002/2174). Available at: http://  Regulations 2002  (SI www.legislation.gov.uk/u www.legisla tion.gov.uk/uksi/2002/2174/conten ksi/2002/2174/contents/made ts/made [Accessed: 18 February 2013]

The Health Health and Safety Safety at Work Work etc. etc. Act Act 1974  1974  [as  [as

amended]. Available at: http://www.legislation.gov.uk/  ukpga/1974/37 [Accessed: 18 February 2013]

4.8

see http://www.le http://www.legislation.gov gislation.gov.uk/all?title= .uk/all?title= building%20regulations]

  Institution of Civil Engineers.  A review of, and  commentary on, the legal requirement to exercise a  duty ‘so far as is reasonably practicable’ with specific  regard to designers in the construction industry. London: ICE, 2010. Available at: http://www.ice.org.uk/  Information-resources/Document-Library/So-Far-As-IsReasonably-Practicable [Accessed: 18 February 2013]   Health & Safety Executive.  HSE principles for Cost  Benefit Analysis (CBA) in support of ALARP decisions .  Available at: http://ww.hse.go http://ww.hse.gov.uk/risk/the v.uk/risk/theory/alarpcba. ory/alarpcba. htm [Accessed: 18 February 2013]

4.17   Council Council Directive Directive 92/57/ 92/57/EEC EEC of 24 June June 1992 on on the  implementation of minimum safety and health  requirements at temporary or mobile construction sites  (eighth individual Directive within the meaning of   Article 16 (1) of Directive 89/391/EEC) . Available at: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri= CELEX:31992L0057: EN:HTML [Accessed: 18 February 2013] 4.18   Occupational Safety and Health Health Ordinance. Ordinance. Cap 509 ,  Available at: http://www.legislation.gov.hk/blis_pdf.nsf/  slation.gov.hk/blis_pdf.nsf/  6799165D2FEE3FA94825755E0033E532/  9198BE222266C421482575EF0012128E/$FILE/CAP_ 509_e_b5.pdf [Accessed: 18 February 2013] 4.19   Construction Sites (Safety) Regulations. Cap 59I  59I .  Available at: http://www.legislation.gov.hk/blis_pdf.nsf/  slation.gov.hk/blis_pdf.nsf/  6799165D2FEE3FA94825755E0033E532/ 

CB7ACD5F5F2AF7D1482575EE00356ACA/$FILE/  CAP_59I_e_b5.pdf [Accessed: 18 February 2013] 4.20   Government of Hong Kong Special Administrative Region. Buildings Department.  Technical memorandum  for supervision plans 2009 . Available at: http://www. bd.gov.hk/english/documents/code/TMSS2009_e.pdf [Accessed: 18 February 2013] 4.21   Government of Hong Kong Special Administrative Region. Buildings Department.  Code of Practice for  Site Supervision 2009 . Available at: http://www.bd.gov. hk/english/documents/code/SS2009_e.pdf hk/english/documents/co de/SS2009_e.pdf [Accessed: 18 February 2013]

4.10   Bolam v Friern Hospital Management Committee, High Court, [1957] 1 WLR 583 4.11   Health & Safety Executive ve website. website. Available at: http://  www.hse.gov.uk/aboutus/index.htm www.hse.gov .uk/aboutus/index.htm [Accessed: 18 February 2013] 4.12   Corporate Manslaughter and Corporate Corporate Homicide Homicide Act  http://www.legislati .legislation.gov.uk/ukpga/  on.gov.uk/ukpga/  2007 . Available at: http://www 2007/19/contents [Accessed: 18 February 2013] 4.13   Health and Safety (Offences) Act 2008  2008 . Available at: http://www.legislation.gov.uk/ukpga/2008/20/content slation.gov.uk/ukpga/2008/20/contentss [Accessed: 18 February 2013] 4.14   Construction (Design and Management) Regulations  2007  (SI   (SI 2007/320). Available at: http://www. legislation.gov.uk/uksi/2007/320/co legislation.gov .uk/uksi/2007/320/contents/made ntents/made [Accessed: 18 February 2013] and  The Construction  (Design and Management) Regulations (Northern  Ireland) 2007 . Available at: http://www.legislation.gov. uk/nisr/2007/291/contents/made uk/nisr/2007/291/conte nts/made [Accessed: 18 February 2013] 4.15   The Building Regulations 2010  (SI   (SI 2010/2214).  Available at: http://www.legislation.gov.uk/uksi/2010/  slation.gov.uk/uksi/2010/  2214/pdfs/uksi_20102214_en.pdf [Accessed: 7 March 2013] [Note that similar but separate regulations apply in Scotland and in Northern Ireland,

4.16 Bibl Bibliog iogra raphy phy Confidential Reporting on Structural Safety [CROSS] website.  Available at: http://www.stru http://www.structural-safety ctural-safety.org .org [Accessed: 18 February 2013] Gilbertson, A.  CDM 2007 – Workplace ‘in-use’’ guidance for  designers. C663 . London: CIRIA, 2007 Gilbertson, A.  CDM 2007  –  –  Construction work sector guidance  for designers. C662 . London: CIRIA, 2007 Health & Safety Executive.  Economic Analysis Unit (EAU)  appraisal values . Available at: http://www.hse.gov.uk/economics/  eauappraisal.htm [Accessed: 18 February 2013] House of Commons. Science and Technology Committee. Scientific advice, risk and evidence based policy making. Volume 1: Report, together with formal minutes. HC 900-I . London: The Stationery Office, 2006. Available at: http://www. publications.parliament.uk/pa/cm200506/cms publications.parliam ent.uk/pa/cm200506/cmselect/cmsctech/  elect/cmsctech/  900/900-i.pdf [Accessed: 18 February 2013] House of Lords. Select Committee on Economic Affairs. Government response to the management of risk. HL Paper  The Institution of Structural Engineers  Risk in structural engineering

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4.15

 

4.16

Legal background 249 . London: The Stationery Office, 2006. http://www. publications.parliament.uk/pa/ld200506/lds publications.parliam ent.uk/pa/ld200506/ldselect/ldeconaf/249/  elect/ldeconaf/249/  249.pdf [Accessed: 18 February 2013]

House of Lords. Select Committee on Economic Affairs. Government policy on the management of risk. Volume I:  Report. HL Paper 183-1. London: The Stationery Office, 2006.  Available at: http://ww.publica http://ww.publications.parliament.uk/ tions.parliament.uk/pa/ld200506/  pa/ld200506/  ldselect/ldeconaf/183/183i.pdf [Accessed: 18 February 2013] Iddon, J. and Carpenter, J.  Safe access for maintenance and  repair: guidance for designers. C686 . 2nd ed. London: CIRIA, 2009 Institute of Doctors and Health & Safety Executive.  Leading  health and safety at work: leadership actions for directors and  board members. INDG417 . Available at: http://www.hse.gov.uk/  pubns/indg417.pdf [Accessed: 18 February 2013] Rail Safety and Standards Board.  Safety Decisions Programme:  the route to ‘Taking Safe Decisions’ . London: RSSB, 2007.  Available at: http://www.rss http://www.rssb.co.uk/SiteCollectio b.co.uk/SiteCollectionDocuments/pdf/  nDocuments/pdf/  vtsic_presentations/RouteTo vtsic_presentations/Ro uteToTakin TakingSafeDecisions.pdf gSafeDecisions.pdf [Accessed: 18 February 2013] Wright, I. ‘Risk and liability for the structural engineer: a legal perspective’. The Structural Engineer , 81(14), 15 July 2003, pp23-35

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5

How to mana nag ge risk 

5.1

Intr In trod oduc ucti tion on

 This chapter describes methods and approaches for managing risk. Like Chapter 3, it is written from a structural engineering perspective, is not intended to be specific to any country and is not a definition of  legal requirements. Except where noted otherwise, this guidance can be applied by structural engineers who are responsible for any stage of the structure’s life cycle.

5.2

First Fir st ide identi ntify fy the haz hazar ards ds

 The first step in risk management is to identify the hazards. Particular effort should be put into identifying the project specific and unusual hazards. Some of  the most serious incidents have occurred because a hazard hazar d was not ident identified ified.. Depen Depending ding on the type of  structure and the surrounding environment, for example, willimpact containofhazardous materials iforthe be structure at risk from vehicles, other people and professions should be involved. More formal approaches to hazard identification may be appropriate for complex structures or environments, and are discussed later in this chapter. The flowchart in Figure 12 of Reference 5.1 may be found useful.  To identify less obvious hazards, an engineer might  To ask ‘what is unusual about this project?’ or ‘what could go wrong?’ In this context, a hazard is not just something that might harm people, but also something that could seriously disrupt the project. Hazards may be intrinsic to the project, such as building over water, or may be introduced by the design, the construction method or the use of the structure. Hazard identification is not, therefore, a one-off task, but should be repeated by the relevant engineer as the structure progresses through its life cycle.  The use of pre-formulated checklists can provide a false sense of completeness; brainstorming the hazards from scratch is less likely to miss an unusual hazard or a particular vulnerability.

5.3

Apply App ly the ris risk k mana managem gement ent hie hiera rarch rchyy

 There is a broadly accepted hierarchy for managing hazard and risk to people, with removal of the hazard as the first priority and mitigating the consequences as the last. There are several variants, used in different industries and in different countries. Eliminate, reduce, inform, control (ERIC) has been recognised as best practice for construction work in the UK 5.1, 5.2. For structural engineering, the following hierarchy is recommended, subject to any local regulations:

Eliminate First consideration should be given to avoiding the hazard in the first place: – by elimination (remove the hazard that is the cause of the risk); or

– by substitution substitution (repl (replace ace the hazard with something something less dangerous).  As at all stages in risk management, action should be proportio prop ortionate nate to the risk. There is littl little e value in eliminating a relatively minor or easily managed hazard at disproportionate cost, or if doing so creates further hazards. Reduce If the hazar hazard d cannot reas reasonabl onably be avoided the risk should be reduced, by those responsible, using one or more of the following, in order of  preference: (1) Expos Exposing ing people people to less of the hazard (e.g. reduce the amount of work at height). (2) Measur Measures es that protect everyone everyone (e.g. provide provide working platforms).

(3) Physic al barriers barrie rs ebetween people ces andifhazards. (4) Physical Mitigation Mitiga tion (reduc (reduce the consequences consequen the risk  occurs, e.g. provide safety nets). (5) Perso Personal nal protective protective equipment equipment (PPE). In any area with limited access, measures should be considered to facilitate evacuation and rescue in the event of an incident or incapacity, and these should take account of any remaining hazards. Inform and Control If it is not feasible to eliminate or reduce risk, information about the risk should be passed on so that the risk can be controlled. Where responsibility for the process passes between parties, such as from designer to constructor or constructor to user, the first party should inform the second about any risk which would be unusua unusual,l, difficult to manage or would not be obvious. Typically, a designer will apply the first three stages of ERIC but will have no control over site activity. Subject to the constraints of the design, which may limit the opportunity for elimination, a constructor can apply all four stages.  A user of the structure may only be able to apply controls.

Finally, if the risk cannot be reduced to a low level, consider whether the proposed activity is worth the risk. This may mean going back to the conceptual stage and approaching the whole project, design or construction sequence in a different way. It is important to emphasise that good risk managers will often apply these measures subconsciously and automatically; for example, by never introducing a hazard hazar d in the first place. place. The objective objective of risk  management is not to score points by demonstrating how much risk has been removed; rather, it is to scrutinise nise the proj project ect to see whether any more risk  can reasonably be removed.  The opportunity to apply different types of risk  reduction is shown in Figure 5.1. This is only The Institution of Structural Engineers  Risk in structural engineering

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5.4

How to manage risk  

      y        t         i       n       u        t       r       o       p       p         O

Key    To elimina eliminate te   To reduce

 

Concept design

Detailed design

Construction

To control

Operation an and maintenance

Figure 5.1   Opportunities for risk reduction

 Appointment stage

Management issues

Design

Construction

Use

Maintenance

Decommission

Clear brief in place before commencement

Competence and resource of team

Metho Met hodd of ana analys lysis is

Sitee spe Sit specif cific ic haz hazard ardss

Design re Design recor cords ds retained and updated

Maintenance requirements outlined to client?

Understand original design and modifications

Compatibility with others in team

Meth Me thod od of de desi sign gn

Ground Grou nd co cond ndit itio ions ns and existing services

 Advice sought on change of use

 Access for maintenance

 Adequate information?

Conflict with statutory duty?

 Analysis model

 Adequate time?

Co-operation measures

Review and checking

 Adequate fee?

Co-ordination measures

Construction/ erection strategy

Temporary works

Competent to do the work?

Information required

Maintenance strategy

Falsework 

Special risks (lessons Information flow from history?) procedures

Special or innovative structure?

Form of engagement?

 Accidental loads

Societal risk appetite

Demolition of similar structures Asbestos and other toxic materials

Beyond codes? Site wide issues Procurement Note  This table indicates one way an organisation could capture typical activities to reduce risk. It is intentionally incomplete; appropriate appropriate content may be selected from elsewhere in this Report  to  to suit the structure and activity concerned.

Figure 5.2   Prompts to consider consider in hazard elimination and risks reduction

indicative, as in practice opportunities will vary between projects.

5.4

How Ho w to to do do ‘ri ‘risk sk as asse sess ssme ment nt’’

Figure 5.2 shows an example of a list of prompts

5.4. 5. 4.1 1

Gene Ge nera rall

which could be used in risk reduction. The appropriate prompts will vary depending on the user and the stage in the construction process.  The example does not show recommended actions; it is for the engineer to determine what is appropriate.

‘Risk assessment’ is often used to mean both the process for managing hazards and their potential consequences and for the document produced to record that process. ‘Risk management’ is a better description of the activity but since ‘risk assessment’

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How to manage risk

Identify hazards

Can the hazard be eliminated?

 Yes

Do it

No Can the hazard be substituted or isolated?

 Yes

Do it

No

Can risk (probability or consequence) reasonably be reduced?

Can you do this?

 Yes

Yes Do it

No

No Inform whoever should do it

Is the residual risk  acceptable?

 Yes Next hazard

No

Think again

Figure 5.3   Flowchart of a risk management process

this Report   Report  uses   uses the terms is in common use, this interchangeably. It is about much more than assessing risks and also includes eliminating hazards, reducing risks and communicating information about residual dual risks. It is not about production production of piece pieces s of  paper. It is about understanding what might go wrong, and how to prevent it, or at the very least to minimise the damage. 5.4.2 5.4. 2

Process Proce ss for risk mana manageme gement nt

 A flowchart for risk management is shown in Figure 5.3. This should, if possible, involve client, designers and constructors. Each should be responsible for their own scope of work but should co-operate and exchange information. Structural engineers should try to ensure that the client understands that early appointment of the constructor and the extension of the designer’s contract into the construction phase will facilitate communication and hence risk reduction. This is the case for risks to health and safety and for risks to the successful completion of the project.

Each hazard, and then each remaining risk, should be considered using the hierarchy in Section 5.3. How this is done will depend on the stage in the structure’s life cycle, and how much has already been decided. The more serious the risk, the more effort should shoul d be put into managing managing it. If carefully carefully chose chosen, n, action to reduce one risk may reduce several others. On the other hand, care should be taken to avoid introducing new hazards unless that reduces the overallll risk. While a good risk assessment overa assessment at the design stage will make the constructor’s risk  assessments much easier, the nature of the project will have a major influence on the risk. For example, constructing constr ucting a city centre deep basement on a cramped site while retaining the original building fac¸ ade will require very thorough risk assessment by the constructor, however good the design is. 5.4.3 5.4. 3

Documenti Docu menting ng the risk asse assessme ssment nt

 There are two main reasons for documenting the work done. Firstly, to produce a risk register to communicate the residual risks. Secondly, as an The Institution of Structural Engineers  Risk in structural engineering

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5.4

 

5.4

How to manage risk  

Ref no. Hazardous Hazardous activity

Hazard

Measures taken to eliminate hazard

Measures not considered reasonably practicable

Information provided (or residual hazard(s))

Construction

Lack of of competence

Demonstration of competence required in tenders

Ground conditions

Contamination, slope or structure instability

Ground investigation completed, results issued

Existi Exi sting ng servi services ces

Dangerr to work Dange workers ers,, disruption to supply

Design based on service drawings

Full survey at all depths prior to commencement

Service drawings provided, CAT scan to be done before each excavation

Temporary works

Instability

Various

Work at height

Load Lo ad de delilive veri ries es

Falling Falli ng or sw swin ingi ging ng load

Provision made for lifting

Working in a confined space

 Asphyxiation, noise, inundation, etc.

Structure designed for construction without creation of confined spaces

Placing concrete

Dermatitis

Some elements precast in factory conditions

Erecting structural steel

Fallin Fa llingg from from heigh heightt

Number of Number of connec connectio tions ns at Eliminate all height minimised connections

Masonry

Manual handling

Block sizes limited

Date issue raised

 Action required by

Date actioned

Detail design by fabricator should include lifting provision

Note  This table indicates one way an organisation could communicate residual residual risk. It is intentionally incomplete; the risks relevant to the project need to be identified by the project team. The risks listed here are only to illustrate the format.

Figure 5.4   Example format for a residual residual risk register  register 

auditable trail for internal reference and to demonstrate to the authorities, if necessary, that due process was followed.  Although the residual risk register and the auditable trail are usually combined in a single document, it is useful to consider these two functions separately, to understand which information should be included.  The risk register is primarily required for communication. It should describe non-obvious, unusuall or significant unusua significant hazards so that the risks can be managed and should list the risk reduction measures which require implementation by others. Too much data which is obvious will reduce its clarity. If a hazard has been eliminated but could be reintroduced by later actions, e.g. the steelwork has been designed so that no connections connections are required red at heigh height, t, this should be made clear. The risk register should be a live document document that will change and evolv evolve e durin during g the design desig n and construction. construction. An examp example le of a risk  register is included as Figure 5.4.  The auditable trail is primarily required as a record. It should include all hazards, even those which are obvious obvio us or have been eliminated, eliminated, and all risk  reduction measures already identified or implemented imple mented.. It rema remains ins live only for the activ activity ity 22

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

concerned, i.e. the auditable trail for design is fixed when the design is complete. Risks to health and safety may be included in the same register as risks to successful completion of the project, or each could be in a separate register. On larger projects, registers can be divided by phase or area; whichever is clearest and has least opportunity for misunderstanding at interfaces.  A document on its own cannot reduce risks; only the intelligent use of its conclusions can do that. The Industry Guidance for Designers5.1, published by ConstructionSkills and supported by the Institution of  Structural Engineers and HSE contains a section on hazard hazar d elim eliminati ination on and risk reductio reduction, n, whic which h incl includes udes as Sectio Sec tion n 2.7 som some e sug sugges gested ted hea headin dings gs forrecor forrecordin ding g the process and outputs of a risk assessment. It does not suggest quantifying risks, either numerically or as high/  medium/low, as a necessary part of a risk assessment.  The Institution shares this view. It may be useful to draw attention to the estimated probability or consequence, particularly if severe or higher than usual for the hazard, butt th bu ther ere e is no va valu lue e (e (eve ven n if it co coul uld d be do done ne ac accu cura rate tely ly)) in scoring risks before and after risk reduction. Marking risk information onto drawings can be a good way to communicate this from designer to

 

How to manage risk

constructor, but such information should be meaningful. There is no benefit simply listing hazards the constructor will be already aware of, or making bald statements statements to the effect that risks have been assessed and reduced without saying what the residual risks are. 5.4.4 5.4 .4

Whatt to do and Wha and not not to do in in risk risk asses assessme sment nt

Do – Demonstrate that risks can’t reasonably be reduced further.

actions, errors and material failures appropriate to the structure considered. To conduct the HAZOP effectively, it is a prerequisite that a ‘design’ already exists and that the team debating it are knowledgeable about that design.  A HAZOP may identify the potential for hazards such as explosions. If the risk cannot be eliminated, it should be taken into account in the overall plant layout. Building structures to resist explosions can be very expensive, and risks can be reduced simply by careful siting so that the consequential damage is

– Think about the hazards and risks early in design. – Design so that there is at least one safe way to erect the structure. If it is not obvious explain what provisions the design makes for construction. – Tell anyone involved in the work what the residual risks are. – Talk to the other parties (client, designers, main contractor, subcontractors, fabricators, suppliers) – try to minimise contractual barriers to communication. – Consider risks over the life of the structure. – Record, for the owner/operator, how the design minimises lifetime risk. – Use risk assessment as a process to understand and record risks in that project. – Put an amount of work into risk reduction which is proportionate to the risk.

limited. The number of people killed in the explosion and fire on the Piper Alpha oil platform in the North Sea5.4, 5.5 (1988) would have been much less if the accomm acc ommoda odatio tion n mod module ule had not bee been n sit sited ed so clo close se to the main gas pipes, and in Texas City 5.6 (2005) the 15 engineering staff would not have been killed if their temporary offices had not been sited close to a major refinery plant.

Don’t – Focus on demonstrating that the risk is lower than it was initially (this only tests the starting position). – Produce paper to tell constructors things they already know. – Treat risk assessment as a process to produce a document. – Instruct others how to erect a structure. – Carry out risk asses assessment sment just to prot protect ect against liability – do it to reduce risks.

to designs been authorsthat of  themost code, and thehave results settaken downbyinthe a manner can be applied in design. By using a code of  practice, the engineer implicitly accepts those  judgements, in many cases without fully understanding the basis for them, or the limits on their application.

5.5

HAZOP

For structures containing hazardous materials, with complex performance demands or within an unusual environment, a formal risk analysis using the principles of a hazard and operability (HAZOP) study may be worth considering considering,, either as written or with modifications. This process is unlikely to be applicable to ordinary structures, but an understanding of it may be useful to engineers who are developing their understanding of risk. 5.3

HAZOP was developed by Trevor Kletz in 1983 as a tool for the chemical process industry. The process uses a systematic approach to identify any possible deviations from the design intent, and any consequent hazards. It is of great value, particularly for assessing any complex system where a mixture of engineering discip dis ciplin lines es has bee been n dep deploy loyed ed and whe where re pla plant nt fai failur lure e is a possibility. It was not developed for use in structural engineering, but the principles on which it is based are applicable to any engineered system.  As originally conceived, HAZOP was based on key words that reflected the origins of the approach in process industries, but its use in those industries (including nuclear power) has expanded to review all types of engineering systems. Appropriate key words should be selected to represent potential loads,

5.6

Code Co dess of pr prac acti tice ce

Codes of practice generally provide guidance to designers. Many contain the stipulation that they should be used by qualified and experienced engineers. In effect, the judgements that are common

In particular, codes assume that the structures they are applied to are ‘normal’ structur structures. es. Designers of  unusual structures need to understand this; for example, clad, framed structures do not usually suffer fatigue from wind induced oscillation, and thus most building codes do not address the phenomenon. Slender and/or exposed structures such as chimneys, masts or some sculptures may need design provisions beyond the code. In addition, every code of practice makes assumptions about the acceptable level of  safety5.7. These are set to meet the expectations of  society, which can vary from country to country depending on the economic balance. The Eurocodes have therefo therefore re include included d Nationally ly Determined ned Parameters to enable each nation to set the safety factors deemed appro appropriate priate for their localit localityy and society. society. In the UK, codes of practice are not mandatory but those listed in the Approved Documents that support the Building Regulations5.8 are deemed to satisfy the regulations; the designer may choose another method but is respo responsibl nsible e for for showi showing ng that the design design is safe. In the USA, the courts tend towards a similar interpretation. In some countries, including Spain and Italy, non-compliance iance with codes of practice is in itself a criminal offence, irrespective of whether failure occurs.

5.7

Risk Ris k man manage ageme ment nt fra frame mewor work  k 

For ongoing operations it can be beneficial to have a risk framework that identifies all potential risks, obtains data on their likelihood and is regularly The Institution of Structural Engineers  Risk in structural engineering

  23

5.5

 

5.8

How to manage risk  

reviewed wed to detec detectt trends and changes in tren trends. ds. An revie example of this is the UK Rail Safety and Standards Board’s (RSSB) Safety Risk Model5.9. This provides risk data covering the management of railway risks and includes details of near misses, which are particularly important in assessing the risk of low probability/high consequenc conse quence e events events.. It also uses the conce concept pt of  equivalent fatalities to address the risk of ‘minor’ or ‘major’ injury. There is no similar framework for structural engineering risk, but the UK’s CROSS (Confidential Reporting on Structural Safety) 5.10 scheme captures and collates information on matters

the particular particular proj project ect and may highlight highlight to the clien clientt and designer where gaps in experience/knowledge appear and where those gaps may be filled by specialists.

of concern to structural engineers and permits monitoring of trends. CROSS was established by SCOSS (Standing Committee on Structural Safety, www.structural-safety.org) in 2005 and publishes quarterly newsletters for structural engineers.

reduce risks due to misunderstandings or errors.  They rely on a careful definition of good practice, intelligent application and continuous improvement. It is important that users recognise recognise that QA does not replace engineering judgement, but only provides a framework to support it. If the rules are too complex or inflexible they will tend to be followed blindly, with a risk that the wider issues are overlooked. Many QA  systems focus on detail rather than ensuring that competent people and systems are used and, as a result, can become bureaucratic and ineffective.

5.8

The imp impor ortan tance ce of com compet petenc ence e

Risk management management relies on the judgement judgement of the engineers engin eers involved; involved; it canno cannott be carried out by rote rote.. It is therefore important that engineers and the organisati organ isations ons they work for are compe competent. tent. In the UK, there is a legal requirement for competence under the CDM Regulations. Guidance was published in 20065.11 and remains good advice, although the

5.9 5. 9

QA an and d ch chan ange ge co cont ntro roll

Systems for management of work, otherwise known as quality assurance (QA), are a valuable way to

Control of changes to the design is a key role for QA.  All changes should be authorised by the person in charge of the design. Every proposed change should be checked against the desi design gn intent, to ensur ensure e that the change does not invalidate design assumptions or other parts of the design. If a change remains

5.12

regulations were revised in 2007

.

Competence requires an understanding of what has to be done and the experience and ability to do it.  Apart from formal training and education, competence is usually obtained by on the job training; working under the guidance of senior engineers. All structural engineers should aspire to raise their personal level of competence, and members of the Institution have a professional responsibility to develop their skills through Continuing Professional Development (CPD)5.13. Management of structural engineering risk typically requires experience of design and construction. The abilit abi lityy of des design igners ers to red reduce uce ris risk k is fre freque quentl ntlyy lim limite ited d by a lack of construction experience. Learning constantly from what has gone wrong elsewhere is an essential risk reduction exercise. In any organisation there is bound to be a gradation of skill and experience and the risks inherent in that diversity need to be managed. It is therefore important that less experienced engineers are allowed to develop their skills and experience in risk  management, subject to review by senior engineers.  This applies not just in the design office but on site; safe management management of both temporary temporary works and the stability of part finished structures requires both structural engineering competence and a willingness to take firm action in the face of commercial pressures if risks are not recognised.

within design and is executed carefully, shouldthe carry little intent, risk. Changes that modify the it original intent require more detailed scrutiny. There remains a need for judgement, as serious accidents, such as the Hyatt Regency Hotel 5.14 (1981, see  Appendix A.15), have occurred due to design changes which were far more significant than were realised by those who made them.

5.10 Inde Indepen penden dentt revi review ew Independent ‘third party’ review of designs is often used where risks are high, and could be applied ed beneficially on many projects. In the UK, it is standard practice for railway bridges, for highway structures (Category 1, 2 and 3 independent checks) and for nuclear facilities. A Guidance Note 5.15 has been issued by SCOSS describing some of the features and benefits of one approach to independent review. Even when independent review is not a legal or contractual requirement, a review of the overall design by a third pair of eyes, independent of the original designer and verifier, should be standard practice for all designs

5.11 Conclu Conclusions sions and reco recomme mmendatio ndations ns Many clients formally review the competence of the organisations they employ or propose to employ on construction projects. This minimises their risks and in some countries, including the UK, is a legal requirement. It is usual to measure the qualifications

Project specific and unusual hazards should be identified early and kept under review. A hierarchy of  risk reduction measures should be applied during

and relevant experience of individual key members of  staff,, as well as the corp staff corporate orate competence competence of the organisation, as demonstrated by its staff, structure and performance. Self assessments by designers and suppliers enable informed decisions by clients. These assessments should include resource availability for

both design and construction. construction. It is more important to carry out a thorough and proportionate risk  assessment than to document it, but communication of residual risk is essential. Drawings are a good medium to communicate construction risk. These activities ties should be focus focussed sed on what is usefu usefull to

24

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

 

How to manage risk

reduce real risks and not on producing paperwork; listing trivial or well understood risks is neither necessary nor useful. Demonstrating that risk has been reduced compared to the original design only illustrates that the concept was flawed; the point is to end up with risk reduced to the lowest reasonable level. Codes of practice are a useful guide, within their scope, but competence and the ability to apply engineering judgement are essential.

5.13   Engineering Council. Council. UK-SPEC   UK-SPEC  [UK   [UK Standard for  Professional Engineering Competence]. Available at: http://www.engc.org.uk/ukspec [Accessed: 18 February 2013] 5.14   Marshall, R.D. et al. al. Investigation  Investigation into the Kansas City  Hyatt Regency walkway collapse. NBS Building Science  Series 143 . Washington, DC.: NBS, 1982 5.15   Standing Committee on Structural Structural Safety. Safety. Independent   Independent  review through peer assist. SCOSS topic paper SC/09/  034 . Available at: http://www.structural-safety.org/  topicpapers [Accessed: 18 February 2013]

5.12 Ref Refere erence ncess 5.1

  CITB-ConstructionSkills. CITB-ConstructionSkills. Industry  Industry Guidance for  Designers , King’s Lynn: CITB-ConstructionSkills, 2007.  Available at: http://www.cskills.org/uploads/CDM_ ls.org/uploads/CDM_ Designers4web_07_tcm17-4643.pdf Designers4web_07_t cm17-4643.pdf [Accessed: 18 February 2013]

5.2

 

Carpenter, J. ‘Risk management with ERIC’. Carpenter, ERIC’. The   The  Structural Engineer , 88(7), 7 April 2010, pp20-21

5.3

 

Kletz, T.A. T.A. HAZOP  HAZOP & HAZAN: identifying and assessing  process industry hazards . 4th ed. London: Taylor &  Francis, 1999

5.4

 

Crawley, F. ed. ed. Piper  Piper Alpha: lessons management  . Rugby: IChemE, 1990for life cycle safety 

5.5

 

Lord Cullen. Cullen. The  The Public Inquiry into the Piper Alpha  disaster . London: HMSO, 1990

5.6

 

Fatal Accident Investigation Report: Isomerization Unit  Explosion Final Report . Available at: http://www.bp. com/liveassets/bp_internet/us/bp_us_english/S com/liveassets/bp_int ernet/us/bp_us_english/STAGING/  TAGING/  local_assets/downloads/t/final_report.p local_assets/down loads/t/final_report.pdf df [Accessed: 18 February 2013]

5.7

 

Standing Committee on Structural Safety. Safety. The   The   Assumptions behind the Eurocodes .  SCOSS topic  paper, Nov 2009 . Available at: http://www.structuralsafety.org/topicpapers [Accessed: 18 February 2013]

5.8

 

The Building Regulations 2010  (SI   (SI 2010/2214).  Available at: http://www.legis http://www.legislation.gov lation.gov.uk/uksi/2010/  .uk/uksi/2010/  2214/pdfs/uksi_20102214_en.pdf [Accessed: 7 March 2013] [Note that similar but separate regulations apply in Scotland and in Northern Ireland, see http://www. legislation.gov.uk/all?title=buil legislation.gov .uk/all?title=building%20regulations] ding%20regulations]

5.9

 

Rail Safety and Standards Board. Board. Safety  Safety Risk Model .  Available at: http://www.rss http://www.rssb.co.uk/srmodel.asp b.co.uk/srmodel.asp [Accessed: 18 February 2013]

5.10   Confidential Reporting on Structural Structural Safety Safety [CROSS] [CROSS] website. Available at: http://www.structural-safety.org [Accessed: 18 February 2013] 5.11   Carpenter, Carpenter, J. J. Developing  Developing guidelines for the selection of  designers and contractors under the Construction  (Design and Management) Regulations 1994, HSE  Research Report 422 . Sudbury: HSE Books, 2006.  Available at: http://www.hse. http://www.hse.gov.uk/rese gov.uk/research/rrpdf/  arch/rrpdf/  rr422.pdf [Accessed: 18 February 2013] 5.12   The Construction Construction (Design and Management)  Management)  Regulations 2007  (SI   (SI 2007/320). Available at: http://www.opsi.gov.uk/si/si2007/20070320.htm .gov.uk/si/si2007/20070320.htm [Accessed: 19 February 2013] The Institution of Structural Engineers  Risk in structural engineering

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5.12

 

6

Stat St atis isti tica call and and pr prob obab abil ilis isti ticc met metho hods ds

6.1 6. 1

Intr In trod oduc ucti tion on

 This chapter provides an overview of the numerical background to risk management. It is mainly relevant to structural stability rather than hazards on construction sites. It is written for practising structural engineers, not risk specialists, and is not intended to be academically rigorous. A bibliography is provided for further study.

6.2 6. 2

Back Ba ckgr grou ound nd

Day to day structural engineering does not generally require direct application of statistical methods. Indirectly, however, statistics have a significant influence, being used extensively in assessing and describing material strengths and in describing the likelihood of random loadings such as wind. For designs that meet the requirements of a code of  practice, safety factors probability failure to athe low enough levelreduce to takethe account of theof  uncertainties in design, loading and material quality.  An understanding of statistics and probabilities can be useful in decis decision ion making and in assessing the capability of existing buildings where code shortfalls have been identified. More widely, probabilistic approaches can provide useful insights for comparing risks, providing perspective to help decide whether a risk is tolerable, to decide which risk requi requires res most attention or to support a decision that the risk has been reduced to an acceptable level. Since the magnitude of many loads on a structure is fundamentally uncertain, they can only rationally be defined defi ned in terms of prob probabili abilities. ties. An examp example le is the 1 in 50 year wind, defined as the wind speed with a 1 in 50 probability of being exceeded at least once during a year. This gives a 64% chance that it will be exceeded at least once in 50 years. Low probability events do happen and this wind speed could be exceeded more than once in the period or even in the first year. For structures where very high reliability is sought, a 1 in 10000 year event might be used6.1.  There is a debate about the use of probability as the sole way to deal with such low frequency natural events eve nts,, com compar pared ed to the res resili ilienc ence e app approa roach ch des descri cribed bed in Section 3.10.  The use of numerical assessment can be deceptive, suggesting a degree of precision that rarely exists. A  probability might be calculated as 1 in 10000 per year (often written as 10 4 per year), but due to the many assumptions which have to be made in the assessment it might in practice lie between 102 and 6

10 . Hence these methods are somewhat approximate and should not be allowed to dominate any risk assessment; assessment; the techniques techniques are best used in conjunction with deterministic methods. Moreover, because this field is so specialised, the preparation of  probabilistic numerical arguments is best left to 26

  The Institution Institution of Structural Structural Engineers Engineers  Risk in structural engineering

experienced individuals. In advanced structural engineering, ‘reliability’ can be assigned a numerical value6.2. BS EN 19906.3, the head Eurocode, offers a methodolog metho dologyy for speci specialist alist calcu calculatio lation n of an appro appropriat priate e load factor taking account of views on the accuracy of  analysis, confidence on material properties and so on.

6.3

Quanti Qua ntifyi fying ng pro probab babili ility ty

 There are various ways of quantifying probability probability.. In everyday language the chances of winning a lottery might be considered as 1 in 14 million per ticket purchased, or the probability of dying from smoking as 1 in 200 per year. In engineering practice, rational methods of defining ‘safety margin’ or ‘probability of  failure’ can be explored using numerical methods and, in some branches of engineering, such methods are used quite widely. If enough similar items are in use, the proportion that fail can be counted and used to calculate the probability of others failing in the future. a complexfor electrical system, with known Thus, failurefor probabilities the separate items within that system, it is mathematically possible to define the overall level of reliability or, looked at the other way around, the overall probability of failure.  This can be compared with what might be considered an acceptable risk. This would also need an understanding of the way components interact, such as the possibility that one failure could trigger another, or that one event could cause multiple failures.  To apply this process to structures, there would need  To to be some basis for counting actual failures. This is not really available, certainly for the whole structure.  The reliability of electronic components can be calculated easily, as thousands are made to the same design. Few structures are identical and as explained elsewhere in this  Report , many of the causes of  failure are independent of the design code used or the precise details of the structure. One area where failure statistics are used to good effect is in preparedness for emergency response. In most parts of the world, buildings follow a common form, e.g. in the UK, the brick built three bedroom semi-detached house. In an area of high seismicity, there will be a historical understanding of the response of the local type of houses or bridges to earthquake loading of  certain intensities and it is possible to make an estimate as to how many might be damaged in a particular earthquake and hence decide what preparedness could be undertaken. This approach becomes more effective as more data is available, such as through improved seismology and satellite based damage surveys. Calculated probabilities will only be correct if the data used to generate them is correct. For example, it may be assumed that concrete meets the specified characteristic strength; if, as often happens, it is over strength, the probability of failure may be lower (provided the amount of reinforcement is adequate). It is usual to calculate the probabilities using what are

 

Statistical and probabilistic methods

known as ‘conservative’ values, that is, the value is chosen cautiously, so that the final probability of  failure is probably an overestimate. It is often argued that conservative values should be used where there is uncertainty, but this approach requires care. Risk  management often involves compromises, comparing one option against another to see which has the lowest overall risk 6.4, but inconsistent levels of  conservatism could overestimate one side of the balance, and so skew the judgement.

6.4 6. 4

Safe Sa fety ty fa fact ctor orss

and mortar used. This approach recognises the overall lower probability of failure if workmanship is controlled more closely. Partial factors are also varied for different load combinations (e.g. dead þ live þ wind) to reflect the lower overall probability of that combination of  circumstances arising. It is important to be aware that some uncertainties, such as the accuracy of our models of structural behaviour, do not have a specific partial safety factor but are included within other factors. This means that even if the load is known exactly, a partial factor for load of 1.0 may be inappropriate, as the factor also covers other, unstated aspects which are still uncertain.

 To avoid the need for enginee  To engineers rs to apply probab probability ility theory in routine design, structural design generally includes a safety factor or load factor (usually built up from partial factors) which ensures that the probability of overall failure is low enough. The value of the factor was traditionally subjective, based on collective historical experience, but there is now an aspiration to derive the factors statistically or by reliability theory.  A structur structure e with design factor factors s lower than in the design code is not necessarily unsafe, but it should be expected to have a higher probability of failure than one that meets the code. Excluding gross error, structural failure would not normally occur unless the combined probability of adverse variation in applicable

6.5

loading, loadingetc., configuration, quality,too high. workmanship, all becomematerial coincidentally

aftermath of severe severe temperatures and severe flooding evenwind, in countries with a well-established infrastructure such as the UK and USA. A practical question engineers have to address is to determine the likelihood of such events and then decide what resources can be afforded to defend against them. Techniques such as quantitative risk  assessment (QRA) are available to predict the (numerical) magnitudes of rare natural events and to predict the likelihood of process plant failures.

Structures meeting the code requirements, therefore, have an acceptably low probability of failure. If very high reliability is sought, this can be effected by increasing the load factor, using reliability theory to obtain a specific increase. Conversely, if an increased probability of failure is acceptable, perhaps because a structure need only have limited functionality after the event, then the required load factor may be reduced. For example, a building under construction would be unoccupied and construction work would usually stop in very strong winds, so the design wind loading during construction may be based on a two year recurrence period rather than 50 years. ‘Time at risk’ may be considered as a reason to reduce the factor of safety. This may be valid for transient risks or those that could occur during only a small fraction of the life of the structure and where it might be disproportionate to use the usual factor. When designing for a specific situation that only lasts a short time, such as the construction construction phase example in the previous paragraph, this may not be valid, and each case should be taken on its merits. For the people involved, construction is usually a full time activity, on one site after another, so increasing their risk would be unjustified. Confidence in material properties clearly affects the selected value of a load factor. Material testing is bound to show a scatter of results and this is managed by using such concepts as the 95% confidence level, which typically defines the characteristic strength. In some codes, the partial factors are varied explicitly to take account of known factors affecting the probabilities. For example, in UK masonry design codes (both BS 56286.5 and BS EN 19966.6 with UK  National Annex) the partial factor for materials depends upon the workmanship and quality control of blocks

Low Lo w pr prob obab abili ility ty ev even ents ts

For events with low probability but high consequences, the reliability provided by standard design desig n code codes s may not be adequate. This becomes more relevant as the population of the world grows and the number of people living in areas vulnerable to infrequent but severe natural hazards such as floods and earthquakes increases. Coupled with the wider development of an engineered infrastructure, there is a growing belief among the public that the human consequences of such disasters are avoidable. Nevertheless, communities still have to cope with the

 An early use of QRA 6.7 was in the study carried out in 1978 to assess the risk that the chemical plant on Canvey Island posed to London6.8. Major accidents to such plants do happen, such as the failure at Flixborough6.9 (1974) or at Buncefield6.10 (2005). How can engineers decide which modes of failure are possib pos sible, le, wha whatt the pro probab babili ilitie ties s of tho those se fai failur lures es are and hence the risk they pose in terms of their location relative to populated areas? These risks can be expressed in numerical terms. In areas where the population are at risk from flood, an event with a statistical probability of occurrence, one way of  assessing the likelihood of damage, or the required height of a flood protection system, is to assess the risks using statistics and numerical values against a target acceptance value. In London, a quantitative flood risk assessment is required to review the effectiveness of the Thames Barrier6.11. The consequences of the floods in New Orleans following Hurricane Katrina6.12, 6.13 (2005) illustrate the need. In the nuclear industry, when assessing the safety of  facilities, a combination of deterministic (designing for specified events) and probabilistic assessment (the probability of those events not occurring) is used. Neither method is used exclusively, since both give insights into the overall safety of the plant. Probabilistic methods are often used to define the deterministic events. The accepted ‘safe’ target in the UK is that the probability of significant harm to the public from radioactivity should have a probability of  107 per year or lower. In practice, events as rare as The Institution of Structural Engineers  Risk in structural engineering

  27

6.4

 

6.6

Statistical and probabilistic methods

this are very difficult to predict so the design basis for natural hazards such as earthquakes is an event that, by a conservative assessment, has a 104 per annum probability of exceedance 6.1. This may be compared compared to the typical UK loads of 1 in 50 years for building structures and 1 in 120 years for bridges. Use of the 104 event with a conservative approach to design ensures that the combined probability of the earthquake, leading to structural failure, leading to release of significant radioactivity, is in the region of  107 per year. The 104 earthquake for the UK is a significant design loading. Design also considers

to design for the worst possible case. Considering the potential consequences will allow the acceptable probability of occurrence to be calculated; if the probability of the worst possible case is lower than this, the design may be based on the limiting acceptable risk. Numerical probability was used in the design of the river barriers for the London Eye6.17. Given the client’s wish to site the observation wheel on the th e ed edge ge of theRive theRiverr Th Tham ames es,, an anyy ad addi diti tion onal al ri risk sk ha had d to be reduced to a level consistent with a more conventional site. It would be geometrically possible for for a lade laden n vesse vessell to hit the Eye’s Eye’s capsules in certa certain in river

corresponding extremes extremes of environmental loading such as wind and temperature. Similar approaches are used in probabilistic studies of risks to offshore installations, railways or from ship impact on bridges6.14.

states, if this was not prevented by the pontoon and its moorings. The design question was ‘what impact energy should the pontoon be designed against?’ The designers identified one specific vessel that operates occasionally and could, if fully loaded and in combination with specific tide and flood conditions and other specific factors about the vessel’s approach, produce a particularly high impact load. A more likely even ev entt wo woul uld d be fo forr on one e of th the e ve very ry muc much h liligh ghte terr bu butt ve very ry much more frequent tourist tourist vesse vessels ls to hit the ponto pontoon on as they manoeuvre in front of the Eye. Modelling the impact and considering the probability of each event along with the cost of designing and building the pontoon ponto on to resi resist st it allo allowed wed the desig designers ners to reach a  judgement on which approach was reasonably practicable.

Rather than consider a single earthquake return period, with an estimated probability of structural failure in that earthquake, a more advanced approach would be to consider the probability of a range of  earthquake intensities, to calculate the failure probability for each and to combine these. This could be done by numerical integration of the earthquake hazard curve and the structural fragility curve. The difficulty with this approach is that it requires actual fragility data for the type of structure concerned, when subject to the relevant loading, loading, and such data does not exist for unusual structures or areas with infrequent earthquakes. When acceptable risks are expressed in terms of a failure probability per year, care has to be exercised when exposure to the risk is only for a short period.  As the time interval of exposure is short, it might appear that the probability of failure ‘at that instant’ is very low. But if there are many such exposures, the probability that a failure will occur at some stage is much increased. Appendix A.12 discusses the probability of failure in the context of the road/rail accident at Great Heck 6.15 in Yorkshire, UK (2001).

6.6 6. 6

Appl Ap plic icat atio ion n

 The approaches described above may seem esoteric and distant from the real world but have many practical applications. Firstly, consciously looking at any structure in such terms allows rational decisions to be made on where to spend money to improve safety. Thus, putting the whole problem in terms of  the uncertainties, the effect of changing a particular parameter can be investigated to understand how much it costs and how much ‘safety’ it buys or what risk reduction is achieved. Effort should always be focussed on the most significant risks. Similarly, the cost of designing a facility against a return period of  104 flood as compared to 102 can be investigated. In Holland, the consequences of spring floods on the Rhine overtopping the dykes have led to design risk of  1 in 1250 years, while the sea defences in the west of  the country are designed for a 10 4 event6.16. This does not mean they will not fail in 10 000 years; improbable events can happen. It is also likely that forecasting such a rare event on the basis of 100 years of records will not be accurate. Estimates of extreme values val ues ar are e upd update ated d as kno knowle wledge dge imp improv roves, es, and as the climate changes. Probabilistic techniques can, where appropriate, provide a rational way to show that it is not necessary 28

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

Designing routine buildings for a lower wind loading during construction is rational. But if in the consequences resulting from failure a slightly larger wind were really intolerable it pays to think again, for it might only cost a marginal amount more to reduce the probability of failure. Likewise, it is irrational to design every structure for terrorist attack. In reality, it is necessary to judge the probability of the event and define some design standards accordingly. For most buildings, the likelihood of terrorist attack is low, and therefore the associated risk is tolerably low when the cost in potential loss of life and injuries is considered.  The risk increases when the target has a higher probability of attack and especially where the consequences are of harm to large numbers of  people. In effect, the classes of robustness6.18 used in routine design are standardised judgements taking account of the consequences of failure. Thus, structures are divided up into groups with more care being taken on those structures where the consequences of failure in terms of loss of life are greatest. Guidance on design against accidental loadings, including specific reliability assessment for high consequence events, is given in Part 1-7 of  BS EN 1991, Eurocode 16.19.  Although the nuclear nuclear,, chemical and aeronautical industries continue to apply probabilistic analyses to guide their projects and choices, such analysis for normal structural projects remains unusual. This may be at least partly due to the difficulty of maintaining databases of component reliabilities. The nearest approach is the use of characteristic strength to manage the probability of faults in metal castings, cavities in concrete piles or under-strength concrete.

6.7

Assess Ass essmen mentt of exi existi sting ng str struct ucture uress

Structural engineers are frequently faced with the need to appraise structures that do not comply with ‘modern standards’. This does not necessarily mean

 

Statistical and probabilistic methods

they are unsafe and it may not be in the best interests of society to spend a disproportionate amount of money ‘strengthening’ them. Many older structures were designed with a much larger factor of  safety than today’s structures because of lack of  knowledge about material performance over time and the inability to analyse structures as rigorously as is possible today. Depending on where the ‘real’ performance lies, the actual factor of safety may be very large, or quite small. With a knowledge of how safety is defined in terms of failure probability, it is possible to assess a structure taking into account the actual uncertainties to judge the failure probability. For example, the actual loading may be known accurately and there may be evidence of strength being much better than assumed in desig design. n. In engin engineerin eering g terms it is legitimate, in this judgement, to take benefit from other structural qualities such as redundancy and ductility, but in countries where design codes are mandatory rather than advisory, the legality of such approaches should be checked. Further information is also available in ISO 13822: 20106.20,  Assessment of existing structures, and in the Institution’s Institution’ s report,  Appraisal of existing structures 6.21.

6.8

Conclu Con clusio sions ns and rec recom omme mendat ndation ionss

Normal design codes contain design methods and embedded load factors which experience has shown will usually produce safe structures. Design of  ordinary structures subject to normal loading should be carried out in accordance with design codes and there is no benefit in using probabilistic methods to manage risk in such designs. Probabilistic methods can be valuable for gaining a deeper understanding of the risk resulting from complex structures, containment of hazardous materials or unusual loading. They can also be useful in assessment of existing structures.

reinforced and unreinforced masonry structures . London: BSI, 1996 [Incorporating corrigenda February 2006 and July 2009 and UK National Annex] 6.7

 

Det Norske Veritas. Veritas. A  A Comparison of accident  experience with Quantitative Risk Assessment (QRA)  methodology, Contract Research Report 293/2000 . Sudbury: HSE Books, 2000. Available at: http://www. hse.gov.uk/research/crr_pdf/2000/crr00293.pdf [Accessed: 18 February 2013]

6.8

 

Health and Safety Executive. Executive. An  An Investigation of the  potential hazards from operations in the Canvey Island/  Thurrock area . London: HMSO, 1978

6.9

 

The Flixborough Flixborough disaster: disaster: report of the Court of Inquiry . London: HMSO, 1975

6.10   Explosion on Mechanism Mechanism Advisory Group report . London: Buncefield eld Major Incident Investigation gation Board, 2007.  Available  Availab le at: http://w http://www.b ww.buncefie uncefieldinve ldinvestigati stigation.gov.uk/  on.gov.uk/  reports/buncefieldagr.pdf [Accessed: 18 February 2013] 6.11   Dawson, R.J. et al. ‘Quantified ‘Quantified analysis analysis of the probability of flooding in the Thames Estuary under  Imaginable worst-case sea level rise scenarios’.  Water  Resources Development , 21(4), December 2005, pp577–591. Available at: http://www.hm-treasury.gov. uk/d/atlantis-floodmodellingpaper uk/d/atlantis-floodm odellingpaper.pdf .pdf [Accessed: 18 February 2013] 6.12   Select Bipartisan Committee to Investigate Investigate the Preparation for and Response to Hurricane Katrina.  A Failure of initiative: final report of the Select Bipartisan  Committee to Investigate the Preparation for and  Response to Hurricane Katrina. Congressional Report  109-377 . Washington, DC: USGPO, 2006. Available at: http://www.gpoa http://www.gpoaccess.gov/serialse ccess.gov/serialset/creports/katrina. t/creports/katrina. html [Accessed: 19 February 2013] 6.13   American Society of Civil Engineers. Engineers. What  What went wrong  and why: the New Orleans Hurricane protection  system . Reston, Va.: ASCE, 2007 6.14   Duckett, W. W. ‘Risk analysis and the acceptable probability of failure’. The failure’.  The Structural Engineer , 83(15), 2 August 2005, pp25-26

6.9 6. 9

Refe Re fere renc nces es

6.1

 

Health & Safety Executive.  ve.   Safety assessment principles  for nuclear facilities. 2006 Edition, Revision 1, paragraphh 514. paragrap 514. Available  Available at: www.hse.gov.uk/nuclear/  saps/saps2006. saps/sa ps2006.pdf pdf [Acces [Accessed: sed: 18 February 2013]

6.2

 

ISO 2394:1998: General principles on reliability for  structures . Geneva: ISO, 1998

6.3

 

BS EN 1990:2002+A1:200 1990:2002+A1:2005: 5: Eurocode – Basis of  structural design. London: design.  London: BSI, 2010 [Incorporating corrigenda December 2008 and April 2010]

6.4

 

BOMEL Ltd. Ltd. The  The global perspective in addressing  construction risks. Research Report 458 . Sudbu Sudbury: ry: HSE Books, 2006. Available at: http://www.hse.gov.uk/  research/rrpdf/rr458.pdf [Accessed: 26 February 2013]

6.5

6.6

 

 

BS 5628-1:2005: 5628-1:2005: Code of practice for the structural  use of unreinforced masonry . London: BSI, 2005 BS EN 1996-1-1:2005: Eurocode 6 – Design of  masonry structures – Part 1-1: General rules for 

6.15   Health and Safety Executive. Executive. The  The track obstruction by a  road vehicle and subsequent train collisions at Great  Heck 28 February 2001. 2001 . Sudbury: HSE Books, 2002.  Available at: http://www.rail-reg.gov.uk/upload/pdf/  -reg.gov.uk/upload/pdf/  incident-greatheckfinal-optim.pdf incident-greatheckfina l-optim.pdf [Accessed: 19 February 2013] 6.16   Hoekstra A.Y. and De De Kok, J-L, ‘Adapting to climate change: a comparison of two strategies for dike heightening’. Natural heightening’.  Natural Hazards , 47, 2008, pp217–228.  Available at: http://doc.utwente.nl/59 http://doc.utwente.nl/59991/1/  991/1/  Hoekstra08adapting.pdf [Accessed: 19 February 2013] 6.17   Beckett, T. T. ‘The British Airways London Eye. Part 5: Pier and impact protection system’. The system’.  The Structural  Engineer , 79(2), 16 January 2001, pp34-35 6.18   Institution of Structural Engineers. Engineers. Practical  Practical guide to  structural robustness and disproportionate collapse in  buildings . London: IStructE, 2010 6.19   BS EN 1991-1-7:2006: Eurocode 1: Actions Actions on  structures – Part 1-7: General actions – Accidental  actions . London: BSI, 2010 [incorporating corrigendum February 2010] The Institution of Structural Engineers  Risk in structural engineering

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6.8

 

6.10

Statistical and probabilistic methods 13822:2010: Bases Bases for design of structures –  6.20   ISO 13822:2010:  Assessment of existing structures . Geneva: ISO, 2010 6.21   Institution of Structural Engineers. Engineers. Appraisal  Appraisal of existing  structures . 3rd ed. London: IStructE, 2010

6.10 Bibl Bibliog iogra raphy phy Blockley, D.I. The D.I.  The nature of structural design and safety . Chichester: Ellis Horwood, 1980 CIRIA.  Rationalisation of safety and serviceability factors in  CIRIA. Rationalisation structural codes. CIRIA Report 63 . London: CIRIA, 1972 Diamantidis, D. ed. Probabilistic ed.  Probabilistic assessment of existing  structures: JCCS report . Available at: http://www.rilem.org/gene/  main.php?base=500219&id_publication= main.php?base=500 219&id_publication=96 96 [Accessed: 18 February] Kletz, T.A. ‘Process industry safety’. In Blockley, D I. ed. Engineering safety. London: safety.  London: McGraw-Hill, 1992, pp347-368

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  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

 

7

Risk in design

7.1

Intr In trod oduc ucti tion on

7.1. 7. 1.1 1

Over Ov ervi view ew

 This chapter discusses what should be done at the design stage of a project to manage risk over the life of the structure. Many of the decisions taken at this stage can have a major effect on later risks. This is not limited to structural design, nor to the scope of  the consulting engineer. Topics such as procurement, programme and construction strategy are relevant to minimising risk. 7.1.2

What does the desi design gn stag stage e incl include? ude?

Design is a key activity for structural engineers and is much wider than just the method methodical ical process process of  calculation and drawing production to meet code requirements. Designers of both permanent and temporary works have an obligation to make sure that their design meets the functional demands of the project brief, performs adequately and will be safe to construct, operate and eventually demolish. Design, therefore, means any part of the whole process of producing a structure through from concept to completion, except the physical process of construction. Most structural engineers are involved in this in some way, even those not calling themselves designers. Preferably, design should involve clients and construction staff. The process often requires interaction with other disciplines and encompasses concept, functionality, evolution of  structural form, calculations, drawings and specifications, and procurement. Design is both a theoretical skill and a practical skill.  The design stage involves looking ahead to construction and use to see what can be done to reduce future risk. This may take the form of  planning, such as designing for a construction sequence that provides weather protection as soon as possible, carrying out good soil investigation well in advance, or choosing a particular design solution to reduce risk. For example, piling can eliminate excavation in unstable ground. Careful tendering and selection of contractors can reduce many risks by ensuring adequate competence. Restrictions on subcontracting can also reduce the number of  interfaces.  The initial stages of the design process are the best time to take strategic action to manage risk; if this opportunity is missed, the deficit cannot be made up during construction and use.

harm occurs. These are not the only liabilities; contractual claims for delay, additional costs or failure of the structure to perform satisfactorily can be large.  The costs of changing the design or of defending claims for damages can be out of proportion to the cost of the work. Risk management should not be seen as a cost but as an essential activity to control costs. 7.1.4 7.1 .4

Whatt can go wro Wha wrong? ng?

 There are many opportunities for things to go wrong during design, even when both individuals and their organisations are fully competent. The majority of  structures are unique and large amounts of data are both consulted and produced. The range of  knowledge individual engineers need in order to identify errors is wide. While the technical press often reports failures, the root causes are not usually known until later, when the failure is no longer newsworthy. Typical errors include failure to understand the project requirements, an important feature being overlooked, mistakes in calculation or drawing, an imperfect understanding of the loads or load path and imperfect technical understanding (e.g. of concrete durability or dynamics). Failures also occur as a result of poor communication, either with other disciplines such as services engineers or architects or between the designer and the (yet to be appointed) constructor constructor.. Given the complexity of the task and potential consequences, the structural engineering profession has developed a culture of independent checks within the design office. In recent years, some people have thought this unnecessary or commercially unjustified. This is unfortunate; modern codes, contractual models and computers have introduced more complexity and more opportunity for error while reducing the margins in design. Checking remains an essential part of design. 7.1.5 7.1 .5

Manag Ma naging ing unc uncert ertai ainty nty

 A key aspect of design ‘risk’ is to take the opportunity to minimise the effects of the uncertainties inherent in the design and construction process. For example, delay can occur due to the unavailability of materials or application of new technology. While these risks are not created by the structural engineer, there is an opportunity to eliminate them by proper consideration during design.

If risks are not properly managed at the design stage,

Work in the ground is fraught with risk and the term ‘unforeseen ground conditions’ is familiar to every experienced engineer. This risk can be minimised by site investigation and by choosing a foundation solution which reduces the effect of any remaining unknowns.

the worst case would be that people would be harmed.. That could result in both criminal harmed criminal and civil liabilities extending not only to those who ‘caused’ the harm but those who failed to prevent it. Criminal liability can also be incurred if legally defined processes for designers are not followed, even if no

 A useful tool is to test the sensitivity of the design by varying the parameters over the range of any uncertainty. Structures may be sensitive to variations in load, errors in construction geometry, defects in materials, and many other changes. The

7.1.3

The cons conseque equences nces of desi design gn stag stage e error errorss

The Institution of Structural Engineers  Risk in structural engineering

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7.2

Risk in design

key issue is that sensitive configurations are often fixed at the concept design stage, and should therefore be identified and eliminated or reduced at that stage. Questions to ask might include: – Is the structure so sensitive to the prediction of  wind speed that a minor change will render the design unsafe? – Is the deformation or alignment of the structure really critical? – Can the dynamics cs be predicted cted with conf confidenc idence? e?

the structure will be analysed, designed and detailed to achieve the overall design objectives. It should identify a structural form that is functional in-service and capable of being built without undue site risk.  The document should be agreed by relevant internal and external parties and periodically reviewed to confirm confi rm that it remains relevant relevant and that the design still follows it.  The design work should be executed in stages, with interim checks to make sure no serious mistake is carried forwards7.2. If there is a major analysis on which

– Will any minor change in any of the design assumptions (such as tolerances) render the structure weak, unstable or not functional?

all subsequent member design is based, then it is common sense to check that before handing it out for detail design.

Many risks are traditionally avoided by making the structure insensitive; for example, by using ductile steel and proper member and connection design to ensure ductile behaviour so that the mode of failure in overload is controlled. A general assumption is ‘the stronger the better’. In certain circumstances, however, such as impact, blast protection or in seismic design, structural performance is based on controlled failure. In such circumstances, over-strength in the wrong place can be detrimental.

Every analysis, and especially every complex computer analysis, should be fit for purpose. Any computer analysis should have some hand checks to make sure its predictions are of the right magnitude. Computers can produce very precise output, but precision is not the same as accuracy. The predicted behaviour of the computer model is only relevant if  the model is a good representation of the structure.  The Institution has published a report on the use of  computers7.3.

 An example of sensitivity is a lightweight temporary large television screen that had an apparently sound margin of 1.5 against overturning at the low wind

 All design calculations should be self-checked by the designer and then checked separately within the designer’s organisation. Even where they exist, it is

speed thought appropriate forbut design. considered ‘adequately safe’, failedIt was catastrophically7.1. In the investigation it was observed that a wind speed increase from 15m/s to 17m/s increased the wind force by 1.28 [ (17/15)2], removing most of the margin.

not acceptable to rely onasstatutory external controls or independent reviews the primary check.

¼

 Another approach to uncertainty is to choose design values on the safe or ‘conservative ‘conservative’’ side side.. Care needs to be taken to establish what is conservative with properties such as the co-efficient of friction. If sliding is to be prevented, it is conservative to choose a low value, but if slidi sliding ng is benef beneficial icial (such as when dragging a load), a high value should be chosen. Similar issues occur with stiffness, depending on whether deflection or strain controlled load is of  interest. If in doubt, a ‘best estimate’ value should be used and then varied in each direction direction to under understand stand the effect. 7.1.6 7.1 .6

Projec Pro jectt ris risk k awa awaren reness ess

 Awareness of the specific risks on a project can do much to help eliminate or reduce them, or to ensure they are allocated to the right person or organisation.  This is not a matter of shifting blame, more of  ensuring that action is taken when required and providing a focus on what is important. It is a sensible precaution at the beginning of a contract for all parties to recognise where the risks lie and respond accordingly. For example, if the programme for the design desig n of a steel framed framed build building ing is criti critical cal there is absolutely no point in shaving off a tonne of steel, for the sake of least weight, if doing so risks delay and far more cost.

7.2

Managi Man aging ng the des design ign pro proces cesss

For any project, there should be a clear ‘basis of  design’ that records all the key data and defines how 32

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

Ideally there should be concept checks, approximate manual checks (using simple formulae such as ‘WL/8’) targeted to ensure freedom from gross error, followed by standard checks of structural elements.  The systems that provide global stability should be clearly identified. Staff should be trained to set out their work so that it is obvious where all the data comes from, to assist both checking and any subsequent changes. Calculations should be set out so that the output to the drawings is clear. Detailed methodical checks on all drawings and Building Information Models should be supplemented with a look over the details by an experienced eye to see if they look right. These should be suitable for their purpose, purpose, i.e. to communicate communicate the design to the constructor. At this stage, a check should be made that sensible precautions have been taken to assure structural robustness (see Section 7.6). In developing the structural scheme, designers should have regard to the future difficulties of inspection or the potential for lack of durability. A designer’s professional duty is to assure best value for money (not neces necessaril sarily the least initial cost). Design office management should assure that the mechanics of the design process are carried out under controlled procedures that minimise the risk of  error. It may be a platitude to suggest that the designer desig ner should ‘get it right first time’ but it remains the best aspiration and a company’s QA system should be geared towards achieving that objective and certainly towards preventing gross error. Design reviews should be held periodically to check  that the design matches what is required, that it ‘looks right’ and to allow peer review of the  judgements made by the designers.

 

Risk in design

7.3

Clarit Cla rityy of res respon ponsib sibilit ilityy

7.5

Clarit Cla rityy of des design ign req requir uireme ements nts

 There is a significant risk on very large projects that the design process becomes so fragmented that no one party retains a clear overview of the whole. Where more than one organisation is involved in design, it is essential to make the division of  responsibility clear. There should be one engineer responsible for overall stability and one lead designer to set the demands on all subordinate designers.  Thus, for example, the lead steel designer has to

Risk reduction starts at concept stage. Questions should firstly be asked about the design information: – What exactly is the structure required to do? – What are the key drivers in design that have to be achieved? – What loads is the structure required to carry? – How much uncertainty is there in these parameters? – Is the technology available to match the

define the standard to which connections are designed; the lead concrete designer has to set the standard to which, say, precast units are designed.  This is to avoid the risk of misunderstanding which is inherent across the interface. Many clients prefer to place a concept design contract which terminates when the design goes to the fabricator or constructor, making direct communication impossible. The risks in this approach should be pointed out to the client; these may include confusion or inadequate attention to detail or the presumption that some preceding party has taken care of  important aspects. Equally, there are commercial risks to the second party in accepting responsibility for a concept that might be flawed. If the concept designer’s involvement does terminate, overall responsibility should be formally handed over after making sure that the recipient is competent to take it

aspirations? – Are the desig design n processes processes being used proven enough to deliver the goals with confidence? – Is the form of structure robust and insensitive to the accuracy of the design assumptions? – Have all credible modes of failure been considered? – Is overall stability stability absol absolutely utely dependent dependent on any one single point? – Is the struct structure ure buildable buildable within budget?

on. 5.8 discusses as aSection risk management managem ent tool.the value of competence

recorded in writing.

In many countries, the law requires certification of the design. In the USA, design must be supervised by a licensed structural engineer. In England and Wales, there is a duty on local authorities. In Scotland, licensed independent engineers may issue certificates. The certifying engineer should assess all aspects and interfaces thereby giving the overview that has sometimes been missing in the past.  There needs to be special care if the programme is tight. There should always be adequate programme time to implement the chosen approach, but there are occasions when rapid action is required to assure safety or to achieve a client’s prime objective. On those occasions, the design concept should fit the time available; it is much less risky to produce simple robustt desi robus designs gns than it is to attempt the elabo elaborate. rate.

7.4 7. 4

Desi De sign gn ch chan ange gess

It is frequently the case that the client or end user does not really know the functional functional deman demands ds or the necessary design data with precision, and one of the skills structural engineers should deploy is the ability to define the design information required and ask for it. If the information information is not available, available, then mutual mutually ly understood bounded information (like worst case loading) should be agreed. All agreements should be

Most new structures require foundations and there is always inherent uncertainty in the assumptions underpinning the design. As numerous court cases will testify, it is a false economy to skimp on site investigations. Every effort should be made to verify essential design assumptions and, if this is not possible until actual construction, it should be made quite clear who is carrying the residual risk. Likewise, in refurbishment projects, there is often great uncertainty about the condition of the existing structure and, equally often, lack of opportunity to probe it destructively before alteration on site. If that is the case, the design team should have a strategy, supported by sufficient time and money, for onward investigation and for making changes as the picture clarifies; there is a risk that gross deterioration will be discovered.

7.6

Designi Des igning ng rob robust ust str struct ucture uress

7.6.1

Principle Prin cipless of robu robustnes stnesss

Design changes occur on most projects, although excessive changes during the design development phase are a frequent cause of commercial dispute on cost and delay, posing risk to those who make them or those who fail to manage them. There are often changes in the loading and dimensional information given by other team members to the structural engineer. To minimise the risk of a drawing passing to construction with the wrong information, there should be a strict design change procedure. Throughout a structure’s life, certainly on major structures, there

Robustness is a valuab Robustness valuable le attribute for any structure particularly where the consequences of failure could be high, for robustness will assist resistance to faults and hazards of all kinds. It can be defined as: the ability to resist unplanned events without disproportionate or catastrophic failure. Robustness is, therefore, useful in desensitising the structure to the consequences of error or unforeseen risk. Reducing the risk of progressive collapse is one benefit of robustness and is a design obligation.

should be a design authority with responsibility to assure assur e the safe safety ty of any prop proposed osed changes. changes. It is fairly obvious to engineers that to execute changes safely during the life of the structure, reliable as-built records are required, but the benefits may need to be explained to the client who will need to retain them.

 Awareness of progressive collapse developed in the UK as a result of the Ronan Point collapse in 1968 (see Appendix A.19). More recently, international recognition of this issue has increased following a number of terrorist incidents in which the robustness The Institution of Structural Engineers  Risk in structural engineering

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7.3

 

7.7

Risk in design

 The Institution’ Institution’s s report on robustness7.6 can be consulted for an elaboration on the principles and application of structural robustness. This does not cover specific UK requirements for Class 3 structures, which require dedicated risk assessments. The Institution has prepared a further report, Manual report,  Manual for the systematic risk assessment of high risk structures  against disproportionate collapse7.7. Guidance can be found in The in  The Structural Engineer 7.8. 7.6.2

Designin Desi gning g for accid accidenta entall load loadss

Most structures are designed to withstand known actions such as dead or live load. Some may also require design for less predictable effects such as impact or explosion, using a combination of  robustness and energy absorption. Guidance on accidental load cases and associated design strategies can be obtained from BS EN 1991-17:2006 Actions 7:2006  Actions on structures. Accidental actions7.9 (EC1 Part 1-7).

Figure 7.1   Glass plate plate carrying ’sea’ ’sea’ around ship ship

of the structure, or otherwise, had a significant effect on the level of damage. Stability is often taken to mean resistance to collapse but its full meaning is more subtle; the principle being that a minor change change in any of the assumed conditions should not cause a disproportionate change in state.  An example is Brunel’ Brunel’s s ‘SS Great Britain’ (Figure 7.1), surrounded in dry dock by an artificial ‘sea’ consisting of shallow water supported on horizontal glass plates that span between steel grillage beams 7.4. As a brittle material, with people walking beneath it, the glass is in a sensi sensitive tive position, position, so is much thicke thickerr (21mm (21mm)) than needed purely on stress grounds. It is often possible to provide robustness in a structure without adding substantially to cost and providing robustness is generally a cheaper and more effective way to reduce the risk of failure than by using heavier sections to provide more strength. Robustness is not obtained by designing for more load cases, or by using a higher factor of safety. To develop robustness when designing either permanent or temporary works, consider the following principles: – Ensure there is capacity for horizontal load transmittal (including notional loading) and clear load paths down to ground level. – Use materials and structural forms which are ductile and whose capacity is insensitive to changes in load, geometry or material properties. – Tie all the elements of the structure together together.. – Provide alternative load paths where possible. – Allow ‘non-structu ‘non-structural’ ral’ elements elements to carry load in an emergency.  These principles are among those defined in Section 2 of BS EN 19907.5 (Basis of structural design). This is both a useful summary of the principles of structural design, and an authoritative reference should one be required (for example, to support explanation to a client over the need for robustness). 34

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

Section 3 of EC1 Part 1-77.9 describes design strategies for identified accidental actions (loads) such as explosions and impact, and for unspecified causes.  This distinction is very useful. For identified actions, a hierarchy of design methods is given, aimed at reducing the probability of occurrence, reducing the severity of damage or reducing the consequences of  failure. For unspecified actions, Annex A of EC1 Part 1-7 defines risk classes, also known as consequence classes, for different types of buildings and levels of  use.. The use These se des descri cribe be bui buildi ldings ngs wit with h rel relate ated d act activi ivitie ties s and occupancy levels (as appropriate) in four broad groupings which vary from lower risk up to higher risk. National building regulations may use similar categories. For each class, rules are defined to limit the consequences of localised failure.  Annex B of EC1 Part 1-7 gives specific guidance on risk assessment in the context of accidental loads. In the Eurocode, ‘mitigation’ is used to mean risk  reduction by any means, and not to mean reduction of the consequences if the event occurs, as it is used in this Report. this  Report.  As a design code, EC1 Part 1-7 gives prescriptive design rules. It would be very difficult to write such a design code around the subjective, judgement based approach to risk assessment which is recommended in this Report  this  Report . There is a place for both approaches, depending on the type of risk and the knowledge and experience of the designer.

7.7

Designi Des igning ng for con constr struct uction ion

7.7.1 7.7 .1

Builda Bui ldabil bility ity

 A standard structural engineering approach is firstly to determine a struc structural tural form. In making this choice, regard should be given to construction needs. Apart from a basic duty to assure designed structures are ‘buildable’, it may well be that the worst loading conditions or some particular loading or stability condition occurs during site assembly. The risks of  this being found out too late, or not being comprehended by the constructor are that unwarranted unwarr anted safety risks may be imposed (with the

 

Risk in design

liabilities that flow from that) or perhaps the imposition of extra cost and programme delay which give rise to dispute. If the construction skills required are at all unusual, early co-operation with a constructor is a key risk reduction policy. Many countries have introduced legal responsibilities to design for safe construction, and in particular to co-ordinate design to ensure safety. There is a perception among some engineers that these duties can be met by specifying certain routine precautions (safety nets, reducing component weights etc.). While these may be valid mitigation measures, they are only a small part of the story; the real issue is that the whole design should be properly conceived. While this Report  this  Report  aims   aims to recommend internationally valid good practice, not to interpret or restate national legislation, useful guidance can be found in the  Approved Code of Practice7.10 to the UK’s CDM Regulations7.11, 7.12. Guidance has also been produced in the UK by industry and published by ConstructionSkills7.13; Section 2.6 of which contains specific advice on designing out risks. Structures have to be safely ‘buildable’. Structures ‘buildable’. As a minimum, minimu m, the load loading ing and stabi stability lity states that are likely to exist durin during g construction construction need to be addressed and a strategy devised. There is no firm rule about how these temporary states should be catered for, i.e. by strengthening the permanent design or by providing temporary works, a good design will make a proper judgement andbut document the information both to manage commercial risks and eliminate or identify site safety risks. These actions might be as simple as ensuring that the floor capacity allows mobile elevating work platforms (MEWPs) to be used or they might be as sophisticated as designing bridge girders for the stress states inherent in a cantilever bridge launch. For anything other than routine structures, there should be a close connection connection between between the method of  constructi cons truction on and the design of the structure; structure; both in its mode of erection and for the structure’s strength and stability in temporary states. Figure 7.2 shows the London Eye, where construction was fundamental to the concept. Construction sequences and interim stability states require particular attention in construction of deep basements, retaining walls, steel framed buildings and hybrid structures where, for example, the stability of the whole relies on a concrete core. Not all risks are those linked to safet safetyy or structural structural failure. Commercial design disputes are far more common. For example, in steelwork design there have been many disputes after concept design, where main member sizes have been chosen on the basis of minimum weight but have proportions such that connections could not be economically made or have required excessive stiffening within the connection zone. This risk is exacerbated if, as is typical in the UK, connection design is a separate activity activ ity to member sizing with the resu result lt that conceptual designers fail to gain adequate experience. In concrete design, there have been many examples of poor detailing or proportioning having little regard for the practicalities of placing and compacting concrete around congested reinforc rein forcement, ement, any of which might lead to loss of  capacity, excess cracking or poor durability with consequent claim. There are cases in precast concrete design where significant failures have

Figure 7.2   London Eye

occurred due to inadequate bearing or lack of  consideration for overall stability.  A key aspect of ‘buildability’ is the specification of  achievable tolerances. Disputes and conflicts over lack of realism in choosing tolerances, especially at interfaces, may be perhaps the most common of all. In the worst cases these have led to lack of  functionality with repercussions for the design team. On the one hand, the design team should appreciate the difficulties of site work and avoid specification of  tolerances that are unnecessarily tight or impossible to achieve. On the other hand, the constructors constructors should meet the specified requirements, and discuss any difficulties with the designer. Where it is necessary to define close tolerances, the drawing should make clear where and what these are and include references to any relevant specification clauses. 7.7.2

Designing Desi gning out cons construct truction ion haza hazards rds

It is incumbent on all members of the professional team to be aware of typical site hazards associated with common construction processes. These include hazards linked to, among other things, excavation, concreting, steelwork erection and building masonry.  The processes of demolition and refurbishment also need to be understood.  There have been incidents of site injury (including death) which could have been avoid avoided ed by bette betterr design, perhaps as simply as not requiring workers to struggle with the lifting of heavy objects. Some failures have occurred simply because the design team has fail failed ed to consi consider der or communicate communicate the risks properly. Designers should consider every person affected by the work activity. The key people at risk are the operatives rather than their managers, and the best place to record health and safety information is on those working working drawings ngs which will be used by both construction operatives and maintenance personnel. The Institution of Structural Engineers  Risk in structural engineering

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7.7

 

7.8

Risk in design

Significant risks can be listed in an eye catching box on a related drawing. This is sometimes known as a safety, health and environmental (SHE) box. It is also recommended that information on risk reductions taken from the design team meetings and the risk  register together with the drawings describing significant risks, are included in the lifetime records for the structure. Where the construction information is conveyed via computer models using Building Information Modelling (BIM), specific consideration should be given to the way risk information is communicated.

abstracted for onward dissemination on the drawings. 7.7.4

Designin Desi gning g for for unfam unfamilia iliarr envir environme onments nts

When designing structures to be built in an unfamiliar environment, such as another country, the design organisation should be competent to do so. This will require information on local construction methods, materials and practices and may require employing local engineers. It should not be presumed that familiar materials are available. An operation which is

Ideally, the design for permanent access to the upper parts of the structure should allow construction to be sequenced so that workers are protected; e.g. so that stairs can be erected in time to avoid need for temporary access provisions. The structure should be designed to support all the loads which may be placed on it, including materials, plant and equipment during construction and installation. See Table 4.1 (Representation of  .14 construction loads) of BS EN 1991-1-67 .14 .

quite safe with a workforce who are familiar with it and well trained may be quite risky if used by those who are unfamiliar. The capabilities of the available constructio const ruction n plant may not match what the designer is accustomed accustomed to, with the risk that some operations operations might not be feasible; conversely, faced with what is for them a novel desig design, n, operatives operatives might attempt to use an unsaf unsafe e appro approach. ach. Caution needs to be exercised as the construction quality achievable in the country of design may not be achievable in the country of construction.

 The risks to workers are not just injury but also to health7.15. Designers should avoid specifying materials that may be harmful to health, and be wary of details that require construction processes, such as use of  vibrating machinery. Internationally, there are many examples of good practice; in Japan, self-compacting

7.8

concrete is used extensively, avoiding the need for mechanical vibration. 7.7.3 7.7 .3

Commun Com munica icatio tion n of ris risk  k 

It is not the purpose of this  Report  to  to describe the communication skills that a structural engineer should possess. In terms of risk, however, inadequate data is known to have allowed the wrong materials to be used, clashes between concrete and steel, clashes between services and structure, confusion about what was propping what up at any stage, etc. Production of clear and complete information is just as important as skills in calculation. calculation. Advice may be found in the IChemE publication, Communication publication,  Communication .16 skills for engineers and scientists7 .16 .  The design team should consciously consider the th e information they are supplying, with particular attention being paid to data where errors might not be obvious until too late (such as key setting out data or key materials). They should also ensure clarity of communication regarding the load paths for vertical and horizontal loading, and which elements provide stability to which; this is especially important in hybrid structures of mixed materials (for example, where a steel frame might be stabilised by a concrete core).

Design for the who Design whole le buil buildin ding g life life cycle

Reducing risks during the life of a building starts at Reducing the concept and design stages. A key need is to identify fy how the structure will age and how materials materials may degrade. There can be severe financial and safety risks to clients and designers consequent on premature degradation. The financial risks arise from both the capital cost of repair and from loss of  income if the structure becomes non-operational. Liability may be high if the causes were foreseeable. Designs should consciously address issues of  durability and include provisions for safe access for maintenance and for inspection of any areas at risk 7.17. Issues such as corrosion in cable anchorages illustrate these concerns and present a good case for giving a higher priority, in the design of certain structures, to the practicability of maintenance 7.18. Where degradation is anticipated, for example, by fatigue fatig ue damage, it is most impor important tant during design to identify areas at risk and ensure these can be inspected in service. Design should also address the safety and health of building users. This might, for example, affect the choice of materials or specification of cladding materials above areas where many people will pass. Where the design allows for installation of  plant and equipment, it should also allow for safely replacing it during the building life, if this is likely to be necessary.

 The identification of hazards and corresponding evolution of design risk reduction in an overall project is best achieved via collective team debate. Risk  thrives at interfaces. During such debate, reports can be received from the various designers and decisions agreed for taking the project forward. A failure to co-ordinate, for example, between architecture, building services and structure for both finished and

 The designer should ensure that all relevant information about the facility is available on hand-over to the owner and/or occupier, including staged hand-over and where there may have been minor works.

installation phases risks all manner of site and operational problems. The results may be a failure of  functionality, fit or even danger if it results in unauthorised site modifications. Conclusions should then be recorded in an updated hazard/risk register. From that register, essential information should be

7.9

36

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

Designi Des igning ng for fut future ure dem demoli olitio tion n

Structural designers should consider making recommendations on how best to demolish what they have designed. The basis of this should be

 

Risk in design

structural engineering philosophy, principles, background and detail for the design. It should be clear what provides overall stability to avoid the risk of  that part being taken out prematurely. The benefit of  ‘as-built’ information should be available as a legacy (in the UK, included within the health and safety file), as well as records of alterations made during the structure’s life time. This may include reports on component degradation.

7.12 Ref Refer erenc ences es 7.1

 

‘Firms fined over screen collapse’. BBC News website, 5 June 2008. Available at: http://news.bbc.co.uk/1/hi/  england/west_midlands/7438044.stm england/west_midla nds/7438044.stm [Accessed: 19 February 2013]

7.2

 

Institution of Structural Engineers. Engineers. Structural  Structural design –  the engineer’s role . London: IStructE, 2011

7.3

 

The Institution of Structural Engineers. Engineers. Guidelines  Guidelines for  the use of computers for engineering calculations . London: IStructE, 2002

7.10 Proc Procurem urement ent and planni planning ng  Apart from design itself, other activities during the design phase can have a significant influence on risk.  These are not always the sole responsibility of the structural engineer, but it is important to influence them and to ensure that the consequences of the decisions are understood by those making them. Selection of the right contract form and a competent constructor can have a significant influence on risk.  All quality quality,, safety and competence requirements in the main contr contract act should cascade into all levels of  subcontracts. It is often preferable to limit the number of levels of subcontract to avoid too many interfaces. It is prefe preferable for the desi design gn contract to remai n in force intorable the construction phase and for construction contracts to be structured such that appropriate periodic checking can be carried out to ensure that design intent is being translated into reality. Subcontractors carrying out design should be required to formalise their own basis of design and construction method statements and submit them for review. This does not necessarily transfer liability, but it increases the chances of exposing error in the constructor’s assumptions and reduces the risk of  gross misinterpretations. While the detailed construction programme may be a matter for the constructor, the programme at the design stage should aim to sequence activities to eliminate instability and minimise hazards to the workers and public. Thus the programme cannot be formulated until the construction process and risks are understood.

7.11 Concl Conclusions usions and reco recommen mmendation dationss  The design phase, particularly the concept design, offers major opportunities for risk reduction. To use these opportunities, designers should be aware of  the uncertainties in design and the practicalities of  construction. Communication and co-operation are vital for the reduction of risks to health and safety over the life cycle of the structure, as well as disruption to the project as a result of bad design. Where designers have a specific legal responsibility to design out risk, as under the UK’s CDM Regulations7.12, this includes risk from structural design errors just as much as risk from failing to give thought to safe construction. The detailed advice in this chapter should be read and understood by all engineers who carry out design of any kind, whether as part of the main design phase or not.

7.4

 

Jofeh, J. and Perry, A. ‘Sea of glass: ass: ‘Refloating’ Brunel’s SS Great Britain’. Arup Britain’.  Arup Journal , 3, 2005.  Available at: http://www.arup.co http://www.arup.com/_assets/_downloa m/_assets/_download/  d/  download468.pdf [Accessed: 19 February 2013]

7.5

 

BS EN 1990:2002+A1:2005 1990:2002+A1:2005:: Eurocode – Basis of  structural design. London: design.  London: BSI, 2010 [Incorporating corrigenda December 2008 and April 2010]

7.6

 

Institution of Structural Engineers. Engineers. Practical  Practical guide to  structural robustness and disproportionate collapse in  buildings . London: IStructE, 2010

7.7

 

Institution of Structural Engineers. Engineers. Manual  Manual for the  systematic risk assessment of high risk structures  against 2013 disproportionate collapse . London: IStructE,

7.8

 

Harding, G. and Carpenter Carpenter,, J. ‘Disproportiona ‘Disproportionate te collapse of ‘Class 3’ buildings: the use of risk  assessment’. The assessment’.  The Structural Engineer , 87(15-16), 4 August 2009, pp29-34

7.9

 

BS EN 1991-1-7:2006: Eurocode 1: Actions on  structures – Part 1-7: General actions – Accidental  actions . London: BSI, 2010 [incorporating corrigendum February 2010]

7.10   Health & Safety Executive. Executive. Managing  Managing health and safety  in construction: Construction (Design and  Management) Regulations 2007 Approved Code of  Practice. L144 . Sudbury: HSE Books, 2007. Available at: http://www.hse. http://www.hse.gov.uk/pubns/pri gov.uk/pubns/priced/l144.pdf ced/l144.pdf [Accessed: 19 February 2013] 7.11   The Construction (Design and Management)  Management)  Regulations 1994  (SI   (SI 1994/3140). Available at: http://www.legislation.gov.uk/uksi/1994/3140/co egislation.gov.uk/uksi/1994/3140/contents/  ntents/  made [Accessed: 19 February 2013] 7.12   The Construction (Design and Management)  Management)  Regulations 2007  (SI   (SI 2007/320). Available at: http://www.opsi.gov.uk/s http://www.o psi.gov.uk/si/si2007/20070320. i/si2007/20070320.htm htm [Accessed: 19 February 2013] 7.13   CITB –  Guidance to the CDM Regulations  [separate  [separate guidance documents for Clients, CDM Co-ordinators, Designers, Principal Contractors, Contractors and Workers]. Available at: http://www.citb.co.uk/enGB/Health-Safety-and-other-to GB/Health-Safetyand-other-topics/Health-Safety/  pics/Health-Safety/  health-safety-legislation/ [Accessed: 12 March 2013] 7.14   BS EN 1991-1-6:2005: Eurocode 1: Actions Actions on  structures. Part 1-6: General actions – actions during  execution. London: execution.  London: BSI, 2010 [Incorporating corrigendum July 2008]

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7.10

 

7.12

Risk in design Engineers. Health  Health and safety  7.15   Institution of Civil Engineers. guidance and legislation . Available at: http://www.ice. org.uk/topics/healthandsafety/Guidance-a org.uk/topics/healthan dsafety/Guidance-and-legislation nd-legislation [Accessed: 19 February 2013] 7.16   Institution of Chemical Engineers. Engineers. Communication  Communication skills  for engineers and scientists . 4th ed. Rugby: IChemE, 2007 7.17   Iddon, J. and Carpenter Carpenter,, J. J. Safe  Safe access for  maintenance and repair: guidance for designers. C686 . 2nd ed. London: CIRIA, 2009 7.18   Health and Safety Executive Executive HM Railway Inspectorate, Inspectorate,  An Assessment by HSE of the structural integrity of the  Forth Rail Bridge report, C10. Sudbury: C10.  Sudbury: HSE Books, 1996. Available at: http://www.railwaysarchive.co.uk/  docsummary.php?docID=2107 [Accessed: 19 February 2013]

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8

Risk Ri sk ma mana nage geme ment nt dur durin ing g co cons nstr truc ucti tion on

8.1

Intr In trod oduc ucti tion on

construction, while in most cases it should receive more.

 This chapter describes actions that should be taken during construction to manage risk. The most serious risk is collapse of the structure or temporary works, or harm to construction workers. On many projects, however, the most frequent risk is that errors in structural engineering or its communication lead to cost or programme over run. Action taken during the design phase to minimise risk in construction is covered in Chapter 7.

8.2

Caus Ca uses es of inc incid iden ents ts

Construction is a complex activity, requiring the integration of many activities across a wide range of disciplines and the translation of an abstract ‘design’ into reality. The underlying presumption at the start of any site work should be that it will be risky: things can that go wrong. Most experienced engineers know projects rarely go entirely to plan. The causes (in effect, the hazards) may be in the design or its communication, such as errors on drawings, misinterpretation of specifications, design changes, failure to eliminate hazards etc., all of which raise the prospect of rework, wasteful expenditure, faulty construction and danger to life or health. The hazards may be in errors by the site staff and suppliers (e.g. incorrect setting out, the use of a wrong concrete concrete mix, missi missing ng reinforcement or just bad workmanship). Some of  the greatest risks on site arise from unplanned activities. Examples of the risks which can be directly minimised minimi sed by good planning planning and manage management ment on site include excessive construction loading on floors not designed for it, improper sequencing of construction, material weakness and temporary structural instability. Most structures are more vulnerable to instability instab ility when in the assembly stage than when complete. Temporary works, e.g. falsework and excavations, often receive less attention than permanent works, even though the loading and construction process may be more onerous but less certain. certa in. The immedi immediate ate safety of worke workers rs on site can be at risk from working at height or in confined spaces and handling or being too close to heavy objects and machinery. Their short or long term health can also be at risk from manual handling, vibration from tools, asphyxiation, infection and hazardous substances. These include not only materials like asbestos which are still found in existing structures, although banned in new construction in most of the world, but also common materials, materials, such as concrete, that cannot easily be eliminated 8.1. Hot

8.3

Reso Re sour urci cing ng an and d pl plan annin ning g

Management of risks during construction starts, as at any time, with hazard identification. The construction work should be planned in terms of  both how and when each activity will be executed, making sure it is then resourced in terms of  competent labour, time and finance. The number of  people employed, and their skills, should permit adequate planning, checking and supervision. If the project is rushed, or under-resourced, short cuts will be taken and both quality and safety will suffer. This does not mean that projects cannot be built to a tight budget and timescale, just that there should be a realistic plan in place setting out the required objectives and how they can be achieved. That plan should be framed around a conscious assessment of the hazards,control including those outsideweather the constructor’s such as adverse or unforeseen ground conditions. Ensuring that materials are delivered to site in the right sequence sequence and quanti quantity ty will not only save money but will reduce risks to both health and safety of  those involved and project completion. At the delivery points there need to be proper facilities for offloading and storing. Surplus material lying around a site can be one of the main contributors to accidents, either as an obstruction, a toppling stack or, if stored on the structure, an overload. The risk of deviating from the planned erection sequence is reduced if deliveries to site follow the ‘just in time’ principle, with a suitable buffer to avoid delay.  The designers of the permanent works should have ensured that there is at least one safe way that the structure could be erected and that the overall stability systems are clear. If the structure is unconventional, or the design anticipates a specific erection method, a description should have been provided. If drawings are not clear, the designer should be asked. If the design contract has been terminated this may require specific arrangements by the client. This does not necessarily mean that the structure has to be erected the way envisaged by the designer, for the constructor will probably have more experience exper ience in this area and is entru entrusted sted with construction for that reason. Planning construction should be based on the broad strategy of making sure the vertical and horizontal loads can be carried at all times and that stability is assured at all times. Thereafter it should be assured that all parts are safe to lift and stay stable while

8.2

work such as welding , flame cutting or melting bitume bit umen n bri brings ngs wit with h it obv obviou ious s per person sonal al dan danger ger but als also o the risk of fire. Some of the biggest fires on record (Broadgate8.3, 1990) have occurred not in finished buildings but during the construction phase. Refurbishment often receives less attention than new

being lifted and that there is safe access for workers to locations inherent in the assembly sequence. After the broad strategy is determined, a detailed appraisal of each activity needs to be made identifying any relevant risks and making sure the workers are properly briefed. Documented method statements, The Institution of Structural Engineers  Risk in structural engineering

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8.4

Risk management during construction

safe systems of work and relevant QA procedures should be produced, and checked.

8.4

Competenc Compet ence, e, ma manag nageme ement nt and welfare

 A key difference between a construction site and almost any other workplace is that a site is always changing. A factory can be set up with safe working arrangements for operation and maintenance and employees can learn to use these. On construction sites it is necessary to review the safety procedures and protective measures continually as construction progresses. Not only does each site vary over time, but different sites require a mobile workforce. As a result, the construction workforce often has a high turnover and includes at least some who have been unable to find other employment. Maintaining competence in such a dynamic situation requires continuous monitoring and proactive work to ensure that all workers have appropriate training and, if not experienced, supervision. Every person working on site should have both general safety awareness training and additional training proportionate to their site responsibilities. As a minimum, everyone in the indus industry should aware of andof  such try risks as thebedangers of common confined hazards spaces and visiting sites alone. Those who are directly involved with structures, such as steel erectors, scaffolders and supervisors should also understand the principles of stability. All workers should be encouraged, or required, to hold an appropriate safety qualification, if one exists. In the UK, the Construction Skills Certification Scheme8.4 issues cards to certify that the holder has demonstrated a knowle kno wledge dge of saf safety ety app approp ropria riate te to the their ir rol role e on the sit site. e. Staff visiting sites (especially if intermittently) should be given inductions on site specific risks that can change on a daily basis, such as evacuation routes. Construction projects range from minor building alteration alter ations s to major bridges. Each brings with it a particular set of hazards and is carried out by a team whose skill levels and experience clearly differ. Large projects will be supported by a full range of specialist roles and skills. On smaller projects, these roles may need to be combined, or specialists may need to be brought in temporarily. To minimise the risks, a combined effort is required by the designers and constructors to make sure the pool of experience assigned to the project has the best chance of  spotting what might go wrong. During construction, adequate supervision by appropriately skilled staff is required and that might include ongoing advice from the design team.

proprietary suppliers, etc. On major projects, a third party role to assess construction methodology can be a powerful insurance policy.  Adequate welfare facilities give staff and operatives the opportunity to maintain hygiene standards; apart from the direct benefits, this helps create a safety culture.

8.5 8. 5

Comm Co mmun unic icat atio ion n

Good communication is vital, including both communication from the design office to the site (for both permanent and temporary works) and within the site itself. Such communication can be by traditional drawings and documentation, or using electronic Building Information Management, but direct dialogue (at all levels) is strongly recommended. Vast amounts of ‘written communication’ carry the risk that vital data will be lost in information overload. Constructors and designers should work together, preferably with the client as well, to ensure that the right information information is produced and conveyed, thus making the site work  efficient effi cient,, rapid and safe. Any chang changes es to the work  shown on the drawing should be referred back to the designer, since the full reasons behind the original design may not be appreciated on site. Communication between parties on itself is crucial. Accidents have occurred duethe to site workers misunderstanding drawings and carrying out work  without checking with a foreman. Construction labour has always been mobile. Where a proportion of the workforce are not native speakers of the language in use, this presents its own obvio obvious us risks. Lapses in communication between different contractors and subcontractors on the same site have also led to accidents, often due to failure to notify others of  hazardous activities. The role of principal contractor in the UK’s CDM Regulations8.5 was partly intended to combat this issue.

8.6 8. 6

Load Lo adin ing g co condi nditi tion onss

 A proper management structure will support a team approach as the safest approach for avoidance of all manner of risks. Within the team it should be quite clear who is resp responsib onsible le for what, at each stage. The main contractor’s role and responsibility for overall co-ordination of site safety is absolutely fundamental.

Critical loading conditions may exist during the construction phase that are entirely linked to the manner of construction. For example, wind loads on bare and incomplete steel frames or large panels of  reinforcement and shuttering, construction loads that exceed the permanent loads and wheel loading from vehicle or crane access are all common issues. It is important when planning construction to establish whetherr the desig whethe design n intent for the perma permanent nent members allowed for the particular erection method proposed. In small scale work, such as refurbish refu rbishment, ment, this may be just to ensure that exist existing ing members have not degraded. In general, the task is to make sure that the evolving structure is capable of  taking the loads at each stage. This may include ongoing ng assessment assessment of the actua actuall rather than the expected condition or loading. Major project events may involve a single sequence of heavy lifts, such as

 This role is particularly vital v ital when there are numerous subcontractors; the main contractor has to control the interfaces interfaces to meet the twin objectives objectives of making sure what is built satisfies satisfies design intent and that safety is assured. This includes liaison with temporary and permanent works designers, specialist

raising ng the London Eye 8.6, the Wembley Stadium  Arch8.7, 8.8 or launching a bridge. Depending on the method chosen by the constructor, such events can stress the structure into states not fully appreciated by the designers for the permanent condition. One exampl exa mple e of a fai failur lure e dur during ing con constr struct uction ion occur occurred red to the

40

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

 

Risk management during construction

tunnel construction at Gerrards Cross8.9 in 2005 (see Figure 8.1).  The tunnel was to allow backfilling over an existing railway to provide the site for a supermarket. The loading conditions applied to the precast arch units would have differed during progressive backfilling from those which would exist after backfill had been completed, although this may not have been the cause of the colla collapse. pse. There were no injur injuries, ies, but the client had to pay substantial compensation to the railway operator for obstruction to the railway and was not covered under its public liability insurance8.10. Permanent works designers often have great difficulty envisaging the loading conditions that may exist during construction and so the construction engineer on site should always verify their proposed methodology. Construction loads may arise, for example, from personnel and hand tools, storage of  construction materials, non-permanent equipment, moveable heavy machinery and equipment, accumulation of waste materials, or loads from parts of a structure in a temporary state. These may exist when material such as concrete is in an immature state. A typical case exists when upper concrete floors of a building are cast supported off the floor below (perhaps propped through multiple floors).

Figure 8.1   Tunne Tunnell at Gerrards Gerrards Cross

stiffness that concrete will eventually provide. In all construction, it is normal for the main stabilising element (such as a core or braced bay) to be constructe const ructed d first and then for the rest of the structure to be progressively erected away from that point. For various reasons, however, the chain of structural integrity back to stability points might be interrupted during construction.

8.7

Sequen Seq uence ce of con constr struct uction ion

 The preparation of drawings or 3D models showing a structure in its incomplete stage(s) should be encouraged, particularly for innovative, complex or unusual structures. These should be studied to understand where the structure obtains strength and stability. For example, a steel truss may be laterally stabilised stabi lised by purlins in its final condition condition but can only be erected with those purlins absent. Bridge beams stabilised by a concrete deck will have to support the weight of wet concrete while lacking the in-plane

In complex projects, a dedicated monitoring regime can be established to assure that all is going to plan. In full size structures it is not practical to measure stresses directly. It is possible, however, to survey displacements and map those against predictions for each stage. When there is correlation, it is an indication that the stresses are acceptable. Such monitoring is essential with certain procedures, notably the excavation of deep basements or tunnelling. Indeed it is central to the success of NATM (New Austrian Tunnelling Method) techniques8.11. The illustrated failure at Heathrow8.12, 8.13 (see Figure 8.2)

Figure 8.2   Recovery from tunnel collapse, Heathrow, Heathrow, UK  The Institution of Structural Engineers  Risk in structural engineering

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8.7

 

8.8

Risk management during construction

resulted at least in part from lack of management of the ground survey information that was collected.  An increa increase se in prefab prefabricati rication on means that building parts can arrive on site fully formed and only require putting into place. The structure may not, however, be stable in some intermediate forms, requiring care in the way individual parts are lifted and temporarily supported.  An erection on sequence should be devised accordingly. ngly.  As well as ensuring stability stability,, selection of the construction sequence provides an opportunity to improve construction access, for example, by constructing stairs and handrails early or by pre-assembling complex items at ground level.

8.8

Tem empo pora rary ry wo work rkss

better to eliminate the hazar hazard d or use colle collective ctive methods of protection that individuals cannot decline. It is human nature to try to avoid things that make the  job more difficult, particularly if the worker has limited understanding of the risk. PPE is often considered to be an encumbrance so the need for it should be avoided whenever possible.  There are, however however,, situations where PPE can provide a real risk reduction, such as asbestos removal and other instances of contamination. In these cases, the hazard is clear and workers will insist on wearing it. The focus in such cases is ensuring that it is adequate.

8.10 Access Access and work area areass including including working at height

Many temporary works failures have been recorded, such as Nicoll Highway8.14, 8.15 (2004). The design of  temporary works can be more complicated and risky than design for permanent works. The design loads and the conditions of installation are often much less certain and the measures for the provision of stability can become confused. Temporary works should be designed properly, not simply erected based on someone’s someone’ s experience8.16. Similarly, foundations for

 There are clear risks if working areas are not provided with safe access and egress. Falls from height are still numerically one of the greatest sources of injury. Working platforms should normally be provided, but there are no universally right or wrong answers. Scaffolding is the traditional way to provide platforms, but its erection involves a risk to the scaffolders. For a one-off task that does not require carrying of tools or materials, a carefully managed ladder may offer least

temporary should of not ignored works or treated lightly. The works appointment a be temporary co-ordinator, as defined in BS 59758.17, should be considered as a means to address any temporary/  permanent works interface issues.

overall risk. For short duration work in difficult areas, roped access may be the safest approach, but this is true only if prop proper er training and equipment are provided.

Falsework, in particular, has been the cause of many incidents. Awareness of the dangers seems to be cyclical. cal. Following ng a number of accidents in the UK, the Bragg Report8.18 in 1974 was the trigger for the production of BS 59758.17, which set new standards for falsework, but concern continued. BS 5975 was updated in 1996 and an HSE construction information circular (CIS568.19 ) was issued to impro improve ve awareness awareness of  the issues. A SCOSS Topic paper8.20 notes that increa inc reased sed use of pro propri prietary etary sys system tems s and subc subcont ontrac ractin ting g has reduced the knowledge base among main contractors and that communication between designers and ere erecto ctors rs can be poo poorr. As par partt of Eur Europe opean an sta standa ndards rds normalisation, BS EN 128128.21 was issue issued d in 2004 and revised in 2008. The UK National Foreword to BS EN 12812 points out that it does not include two recommendations of the Bragg Report, included in BS 5975, namely a minimum lateral stability force and appointment of a temporary works co-ordinator, and reco re comme mmend nds s use of BS 59 5975 75 fo forr Cl Clas ass s A fa fals lsew ewor ork. k. Th This is is an area where published material overlaps and up to date information should be sought.

8.9

Protec Pro tectiv tive e equ equipm ipment ent

Personal protective equipment (PPE) has a high profile as a risk reduction measure, which is probably inconsistent with its real importance in the risk  reduction hierarchy. Many countries, including the UK, place a legal obligation on employers to provide it and certain items are usual usually ly made mandat mandatory ory for workers. Nevertheless, partly because of the difficulty in persuading people to use it, protective equipment should be thought of as the last resort. It is much 42

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

 As with temporary works, there are many examples of access scaffolding collapsing or being unsafe to work from. Tools and equipment need to be maintained mainta ined and read readily ily be avail available able so that a ‘make do’ attitude is avoided. The configuration of simple access acces s scaffolds scaffolds is within the skill of a train trained ed and experienced scaffolder, but complex, free standing or sheeted scaffolds require design by a structural engineer. Collapses have occurred where access scaffolds have been used as temporary works, such as to restrain a fac¸ ade.  The correct use of a mobile elevating work platform (MEWP), sometimes called a ‘scissor lift’ or ‘cherrypicker’, provides a safe working area without the need for scaffolding, but requires planning of the constructio const ruction n seque sequence. nce. A MEWP may be unsuitable if  the site is crossed by trenches, the floor slab is being cast at the same time, the floor capacity is inadequate or the MEWP would be at risk from vehicle impact. Similarly, the need to work in confined spaces can be minimised by careful planning. For example, installation of mechanical equipment into a tank is often carried out from the top, which is inefficient and requires complex arrangements to protect against asphyxiation and allow escape. Leaving out part of a wall until the plant is installed may save time and money, as well as being safer.

8.11 8. 11 Li Lift ftin ing g Lifting anything is always risky, whether by human or mechanical means. Manual lifting poses a risk of 

 

Risk management during construction

operatives sustaining back injury, with an added risk  of trapped fingers or feet. It is essential to have appropriate equipment and training and to limit the load lifted. Crane overturning is a fairly frequent occurrence and is typically caused by overload or due to location on bad ground. All manner of minor lifting devices can be misused. While it is preferable to use mechanical aids than rely on human effort alone it is even better if lifting can be avoided. For example, concrete can be pumped rather than skipped.

8.13 Ref Refer erenc ences es 8.1

 

Institution of Civil Engineers. Engineers. Health  Health and safety  guidance and legislation . Available at: http://www.ice. org.uk/topics/healthandsafety/Guidanceorg.uk/topics/healt handsafety/Guidance-and-legislation and-legislation [Accessed: 19 February 2013]

8.2

 

Puybaraud, M-C. and Barham, R. R. Addressing  Addressing the risk  of fire during the Construction/refurbishment process  by better management . Paper presented at COBRA ’97, Portsmouth, 10-12 September 1997

8.3

 

Fire Safety Engineering Consultants Ltd. Ltd. Structural  Structural fire  engineering: investigation of Broadgate Phase 8 fire. SCI Publication 113 . Ascot: SCI, 1991

8.4

 

Construction Skills Certification Scheme  (no   (no date)  Available at: http://www.cscs http://www.cscs.uk.com/ .uk.com/ [Accessed: 22 July 2013]

8.5

 

The Construction (Design and Management)  Regulations 2007  (SI   (SI 2007/320). Available at: http://www.opsi.gov.uk/si http://www.o psi.gov.uk/si/si2007/20070320.htm /si2007/20070320.htm [Accessed: 19 February 2013]

8.6

 

Berenbak, J. et et al. al. ‘The ‘The British British Airways London Eye. Part 2: Structure’. The Structure’.  The Structural Engineer , 79(2), 16 January 2001, pp19-28

8.7

 

Mann, A.P.Stadium A.P . ‘Design ‘DesignRoof.’ and Fabrication of to thethe new Wembley Presentation Manchester Association of Engineers, 16th November  2006. Available at: http://www.mae.uk.com/  Wembley%20Stadium%20with%20Pictures.PDF [Accessed: 5 March 2013]

8.8

 

Bizley, G. ‘In detail: detail: Wembley Stadium arch’. arch’. Building   Building  Design , 10 June 2005, pp24-27

8.9

 

‘Backfill operation probed in Gerrards Cross tunnel collapse’. New collapse’.  New Civil Engineer , 7 July, 2005 [As at Dec 2012, no formal investigation is believed to have been completed]

Lifting cannot be avoided completely. Prefabrication might require large lifts, albeit fewer of them but large lifts are likely to be treated more seriously. All major lifts should should be planned in detai detaill and that inclu includes des assuring that cranes are used within their capacity (for both load and wind speed) and are supported on stable ground. This includes proper design of  attachment attac hment points to the parts being lifted and controlling and managing the lift under supervision.  The potential for instability and the need to support large and heavy objects from cranes combine to make erection one of the biggest causes of  accidents. Advice can be found in a SCOSS topic paper8.22.

8.12 Concl Conclusions usions and reco recommen mmendation dationss – Take account of the generic guidance in Chapter 5. – Establish procedures for managing, monitoring and updating risks throughout the project. – Identify the safest reasonable approach to each risk  for the site concerned – do not assume that what worked on one site will work on another. – Allocate responsibility for managing risk to the party best able to do so, and ensure that this is understood. – Plan the work, based on the information provided by the desig designers ners and with significant ficant input from the constructor’s skills and experience, to ensure stability and safe access. – Seek the opinion of the designers in review of the proposed approach. It is recommended that design contracts allow for this. – Produce Produce method statements and safe syste systems ms of  work and require subcontractors to produce them for their activities. – Use appropriately competent people to assess the stability of temporary works, including falsework  and access scaffolds. – In major contracts, contracts, or where construction construction quality is critical, consider third party accreditation for the construction methodology (and risk management) arrangements.  All this should be aimed at ensuring that the risk of  gross error is minimised, that the risk of instability and overloading at any stage is minimised and that no worker or subsequent user of the structure is exposed to unnecessary risks to their safety or health.

8.10   Tesco Tesco Stores Stores Limited v Constable and Others [2008] EWCA Civ 362. Available at: http://www.  judgmental.org.uk/judgments/EWCA-Civ/2008/   judgmental.org.uk/judgm ents/EWCA-Civ/2008/  %5B2008%5D_EWCA_Civ_362.html %5B2008%5D_EWCA_C iv_362.html [Accessed: 20 February 2013] 8.11   Nicholson, D. et al. al. The  The Observational method in  ground engineering: principles and applications. CIRIA Report 185 . London: CIRIA, 1999 8.12   Health and Safety Executive. Executive. The  The Collapse of NATM  Tunnels at Heathrow Airport, 20/21 October 1994 . Sudbury: HSE Books, 2000 8.13   Standing Committee on Structural Safety. Safety. The  The Collapse  of NATM Tunnels at Heathrow Airport. SCOSS Failure  Data Sheet SC/06/101. SC/06/101. Available at: http://www. structural-safety.org/topicpap structural-safet y.org/topicpapers ers [Accessed: 20 February 2013] 8.14   Magnus, Magnus, R. et al. al. Report  Report on the incident at the MRT  circle line worksite that led to the collapse of the Nicoll  Highway on 20 April 2004 . Singapore: Ministry of Manpower,, 2005 Manpower 8.15   Standing Committee on Structural Safety. Safety. The  The Collapse  of the Nicoll Highway on 20 April 2004. SCOSS  Failure Data Sheet SC/06/102 . Available at: The Institution of Structural Engineers  Risk in structural engineering

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8.12

 

8.14

Risk management during construction http://www.struc http://www.structural-safety tural-safety.org/topicpapers .org/topicpapers [Accessed: 20 February 2013] 8.16   Carino, N.J. et al. al. Investigation  Investigation of Construction Failure  of the Riley Road Interchange Ramp, East Chicago, Indiana. NBSIR 82-2593 . Washington, DC: National Bureau of Standards, 1982. Available at: http://www. nist.gov/customcf/get_pdf.cfm?pub_id=908826 [Accessed: 20 February 2013] 8.17   BS 5975:2008+A1:2011: 5975:2008+A1:2011: Code of practice ce for  temporary works procedures and the permissible  stress design of falsework . London: BSI, 2011 8.18   Department of Employment oyment and and Department Department of of the Environment. Falsework: Environment.  Falsework: interim report of the Advisory  Committee on Falsework . London: HMSO, 1974 [Final report, known as the Bragg Report, published as Health and Safety Executive. Final Executive.  Final report of the   Advisory Committee on Falsework . London: HMSO, 1976] 8.19   Health & Safety Executive. Executive. Safe  Safe erection, use and  dismantling of falsework, HSE Construction Information  Sheet 56. [s.l.]: 56.  [s.l.]: HSE, 2003. Available at: http://www. hse.gov.uk/pubns/cis56.pdf [Accessed: 20 February 2013] 8.20   Standing Committee on Structural Structural Safety. Safety. Falsework:   Falsework:  Full Circle? SCOSS Topic Paper SC/T/02/01. SC/T/02/01. Available at: http://www.structural-safety http://www.struc tural-safety.org/topicpapers [Accessed: 20 February 2013].org/topicpapers 8.21   BS EN 12812:2008: Falsework – Performance  Performance  requirements and general design . London: BSI, 2011 8.22   Standing Committee on Structural Structural Safety. Safety. Erection  Erection of  steel structures: learning from experience SCOSS  Topic Paper SC/T/04/01. SC/T/04/01. Available at: http://www. structural-safety.org/view-report/sco structural-safety .org/view-report/scoss174 ss174 [Accessed: 20 February 2013]

8.14 Bibl Bibliog iogra raphy phy Health and Safety Executive. Benefits Executive.  Benefits and costs . Available at: http://www.hse.gov.uk/cos http://www.hse .gov.uk/costs/costs_of_injury/costs_of ts/costs_of_injury/costs_of_injury.asp _injury.asp [Accessed: 19 February 2013] Mann, A.P. ‘Construction safety: an agenda for the profession’. The Structural Engineer , 84(15), 1 August 2006, pp28-34

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9

Risk Ri sk ma mana nage geme ment nt dur durin ing g the the lif life e of of a str struc uctu ture re

9.1

Intr In trod oduc ucti tion on

9.3

Risks Ris ks dur during ing the lif life e of of a str struct ucture ure

 This chapter describes considerations for risk  management during the life of a structure, including

Structures may require the attention of a structural engineer during their life for a number of reasons.

alterations, maintenance and degradation mechanisms during life9.1, 9.2. There is a strong link  with asset management9.3 and an appropriate reference9.4 is given for further reading. Also the effects of construction activities9.5 on existing structures should be explored9.6.

Principally, these are proposed changes of use, modifications or when concern arises over the structural condition. Many facilities will undergo modification, typically at intervals of about five years.  This may be due to deterioration or accidental damage, including fire. Modifications may be required to meet statutory obligations, such as access for disabled people or changes to fire regulations. Every modification involves risk, not just from the practicalities of working safely but for the potential for inadvertent changes to the design intent. When engaged to review an existing structure, the engineer should be alert for any  ad hoc  or informal changes to the structure or the loading, as well as those which have been documented. Building occupiers may also seek advice on mainte maintenance nance;; either to maintain or extend the life of the building, to address perceived problems, or as a matter of good practice, possibly

9.2

The Th e lif life e cyc cycle le of a str struc uctu ture re

Structures have a longe Structures longerr life than almost all other human artefacts. During their life, society changes around them and they are often modified for new uses. Humanity cannot afford, in terms of  sustainability, to build new structures where existing ones can be used. New uses, however, and new loadings from uses, will introduce new risks, both during theexisting process proce ss of modif modificati ication on and when in use. Structures will also require cleaning and other maintenance. Good design will make provision for this, but circumstances change and the provisions made when the structure was new may no longer be appropriate. Significant numbers of bridges on UK motorways have had to be upgraded because their vulnerabili vulner ability ty to vehic vehicle le impact has prove proven n to be too great. This was not a consequence of errors in the design, or of experience leading to changes in design practice, but of society changing its view of  which risks are acceptable. Risk assessment in these situations is particularly difficult, as the cost/benefit ratio for reducing the risk by modifying the structure is quite different to what it would have been at the time of the original design, yet there is pressure from society to meet the ‘modern standard’. Managing structures and facilities effectively and safely during their lifetime can provide economic benefits to the owners and users, at many levels, not least in mitig mitigating ating the cost of the drama dramatic tic losses (that society pays for via insurance) if incipient degradation is not detected and acted upon early enough. This is recognised in many business communities and a ‘new’ profession of facilities management evolved towards the end of the 20th century. This was partly because owners and occupiers are realising that management of risks associated with the building or structure can be addressed as part of overall risk assessment for business continuity. Provided the facilities manager has the competence to manage structures, this is a ‘win-win’ strategy strategy,, addressing a potential liability to staff and the public at the same time as protecting the business. In most cases, the cost of a building’s contents or the cost of disruption linked to loss of  function is far more than the cost of the structure itself.

to meet their as landlord tenant. Whatever theobligations initial involvement in theorstructure, however, the engineer should consider making proposals for its future management. A client who has already had to call in an engineer is likely to be receptive to such advice. Ensuring structural adequacy is the most important part of risk management during the life of a structure.  This may entail adhering to regulatory9.7 requirements, either for buildings in general or for specific uses. Taking the UK as an example, Building Regulations are not generally retrospective with regard to structure, but if there is a change of use or a significant modification, they will apply to an existing building. In particular, the requirements to resist disproportionate collapse may apply; see Approved Document A 9.8 to the England and Wales Building Regulations and Part C9.9 of the Scottish regulations. See Section 7.6 for a discussion of robustness and the provisions of BS EN 1991-1-79.10. There are, however, many ways to improve the robus robustness tness of an exist existing ing structure, struct ure, particularl particularlyy if it is required (by the clien client) t) to meet modern standards standards or if there are legal requirements applicable in the location concerned. Every part of a structure will decay from the moment it is completed. Both the construction industry and society in general have had to face up to the risks of  concrete9.11 decay (of various forms) and to absorb substantial maintenance costs as a result. In some cases, as in the Montreal bridge collapse 9.12, 9.13 (2006) in Canada and the Stewarton railway bridge collapse9.14 (2009) in Scotland, unobserved decay has led le d to co collllap apse se.. Th This is ma mayy be of th the e wh whol ole e st stru ruct ctur ure e su such ch as of Pipers Row Car Park 9.15 (1997) or partial as in the failure of cavity walls from tie corrosion. Each year, people are injured (and sometimes killed) simply by parts falling off buildings 9.16, 9.17, 9.18. Monitoring can avoid unexpected failures but even so the economic costs can be high, sometimes extremely so, as evidenced by the consequences of hanger and main cable deterioration on the Forth9.19 and Severn9.20, 9.21 The Institution of Structural Engineers  Risk in structural engineering

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9.4

Risk management during the life of a structure

suspension bridges. Disasters such as the King Cross underground fire9.22 (1987) reveal the vulnerability of  existing infrastructure. The safety lessons from all such events9.23 need to be spelt out9.24, 9.25 and applied to other facilities if further events are to be avoided.

– Fire resistance and escape. – Deterioration, corrosion and ageing. – Dynamic and/or fatigue effects. – Localised failure/collapse. – Progressive/disproportionate collapse. – Serviceability failures, subsidence, falling debris. – Loss of use. – Legal liability.

Even recent history tells us that understandings change. Thus Ronan Point9.26 (1968) alerted engineers to the danger in certain building types of lack  of robustness and possible disproportionate collapse.  The collapse was initiated by the ignition of gas with a resulting overpressure, a hazard which had not been

– What has changed? – Is usage and loading as designed for, including external factors like climate change9.29?

appreciated in design. Following the failure of  structures built using high alumina cement (HAC) concrete in the 1970s, the industry has had to assess significant stock in the light of better understanding about HAC degradation.

– Have standards changed? – Has deterioration occurred as a result of ageing or misuse? – Have new hazards arisen? – Is there new learning from similar structures?

Provisions made by designers are not always understood by later occupiers, and may require reinstatement. For example, it has been known for movement joints to be filled with solid material during decoration, and ‘strengthening’ to be added (on the direct instructions of the owners) to prevent the inevitable cracks.

– What level of risk is acceptable? – Level of degradation in the fabric? – Is proactive or reactive maintenance required? – How much can the client’s business afford (short term survival may outweigh long term benefit)? – What are the insurance implications?

9.4

Risk Ris k ma manag nageme ement nt str strate ategy gy

Management of risks during the life cycle of a build Management building ing should have been considered at the concept and design stages (see Section 7.8). The initial strategy set out by the desig designers, ners, however however, will often need to be modified, for the same reasons reasons that the building 9.27 itself may need alteration , i.e. to make it safer and more resource efficient to operate9.28 and to take account of changing circumstances. Risk management strategies will be required for activities such as refurbishment (long term), maintenance such as painting (medium term) and activities such as window cleaning (short term/cyclical).  To relate building risks to business risks, the strategic  To position of the building within the organisation should be considered. The intended uses and occupancy patterns patter ns of the building should be considered considered in the context of the management arrangements. This will enable the appropriate responsibilities to be put in place for the various aspects of ensuring continuing safe and efficient use, both in and around the building. Risk management strategies should be implemented through project and operational risk  assessments that will take the strategic position into account. Clients may tend to view any activity in terms of short term cost/benefit, without fully understanding the long term risks.  The following points should be considered when establishing a risk/hazard management strategy: – Who may be affected? – To what should the policy minimise risk? (e.g. to human life, to the structure, structure, to busin business ess continuity) – To whom should the policy minimise risk? (e.g. people in/on the facility, people outside the facility, the owners or operators) – What are the areas of risk? – Stability in normal use and in extreme events such as storm or flood. 46

  The Institution Institution of Structural Structural Engineers Engineers Risk  Risk in structural engineering

– What measures are needed to ensure this? – Resou Resources rces to be commit committed ted to imple implementin menting g the policy? – Time scale for action? – Who should be involved? – Interval before next review?

9.5 9. 5

Main Ma inte tena nanc nce e re regim gime e

 An essential part of any risk management strategy will be to put in place a maintenance system. It should define what maintenance and inspection is required, how and when it should be done and which aspects should be given particular attention. Although this should be realistic, to ensure that planned activities are actually carried out, maintenance and inspection are not optio optional. nal. If clie clients nts are short of funds, maintenance is often seen as an area which can be squeezed with no immediate consequences. Lack of  maintenance, however, merely prolongs and exacerbate exace rbates s a prob problem lem which is bound to exis exist, t, with increased long term costs. Engineers involved in maintenance mainte nance work should not accep acceptt work which is inadequately resourced to the point where it cannot be done safely. Failure to carry out adequate maintenance has been implicated in several major collapses, such as the I35W bridge in Minneapolis, USA 9.30 and the Malahide viaduct in Ireland9.31. A contributory factor in the I-35W collapse was lack of redundancy in the original design. In other cases, the risk of lack of maintenance may be economic loss (see Figure 9.1). Different components of a structure have different characteristics and will require a different inspection, approach and maintenance cycle, for example: – Structural materials: concrete, steel, aluminium, timber, glass, masonry, etc. – Fac¸ ade: cladding, curtain wall, external wall tile and renders, windows, structural sealants, etc. – Bearings, fire stops, movement joints, glass balustrades etc. – Non-structural elements: ledges, air-conditioner hoods, equipment/plant supports, etc.

 

Risk management during the life of a structure

It should be possible to inspect, throughout its lifetime, any metal structure subject to varying loads, to assess the likelihood of fatigue damage, unless it is clear from the design that, even if abused, the number of cycles would not lead to damage during any credible lifetime. Provisions made for maintenance may become outof-date. Roof access is a good example; originally, this was the preserve of steeplejacks, who would climb over slate roofs with no protection. Provision for maintenance was brought in with the advent of flat roofs, with parapets or handrails. However, not all roofs were suitable; some which looked accessible on first glance were fragile, and people fell through. Standing seam aluminium roofs were initially provided with attachment points for harnesses, but these are no longer a preferred method of protection. Current thinking9.32 is to design for no maintenance or, for relatively low-rise structures, to provide a suitable surface at ground level for access from mobile platforms. When considering an existing structure, the maintenance strategy should be reviewed to determine whether the approach is still optimal, given the circumstances. In many cases, it will not be practical to use the method which would be ideal for a new structure; equally, it may no longer be justified to use the existing methods.

Figure 9.1   Collapsed I-35W bridge

Relevant information may be found in, for example:

should ensure that the structure which has been

– maintenance manuals – statutory records (e.g. in the UK, the health and safety file), which should be kept up to date.

designed is safe.does It does follow that design an existing structure which not not meet modern codes is necessarily unsafe; it may be, or it may not be.

Operating manuals, including health and safety files should be considered for updating, depending on the nature of the activities undertaken.

For example, many structures in the UK which have been designed to BS 81109.36 may not meet all the requirements of BS EN 19929.37. The change to Eurocodes was made initially to harmonise European trade rather than for structural reasons, and does not necessarily mean that structures designed previously are unsafe. On the other hand, structures designed to meet the wind loading requirements of CP3, Chapter V 9.38, will quite possibly not meet the requireme requ irements nts of BS 6399, Part 29.39 and BS EN 19919.40, and their safety may need to be assessed. These codes are based on a more advanced understanding of wind effects and, for example, their wind and snow maps have been redrawn.

Maintenance should be planned so that it can be carried out safely. Much of the guidance in Chapter 8 on construction risk may also apply to maintenance.  Typical  T ypical risks to people doing maintenance and also those who may be affected by their activities include: – access (modern bridges often include access gantries which themselves require maintenance9.33 ) – confined spaces – hot materials (fire risk), toxic materials – falls from height, including fragile roofs.  A serious accident occurred during modifications to the Avonmouth Bridge9.34 using the access gantry (1999). The gantry was not anchored adequately and blew off the end of the supporting beams, falling to the ground with the loss of four lives. It is possible that this accident might not have occurred if more thought had been put into the way the gantry was operated. Advice on the use of such gantries is given in the Institution’s report 9.33.

9.6

Apprai App raisal sal and ass assess essmen mentt

It is for structural engineers to use their skills and  judgement to decide whether or not an existing structure is adequate for its proposed use, taking all the relevant factors into account9.35. In the era of  sustainabl susta inable e devel developmen opment, t, socie society ty canno cannott affo afford rd to write off existing buildings just because engineers are not prepared to make judgements in the application application of their skills. Judgement, however, is not guesswork, and should be supported by logic and evidence. This may be based on pragmatic first principles engineering and may also make use of the statistical concepts described in Chapter 6.

 Appraisal of existing structures is a topic on its own; risks can be minimised by using appropriate guidance such as the Institution’s guide 9.35.

9.7

Section 5.6 discusses the use of design codes. In the context of existing structures, it is particularly important to understand that design codes are written writte n for design, not for asses assessment. sment. Use of a design code, within the scope of its application,

Management of risks involving existing structures is the direct responsibility of their owners, operators and occupiers. Structural engineers should take any opportunities to recommend to those responsible that a strategy for management of the structure is

Conclu Con clusio sions ns and rec recomm ommend endatio ations ns

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9.6

 

9.8

Risk management during the life of a structure

required and will be economically beneficial. This should cover records, maintenance and inspection and periodic review. When engaged on modification work, and when refurbishment9.41 or repair is required, structural engineers should follow appropriate parts of  the advice given in this  Report  for   for design and construction.

9.8 9. 8 9.1

9.2

9.3

Refe Re fere renc nces es  

 

 

Neale B.S. ‘Maintaining structural safety through a life-care plan and regulation’. Proceedings regulation’.  Proceedings of the 3rd  Forensic Engineering Congress, San Diego, California, September 2003. Reston, 2003.  Reston, VA: ASCE Press, 2003, pp433-440 National Steering Committee for the Inspection of Multi-storey Car Parks. Recommendations Parks.  Recommendations for the  inspection, maintenance and management of car park  structures . London: Thomas Telford, 2002 Neale, B.S., ‘Better value and safety for refurbishment projects through use of new standards’.  Facilities  management and asset maintenance: applying and  extending the global knowledge base: CIB W70 –  Proceedings, Glasgow, 18-20 September 2002. CIB  Publication 277 

9.4

 

Lloyd, C. ed. Lloyd, ed. Asset  Asset management: whole life  management of physical assets . London: Thomas Telford, 2010

9.5

 

Neale B.S. ‘Hazard and risk assessments for  construction: a regulators view’. The view’.  The Structural  Engineer , 73(22), 21 November 1995, pp388-90

9.6

 

BS EN 1991-1-6:2005: Eurocode 1: Actions on  structures. Part 1-6: General actions – actions during  execution. London: execution.  London: BSI, 2010 [Incorporating corrigendum July 2008]

9.7

 

Neale, B.S., ‘To ‘Towards wards ensuring greater structural reliability through legislation’. In IABSE et al. Safety, al.  Safety, risk and reliability: trends in engineering: conference  report, international conference, Malta, March 21-23, 2001.. Zurich: IABSE, 2001, pp759-764 2001

9.8

9.9

 

 

Office of the Deputy Prime Minister Minister..  The Building  Regulations 2000 — Approved Document A:  Structure. 2004 edition incorporating 2010  amendments . Available at: http://www.planningportal. gov.uk/buildingregulations/approveddocuments/parta/  ldingregulations/approveddocuments/parta/  documenta [Accessed: 20 February 2013] Building (Scotland) Regulations 2004 . Edinburgh: The Stationary Office, 2004 (SSI 2004/406)

2006-October 15, 2007. Report  2007.  Report . Available at: http://  www.cevc.gouv.qc.ca/Use www.cevc.go uv.qc.ca/UserFiles/File/Rapport/ rFiles/File/Rapport/report_ report_ eng.pdf [Accessed: 6 September 2010] 9.13   Wood, J.G.M. J.G.M. ‘Implications ons of the collapse of the de la Concorde overpass’. The overpass’.  The Structural Engineer , 86(1), 8 January 2008, pp16-18 9.14   Rail Accident Investigation Branch. Branch. Derailment  Derailment of a  freight train near Stewarton, Ayrshire, 27 January  2009. Rail Accident Investigation Branch Report  02/2010 . Available at: http://www.raib.gov.uk/cms_ resources.cfm?file=/100203_R022010_Stewarton.pdf [Accessed: 20 February 2013] 9.15   Wood, J.G.M. J.G.M. Pipers  Pipers Row Car Park, Wolverhampton:  quantitative study of the causes of the partial collapse  on 20th March 1997 . Chiddingfold: Structural Studies & Design Ltd, 2002. Available at: http://www.hse.gov. uk/research/misc/pipersrow.htm persrow.htm [Accessed: 20 February 2013] 9.16   Standing Committee on Structural Safety. Safety. Confidential   Confidential  reporting on structural safety for Scottish buildings . Livingston: Scottish Building Standards Agency, 2008.  Available at: http://cms.structural-s http://cms.structural-safety.org/as afety.org/assets/  sets/  uploaded/documents/169_SCOTCROSS%20report.pdf [Accessed: 6 March 2013] 9.17   Fitzgerald, T.G T.G.. ‘Failure and rectification rectification of fixings xings of large cladding units’, Structural  units’,  Structural  faults pre-cast & repair concrete ‘87: Proceedings, International  conference on structural faults and repair held 7-9 July  1987, University of London. Vol London.  Vol 2. Edinburgh: Engineering Technics Press. pp383-386 9.18   Royles, R. ‘Repair of large cladding cladding panels on multistorey structures’. Structural structures’.  Structural faults & repair ‘87. Proceedings, International conference on structural  faults and repair, University of London, 7-9 July 1987. Vol 2. Edinburgh: Engineering Technics Press. pp387-400 9.19   Colford, B.R. B.R. and Clark, C.A. ‘Forth Road Road Bridge main cables: replacement/augmentation study’. ICE  study’.  ICE  Proceedings, Bridge Engineering , 163(BE2), June 2010, pp79-89 9.20   Young, J. et al. al. ‘Assessment ‘Assessment of the suspension cables of the Severn Bridge, UK’. Creating UK’.  Creating and renewing  urban structures: tall buildings, bridges and  infrastructure: 17th Congress Report of IABSE, Chicago, USA, 2008 . Zurich: IABSE, 2008 9.21   Cocksedge, C.P C.P.E. .E. and Bulmer, Bulmer, M.J. ‘Extending the life of the main cables of two major UK suspension suspension bridges through dehumidification’. Bridge dehumidification’.  Bridge Structures:   Assessment, Design and Construction , 5(4), 2009, pp159-172

9.10   BS EN 1991-1-7:2006: Eurocode 1: Actions ons on  on  structures – Part 1-7: General actions – Accidental  actions . London: BSI, 2010 [incorporating corrigendum February 2010]

9.22   Fennell, D. D. Investigation  Investigation into the King’s Cross  underground fire . London: HMSO, 1988. Available at: http://www.railwaysarchive.co.uk/documents/DoT_ KX1987.pdf [Accessed: 20 February 2013]

9.11   Neale B.S. ‘The Consequences of poor serviceability – and the way forward: keynote address’. In Byars, E.A.

9.23   Health and Safety Executive, Executive, Collapse  Collapse of a three-storey  building: a report on the accident at Woodthorpe Road,

and McNulty, T. eds. Management eds.  Management of concrete  structures for long-term serviceability . London: Thomas Telford, 1997, pp1-8 9.12   Commission of Inquiry into nto the collapse of a portion of the de la Concorde Overpass, October 3, 48

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 Ashford, Middlesex that occurred on 1 August 1995 . Sudbury: HSE Books, 1999 9.24   Neale B.S., ‘Forensic ‘Forensic engineering in safety safety enforcement – some UK experiences’. In Srivastava, N.K. ed. Structural ed.  Structural engineering worldwide 1998: 

 

Risk management during the life of a structure proceedings of the structural engineers world  congress, San Francisco, 1998 . Amsterdam: Elsevier, 1998, paper T202-2 9.25   Neale B.S., ‘Mitigation gation of failures due to inappropriate loading during construction – a European code’. In Neale, B.S. ed. Forensic ed.  Forensic Engineering – the  investigation of failures: proceedings of the 2nd  international conference, London, 12-13 November  2001.. London: Thomas Telford, 2001, pp83-91 2001 9.26   Ministry of Housing Housing and Local Government. Government. Report  Report of 

9.40   BS EN 1991-1-4:2005+ 1991-1-4:2005+A1:2010: A1:2010: Eurocode 1: Actions  Actions  on structures – Part 1-4: General actions – wind  actions . London: BSI, 2011. [Incorporating corrigenda July 2009 and January 2010] 9.41   Neale B.S. ‘Assessment ‘Assessment of structural safety safety risks’. risks’. Safety criteria for buildings and bridges: conference, Kensington, 1 July 1999. London: 1999.  London: ICE, 1999, pp49-62

9.9 9. 9

Bibl Bi blio iogr grap aphy hy

the Inquiry into the Collapse of Flats at Ronan Point, Canning Town . London: HMSO. 1968 9.27   Fawcett, W. and Palmer Palmer,, J. Good J. Good practice guidance on  refurbishing occupied buildings. C621. C621. London: CIRIA, 2004 9.28   Neale, B.S.,’T B.S.,’Teaching eaching for for enduring structural integrity’. ntegrity’. In Allen, H.G. ed. Civil ed.  Civil and structural engineering  education in the 21st century: proceedings of a  conference, 26-28 April 2000, University of  Southampton . Vol 2. Southampton: University of Southampton, 2000, pp485-496

 Actuarial Profession and Institution of Civil Engineers. Engineers. Strategic   Strategic  risk – a guide for directors . London: Thomas Telford, 2006 ISO. TC 98/SC 2: Reliability of structures website. Available at: http://www.iso.org/iso/standards_d http://www.iso .org/iso/standards_development/technical_ evelopment/technical_ committees/list_of_iso_technical_committees so_technical_committees [Accessed: 18 February 2013]

9.29   Vivian, Vivian, S. et al. al. Climate  Climate change risks in building – an  introduction. C638 . London: CIRIA, 2005 9.30   National Trans Transportation portation Safety Board. Board. Collapse  Collapse of  I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007. Highway Accident Report  NTSB/HAR-08/03 . Washington, DC: NTSB, 2008.  Available at: http://www.dot.s http://www.dot.state.mn.us/i35wbridge tate.mn.us/i35wbridge/ /  ntsb/finalreport.pdf [Accessed: 20 February 2013] 9.31   Railway Accident Investigation Unit. Unit. Malahide  Malahide Viaduct  Collapse on the Dublin to Belfast Line, on the  21st August 2009. Investigation report no. R2010  004. Blackrock: 004.  Blackrock: RAIU, 2010. Available at: http://www. raiu.ie/download/pdf/accident_malahide.pdf raiu.ie/download/ pdf/accident_malahide.pdf [Accessed: 20 February 2013] 9.32   Iddon, J. and Carpenter Carpenter,, J. J. Safe  Safe access for  maintenance and repair: guidance for designers. C686 . 2nd ed. London: CIRIA, 2009 9.33   Institution of Structural Engineers. Engineers. The  The Operation and  maintenance of bridge access gantries and runways . 2nd ed. London: IStructE, 2008 9.34   ‘Gantry fall deaths ‘‘unlawful’’’. unlawful’’’. BBC News website. website. 21 July 2003. Available at: http://news.bbc.co.uk/1/hi/  england/3084859.stm [Accessed: 20 February 2013] 9.35   Institution of Structural Engineers. Engineers. Appraisal  Appraisal of existing  structures . 3rd ed. London: IStructE, 2010 9.36   BS 8110: 8110: Structural use of concrete  [in   [in 3 parts] 9.37   BS EN 1992: 1992: Eurocode Eurocode 2: 2: Design Design of concret concrete  e  structures  [in   [in 4 parts] 9.38   CP3: Chapter Chapter V: Part Part 2: 1972: 1972: Code of of basic data data for  the design of buildings: Chapter V: Loading – Part 2:  Wind loads. London: loads.  London: BSI, 1972 [incorporating amendments issued January, March and June 1986, September 1988 and September 1993] 9.39   BS 6399-2:1997: 6399-2:1997: Loading Loading for buildings: part part 2: code  of practice for wind loads  London:   London: BSI, 2002. [incorporating amendment no.1 and corrigendum no.1] The Institution of Structural Engineers  Risk in structural engineering

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9.9

 

10 Ris Risk k manageme management nt for demolit demolition ion and refurbis refurbishme hment nt

10.1 10. 1 Int Introd roduct uction ion  This chapter discusses risk management during and in preparation for demolition10.1, partial demolition or structural refurbishment, giving guidance on the need to understand the existing structure, managing (deliberate) structural instability and reducing uncertainty10.2, 10.3. Designing for future demolition is covered in Section 7.9.

examples of situations where hidden water penetration has caused widespread corrosion. The potential effects of any degradation on strength and on the scale of refurbishment refurbishment costs can be formidable; it is a risk that has to be recognised and assigned. Risks as a result of working on, in or near to existing operational facilities are such that the risks of  unplanned events leading to personal injury incidents can be higher than those on new build activities. Risks to structure, people and surroundings should be considered10.6.

10.2 The poten potential tial for for unplanne unplanned d events events Structures are often supported by a complex interaction of structural mechanisms, not all of which were intended by the designers. Arching, catenary action and elements that were intended to be nonloadbearing can provide load paths that were not planned. Before any attempt is made to change or demolish a structure, it is important to understand what it up. This requires anisunderstanding understand what keeps is physically present (which often hard ing to of  establish) plus a knowledge of how the components work together and what the load paths are to provide overall stability stability.. Structures designed some time ago may have been designed on principles that are unfamiliar to today’s designers. Before the days of computers, the connection moments in steel frames were often established using empirical rules. Steel being a ductile material, some redistribution occurred under dead load and the initial live load, but for subsequent reapplication of the live load the structure behaved elastically. Although a fully elastic analysis might show an overstress, these structures may be quite satisfactory and need no strengthening during refurbishment. The key is understanding the structure.

10.3 10. 3 The need need to to know know the exis existing ting structure Risk management strategies for work on existing structures need to be as effective as and have a wider scope than those for new build projects. The simple reason for this is that a thorough knowledge of  the structure structure to be worked upon, including the history of use, is essential to minimise risk. This knowledge should be fed into assessments that lead to a comprehensive understanding of structural behaviour, both locally and throughout the structure, under any particular circumstances involving work that affects the structure.  Assessments should take account of known faults in the structure, indications of potential faults and also modifications undertaken during its life time. The aim is to avoid implementing inappropriate work on a structure that will affect structural integrity in adverse ways and that may thus lead, for example, to premature collapse or flying debris. Since the existing records describing the structure

Structural refurbishment and demolition can be  journeys into the unknown, although they should not be. Older buildings often lack any drawings, let alone ‘as built’ records. Many structures have had so many alterations and there is so much stability interdependence within them that it is far from clear what holds what up, so there is the ever present danger of removing a vital component. The classic failure of the West Gate Bridge in Melbourne 10.4 (1970) happened during construction, not refurbishment, but it occurred because the erectors removed certain bolts to ease construction, and, in so doing, precipitated full scale collapse. This is not unknown in demolition and refurbishment.

may not be completely accurate, any planned refurbishment strategy should proceed cautiously.  The structure should be probed and uncovered sequentially and the project budgeting should allow for the possibility of unearthing the unexpected. It is not unknown to uncove uncoverr asbes asbestos, tos, which until the 1970s was used routinely without any awareness of  its health hazards. In the UK there is now a statutory duty to manage existing asbestos and keep a register of any that is present. A lack of  registered asbestos in a structure does not mean that none will be found; although it might change the legal liability, it will not protect the health of  anyone who finds some.

In 2010 CROSS reported on the demolition of two 13-storey large panel concrete tower blocks 10.5. Even though the demolition method was revised following

10.4 Struc Structura turall refur refurbishm bishment ent

each event, not less than four unexpected collapses occurred. occur red. A major contributi contribution on to this was the lack of  adequate ties between the concrete panels.

10.4.1 10. 4.1 Ove Overvi rview ew

Experienced engineers working on refurbishment are cautious cauti ous about what might be found found.. Ther There e are many

Refurbishment of existing structures may require consideration of many of the same aspects as

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Risk management for demolition and refurbishment

design, but with the additional consideration that the structure struc ture will have aged, some details may be unknown, and society’s expectations for a safe structure will have changed. Health issues also need to be considered as well as environmental imperatives. 10.4.2 10.4 .2

Structural Struc tural stab stabilit ilityy

In many cases cases,, any significant significant level of  refurbishment will mean that the structure has to comply with the regulations for new construction. In the UK, this will mean, in particular, the requirements against disproportionate collapse. Non-structural requirements, such as insulation against sound and thermal loss, may also have an influence on the refurbishment process and indirectly on the structure. 10.4.3 10.4 .3

someone in the role of a temporary stability someone ty coordinator.  A flow chart showing considerations for structural stability ity is includ included ed in the British Standard code of  practice for demolition (BS 6187:2011)10.9.

Fire prec precautio autions ns

 Additional measures m ay be required for fire compartmentation, fire stops and means of escape. During refurbishment works, temporary fire protection measures may be required. 10.4.4 10.4 .4

In all cases where removal is planned and deliberate instability is to be introduced to remove material, an assessment for residual stability should be made. In other words, this should be considered a design process. These assessments, and the associated work, should always be undertaken by suitably competent people. Every structure is different; even those that may have been designed to be identical will have been built differently and will have had different histories. Co-ordination of temporary stability is recommended for each occurrence of structural work on a building, with the appointment made of 

Underpinn Unde rpinning ing work workss

Buildings are often extended by means of new basements, or require new foundations for extension works. These may require underpinning of the existing foundations; many accidents have occurred during underpinning. This requires the designer to have a good understanding of the ground and the structure and in particular, requires discipline and competence from those carrying out the work on site.

10.5 Managing Managing (delibe (deliberate rate)) structur structural al instability  An approach should be adopted such that the structural integrity, or safety, is assessed for all stages of removal of any part of a structure. This has become more important for modern structures that are highly efficient in the sense that every component plays a part in stability. Thus, erection of long steel rafters can be difficult since their design requires support of closely spaced lateral purlins, back to a braced system. In reverse, removal of that bracing (or careless removal of the purlins) could initiate an unstable collapse. The trend for modular building systems presents other problems since sometimes all members add to the structural stability. Understanding how a building stands up is a prerequisite to understanding how to take it down. Good examples of the skills required in demolition can be seen in the problems of taking down post tensioned structures as, for example, Marks & Spencer10.7 or Bernard House10.8 in Manchester.

10.6 Reduc Reducing ing uncer uncertaint taintyy Demolition, including partial demolition, is a particular case of the maxim that the effort put into managing a risk should be proportionate to the consequences. Demolition can involve significant uncertainty and a conservative approach should always be taken. Sometimes, it may be most effective to manage the risk by minimising the is consequences; for example, ensuring that nobody in a position where they by could be injured if anything unpredictable happens.  A number of considerations specifically need to be taken into account. These include: – Knowledge of the site. – Decommissioning procedures, even for small projects of any type. – Structural hazards. – Health hazards. hazards. – Protection of the environment, including managing arisings and wastes. – Health and safety of persons on or off site. – Effects of dust (health, contamination, dust explosions). – Safe working spaces and exclusion zones – even for small jobs. – Principles of structural removal or demolition – for the structure concerned. – Avoidance of unplanned collapses. – Temporary, or auxiliary, structures for stability and access. – Demolition techniques – consider possible alternatives. – Materials handling and processing. – Completion of the works – objectives, achievement and review. Interaction of these issues is considered in BS 6187:201110.9.

10.7 10. 7 Gu Guid idan ance ce

It is a useful principle to assume there may be structural instability everywhere whenever the structure struc ture is worke worked d upon. This may includ include e work that may not (normally) be thought of as demolition activities, or perhaps, even conform to all definitions of demolition, demolition, such as remov removal al of mater material ial that is not expected to affect the structural stability in any way.

 A number of guides are available, as listed in the bibliography. Some of these give core advice, while others depend on the particular circumstances that may apply; such as type, height or location of the facility and the extent of  structure being removed. The Institution of Structural Engineers  Risk in structural engineering

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10.5

 

10.8

Risk management for demolition and refurbishment

10.8 Concl Conclusion usionss and reco recommen mmendation dationss

Bussell, M. et al.  Retention of masonry fac¸ ades – best practice  site handbook. C589 . London: CIRIA, 2003

Demolition, whether complete or partial, involves the intentional destabilising of a structure. Unlike construction, the details of the structure and its condition may not be fully known and it is important to ‘expect the unexpected’. Demolition therefore requires structural engineers to have additional competence to deal with the risks involved. To help for the future, however, designers should ensure that for their designs the structural principles and loading

Clarke, R. ‘Role of the structural engineer in demolition’.  The  Structural Engineer , 88(11), 2 June 2010, pp28-33

options and criteria are available to their clients. This should shoul d be with the recommendation recommendation that they are kept, made available to those planning future works and that they are passed onto successive owners and operators, operators, ensuring that they are updated to take account of subsequent works.

10.9 Ref Refer erenc ences es 10.1   Addis, W. and Schouten, J.  Principles of design for  deconstruction to facilitate reuse and recycling, C607 . London: CIRIA, 2004 10.2   Marino Duffy B.M. B.M. et al. ‘Comparative ‘Comparative study of UK and Italian refurbishment sites involving demolition activities and structuralstrategies’. instability:  Proceedings risk factors and health of the 2nd & safety management International structural engineering and construction  conference (ISEC 02), Rome, 23-26 September 2003  10.3   Briggs, Briggs, M. et al. Decommissioning, mothballing and  revamping . Rugby: IChemE, 1997

Report of the Royal Royal Commissi Commission on into the the failure failure of the  10.4   Report West Gate Bridge . Melbourne, Victoria: Government Printer, 1971 10.5   ‘Collapse of large large panel structure buildings during demolition’. CROSS Newsletter , 18, April 2010.  Available at: http://www.struct http://www.structural-safety ural-safety.org/view.org/viewreport/cross273 [Accessed: 20 February 2013] 10.6   Egbu, C. C. et al. ‘Mana ‘Managing ging health health & safety safety in in refurbishment projects involving demolition and structural instability’,  Facilities management and asset  maintenance: applying and extending the global  knowledge base: CIB W70 – Proceedings, Glasgow, 18-20 September 2002. CIB Publication 277  10.7   Roberts, J.M. J.M. ‘Demolition ‘Demolition of Marks & Spencer Spencer,, Manchester (a six-storey commercial building supported by post-tensioned beams)’.  The Structural  Engineer , 77(2), 19 January 1999, pp20-25 10.8   Sellors, G., ‘Demolition ‘Demolition of Bernard House, Piccadilly Piccadilly Plaza, Manchester – January-July 2001’.  The  Structural Engineer , 82(2), 20 January 2004, pp30-34 10.9   BS 6187:2011: 6187:2011: Code of practice for full and partial  demolition . London: BSI, 2011

10.10 Bibliography Bussell, M. et al.  Retention of masonry fac¸ ades – best practice  guide. C579 . London: CIRIA, 2003 52

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Neale, B. ‘Demolition, partial demolition, structural refurbishment and decommissioning.’ In McAleenan, C. and Oloke, D. eds.  ICE  manual of health and safety in construction . London: Thomas Telford, 2010, pp215-232

 

 Appendix A

 A.1

Case studies

Introduction

Many of the concepts discussed in previous chapters may seem abstract and so might be better understood by studyi understood studying ng what has gone wrong in practice. To illustrate this, and to put ideas into context, this chapter recounts some of the more infamous failures and highlights lessons that might be learned. The chapter is arranged according to the concepts illustrated in this Report  this  Report . A key lesson is that there have been, and continue to be, many failures, many of them of high profile structures. With hindsight many of the causes were predictable, so why did they occur? The answer, it seems, is that it is painfully easy to overlook the obvious. The risks certainly exist and part of risk reduction is to look for them. Learning from the past is a key activity, with the purpose purpo se of minimi minimising sing exposure exposure to risk in the future. It is worth reiterating that one of the purposes of  studying ‘near misses’ – sometimes referred to as ‘near hits’ – is that,and statistically, arethat rarely isolated incidents they are afailures warning action might be required. The study of actual failures and incidents demonstrates that many risks which might be dismissed as ‘theoretical’ are actually real and deserve consideration.  The case studies are not all about structural engineering. They are about the way humans have managed manag ed or fail failed ed to manag manage e risk, which is commo common n to all industries. They illustrate the importance of  understanding root causes, identifying the most serious risks and learning from experience. While they are listed by principal cause, such events are rarely due to just one cause and more usually disasters result from the combination of technical and procedural errors.  The information in this chapter is believed to be correct, although some interesting and well-known failures have had to be omitted since insufficient information could be found to describe the events without speculation. Some recent events have been omitted or given limited discussion as liability is still sub judice. judice.  The discussion of each failure is provided solely to illustrate the principles of risk management and is not intended intend ed to repr represent esent the opinion of the authors or the Institution in regard to the allocation of  responsibility or liability.

 A.2

Uncertainty in loading  A.1, A.2

 The first Tay Bridge (1879) collapsed under wind loadin loa ding. g. Whi While le muc much h has bee been n lea learned rned sin since ce the then, n, oft often en by bitter experience, failures due to wind loading or wind-induced movement continue to occur, e.g., the dramatic failures of the Ferrybridge cooling towers A.3, A.4 (1965) or by more recent storm damage,

particularly in hurricanes. Severe damage can be caused by localised wind-induced failure of parts of  buildings and there have been failures of temporary structures where the judgement of short-term wind-loading has proved inaccurate.  The predicted live loading in structures is fundamentally uncertain and can change with time, as a result of both better understanding and changing conditions. Design code requirements for wind and snow loading have changed significantly as further research has been undertaken. Furthermore, climate change is predicted to lead to further changes. Axle loads on bridges change with increases in the legal limits for lorries and total bridge loading increases with traffic density density.. While not a failu failure re in the sense of  structural collapse, and not something that could have been anticipated at the time of design, it would have been better if the strengthening of the Severn Bridge A.5 to withstand increased traffic loads could have been avoided. An interesting question is: ‘To what extent should designers reduce the risk that modifications will be required, by allowing a margin in the original design?’  The live loading defined in standards, such as 4kN/m2 for office floor loading, only appears precise because it is defined as such. The real loads are highly variable and, in practice, the design makes the assumption that probably they will lie below the value taken for design.

 A.3

Extension of technology to an invalid extent

 The Tacoma Narrows Bridge failure A.6, A.7 (1940) occurred primarily because the technology used successfully on previous suspension bridges became invalid on longer spans where aerodynamic effects were disproportionately more important. This highlighted how much there was to learn about wind aerodynamics and yet since that time several structures have suffered from oscillation in wind. In 2006, the roof of the ice-arena in Bad Reichenhall, Germany, collapsed under snow load. 15 people died and 30 were seriously injured. The roof was supported by special 48m long timber box girders.  These were 2.87m deep, although the technical approval for this type of girder limited the depth to 1.2m. A paper A.8 prepared for the public prosecutor also identifies a number of other reasons that contributed to the collapse. These were use of  urea-formaldehyde glue under moist conditions, errors in structural calculations (and failure to check them), a lack of maintenance and vulnerability to progressive collapse.  The long span box girder bridge was introduced over a relatively short period around 1970, with little opportunity for experience to be fed back into design.  The failures during construction at Milford Haven A.9 The Institution of Structural Engineers   Risk in structural engineering

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 A.4

Case studies

(1970) and Koblenz A.10, A.11 (1972) were caused at leastt partl leas partlyy by a failu failure re to under understand stand the load loading ing conditions during construction and the strength of  boxes A.12. A further issue issue was the relations relationship hip between between diaphragm buckling capacity and plate alignment with respect to welding distortion imperfections. This observation led to the Merrison Inquiry A.13 (1973) and rules. Apart from the risk that the structure fails to work  as int intend ended, ed, the fai failur lure e to ant antici icipat pate e tol tolera erance nce dem demand ands s (especially over material interfaces) is one of the commonest causes of contractual disputes with associated delays.  These cases illustrate the difficulties of developing design into new areas. Structural engineering should develop if it is to serve society, but caution is required with structures which are essentially prototypes.

 A.4

Fatigue loading

 An inability to recognise less common loading conditions, particularly fatigue, has been the cause of  many failures. The jack-up barge Sea Gem   A.14 (1965), which found the first North Sea gas in UK  waters, collapsed and sank in the North Sea killing four men. The Public Inquiry concluded that metal fatigue in the th e sy syst stem em lilink nkin ing g th the e hu hullll to th the e le legs gs wa was s to bl blam ame. e. Th The e  A.15

failure the Alexander Keilland fracture. rig (1980) was due toof fatigue and subsequent One of thealso worst series of failures ever, the Comet aircraft A.16, was put down to metal fatigue. ‘B of the Bang’ (2005) (see Figure A.1) was a dramatic 56m tall sculpture, commissioned by Manchester Manch ester City Council to mark the 2002 Commonwealth Games. It consisted of 180 hollow spikes, made from weathering steel, radiating from the top of an inclined support. Wind induced vibration resulted in the tips of several spikes becoming detached. Attempts to modify the dynamic response did not prevent further failures and the sculpture was dismantled. The sculptor and the design and construction subcontractors reached an out-of-court settlement for £1.7 million with the city council A.17.

 A.5

Uncertainty in extreme loading

Most of what is known about earthquake earthquake loading has been derived derived from failure failure studies. As a resu result, lt, the history of seismic code development is one of  gradual increase in the lateral forces that buildings are required to withstand, coupled with detailing rules to avoid brittle failures observed in previous events. Nevertheless, overall seismic forces remain fundamentally uncertain and, generally, designs are configured to address a series of seismic responses based on different earthquake occurrence probabilities. Structures are configured to be ductile, the risk mitigation strategy being that the structures might deform more than expected but at least they won’t collapse. It is not usually economic to design structures to resist earthquake forces and displacement without damage, unless the function of  the structure structure requires this as, for example, example, in the case of nuclear reactors and hospitals in seismic zones.  Although not considered in the design of normal structures in the UK, both the Folkestone earthquake A.18 (2007) and the Birmingham tornado A.19 (2005) resulted in structural damage.

 A.6

Failure to understand materials

 The limitations of construction materials are not always apparent when they are first used. There have been a serie series s of scares on the long-term durability durability of  concrete with degradation by sulphates, chlorides, alkali-silica reaction, thaumasite, mundic, etc. The collapse of roof beams in the Sir John Cass School A.20 (1973) highlighted the danger of high alumina cement conversion and, in the ensuing nationwide investigation, many more cases of  excessively weak structures were discovered. Similarly, the failure of liberty ships in the Second World War A.21 showed the need for the engineering profession to understand brittle fracture in steel. During construction of an elevated ramp to the Riley Road Interchange A.22, A.23 near Chicago (1982), falsework to support cast-in-place concrete was supported on precast plain concrete slabs about 1m 2 and 300mm thick. Of the four defects found to have contri con tribut buted ed to the col collap lapse, se, the pri princi ncipal pal cause cause was the fracture of the unreinforced blocks during placement of  deck concrete. The resulting sudden change in distribution of forces caused the falsework to move out of line and collapse, killing 13 workers. This was the worst construction accident in the history of Indiana.  The Malpasset Dam disaster A.24, A.25 (1959), which killed more than 400 people, was was at least partly caused caused by an inade inadequate quate site investigation investigation that had faile failed d to pick up certain key geological features. The Carsington Dam failure A.26, A.27 (1984) was partly due to the failure to foresee pre-existing shear surfaces.

 A.7

Bang’ sculptur sculpturee Figure A.1   ‘B of the Bang’ 54

  The Institution Institution of Structural Structural Engineers Engineers   Risk in structural engineering

Failure to identify the hazard

 The failure to identify gas explosion as a hazard in the Ronan Point flats A.28 (1968) might be considered an obvious omission, and in that case was compounded

 

Case studies

by ina inadeq dequat uate e tie ties s bet betwee ween n com compon ponent ents. s. How Howeve everr, the explosion at the Abbeystead pumping station A.29 (1984), which killed 16 visitors and injured many more, was less predictable. The explosion was caused by the ignition of a mixture of methane and air that had accumulated in the valve house after the methane had seeped out of the tunnel walls. The designer, constr con struct uctor or and ope operat rator or wer were e all ini initia tially lly fou found nd lia liable ble for damages. In a complex appeal, only the designer was found liable. Two of the three law lords considered that the designer should have foreseen that methane could accumulate accum ulate in a void, while the third considere considered d this was not reasonably foreseeable A.30.  There have h ave been many incidents due to bridge impacts. In the UK, between 1994 and 2001, there were an average of 1500 incidents per year of road vehicles striking railway bridges A.31. Bridge collapses also occur following scour by river currents, for example, the Malahide Viaduct in Ireland A.32, A.33, A.34. Some of the worst cases of bridge failures have occurred following impact damage to the piers by shipping. A vessel collision with an Amtrak bridge in  Alabama A.35 (1993) cost 47 lives and millions of dollars.  An inciden incidentt with a large ship took out a complet complete e section of the Tasman Bridge in Hobart, causing immense disruption A.36, A.37 (1975). It is now normal practice to provide barriers to prevent ships getting too close to bridge piers, but even then the protection can only be designed forthe theClyde largest vessel. The Erskine Bridge over inanticipated Glasgow was severely damaged A.38 (1 (199 996) 6) by the sim simpl ple e er erro rorr of se sendi nding ng an oi oill rig ri g und under erne neath ath who whose se mas mastt wa was s to too o tall fo forr the cl clea eara rance nce at tha thatt tim time e in the tida tidall cyc cycle le.. The da damag mage e cost cost £3 £3.7 .7m m to repa re pair ir wit with h a fur furthe therr £0 £0.7min .7min los lostt re reve venu nue e dueto clo closu sure re..

 A.8

Errors in dynamics

 The Tacoma Narrows failure (see Section A.3) is a clear example. The dramatic failure of the 365m high Emley Moor TV mast A.39 (1969) was another. The mast consisted of a tubular steel section up to 275m with a lattice section above. Failure occurred in strong winds with large quantities of ice formed on the tower and its guys. The cause was held to be dynamic oscillation, rather than the direct effects of the weight of ice, and at that time the phenomenon was not widely understood.  This was an inadequate defence for the designers in subseq sub sequen uentt lit litiga igatio tion n on lia liabil bility ity,, not res resolv olved ed until 198 1983, 3, with a settlement of £3.2 million. More problems occurred on the Millennium Footbridge in London A.40, A.41 (2000), which swayed excessively under crowd loading and it was necessary to backfit the bridge with dampers. The problems with the dynamic response of football stands under crowd loading are equally important. There has been a long history of stand failures such as that at the Pink Floyd concert at Earl’s Court, London A.42 (1994), and the worst cases have been in temporary stands where some have collapsed with loss of life.

 A.9

Errors in stability

 There are examples of error in stability in both buildings and in the ground. The collapse of part of 

Rock Ferry School A.43 (1976) was a lesson in that, while individual parts were designed correctly, the structure struc ture as a whole was unstable and the struc structure ture certainly lacked ‘robustness’. More tragically, the  Aberfan coal waste slip A.44, A.45 (1966) killed 144 people (116 of them children) and was essentially a stability failure resulting from poor design and maintenance; once movement occurred in the saturated coal waste a huge volume surged down the slope. Likewise, the failure of Carsington Dam A.26, A.27 (1984) was a ground instability failure exacerbated by uncertainties in ground conditions. Incidents of crane overloading continue to occur, either as a result of instability of the crane as a whole or instability of some part such as the jib. Most of  these relate to misuse of the crane, such as overloading or incorrect erection, or failure to ensure adequate support to outriggers.

 A.10 Errors in design or detailing One of the causes of the Ramsgate link-bridge collapse A.46, A.47, A.48, (1994) was a design/detailing error made by the designer and missed by the checking organisation. This might not have been fatal had the bridge been provided with some form of  redundancy; as it was, failure of one bearing led inevitably to complete collapse. The designer, checker and cli client ent wer were e all con convic victed ted sub subseq sequen uently tly.. Mor More e det detail ails s of this case are given in  The Structural Engineer  A.49. Cases have been reported of designs where reinforcement meshes have been simply transposed, or inadequately placed on site, leading to collapse or necessitating demolition. A car park in Birmingham, UK required external reinforcement to the slab soffits when it was discovered, some time after completion, that the tension reinforcement required for the short span had been detailed for the long span and vice versa.

 A.11 Deterioration and lack of  maintenance  A gantry girder bridge over the railway collapsed at Clapham A.50 (1965) due to the build-up of rust in overlapping plates bursting the riveted seams apart.  The car park failure at Pipers Row Car Park  A.51 (1997) was probably exacerbated by top-mat reinforcement corrosion. A waitress was killed in Edinburgh A.52, (2000) by falling masonry and this highlighted a more widespread problem leading to a report A.53 on falling masonr mas onryy. As a re resul sultt CRO CROSS SS was com commis missio sioned ned by the Scottish sh Building ng Standard Standards s Agenc Agencyy to gathe gatherr data and report findings A.54, A.55.  A number of major bridges in the UK have had to undergo substantial repairs that have caused significant, but probably unattributed, economic disruption. Two examples are the Kingston Bridge in Glasgow A.56 and the Thelwall Viaduct on the M6 motorway A.57 which were partially closed for several years while repairs and improvements were made.  These were necessitated partly by a failure to anticipate an increase in traffic levels, resulting in the need to increase the load capacity, but mainly as a result of  The Institution of Structural Engineers   Risk in structural engineering

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A.8

 

 A.12

Case studies

defects. In the case of the Thelwall Viaduct in Cheshire, the bea bearin rings gs fai failed led onl onlyy a few yea years rs aft after er bei being ng re repla placed ced as part of a complete re-build of the bridge deck. This failure was compounded by the lack of provision for replacement, meaning that the deck had to be strengthened before it could be jacked up to access the failed bearings.

killed and eight injured. The building used flat slab construction and progressive failure occurred during demolition. Punching shear failure could be seen in the slab. The main cause of failure was excessive debris and other loads on slabs.

 A.14 Human factors  A.12 Identifying significant risks  Ten people were killed when a Land Rover towing a  Ten trailer went off the M62 motorway at Great Heck near Selby, UK  A.58 (2001). The vehicle plunged on to the railway line into the path of a passenger train, which derailed and impacted with a goods train travelling in the opposite direction. Some people at the time described it as ‘a chance in a million’. Considering, however, the number of bridges there are over railways and the statistics regarding the number of vehicles which run off the road, the probability of a vehicle going off the road at a railway bridge at some time is not low.  At this particular site, once that had happened, the probability of ending up on the line was high.  Thereafter,, given rail traffic density, once the vehicle  Thereafter was on the track the probability of being hit by a train was also high. So the total probability was not small and the potential consequence was significant. On the other hand, a tornad tornado o seriously seriously damag damaged ed several houses in London A.59 (2006); yet engineers do not customarily ly design for tornad tornado o damage in the UK  because the probability of any individual property being damaged is small and the cost of protecting all houses would not be justified. For such limited risks, insurance can limit the financial risk to each householder to the average for the nation.  The fire at the Bradford Football Club stand A.60, A.61 (1985) spread rapidly causing loss of life, partly because becau se the crow crowd d could not escape onto the pitch.  The fire highlighted the balance of risk assessment needed; on the one hand in keeping crowds from spilling onto the pitch to preserve public order versus the hazard of preventing them from being able to escape in times of dire necessity.

 A.13 Demolition  A building in Bootle, Merseyside, collapsed during demolition A.62, A.63 (2000). The site was a terrace of  nine three-storey Victorian properties, with basement cellars. The building itself was somewhere between 90 and 100 years of age. One worker was buried under rubble and died. In the opinion of the investigators, the significant causes of the collapse were alterations to the structural structural form of the orig original inal building building and the weakened state of the structure. Both were foreseeable and could have been detected by a structural struc tural survey if there had been one. The actual demolition activity itself left the party wall with little or no lateral support. There were also deficiencies in

 The Moorgate underground train crash A.65, A.66 (1975) killed 43 people and injured 70 more. The cause was never established, although it was clear that the driver had made no attempt to slow the train. It illustrated graphically the need to consider human factors in design. Subsequently, engineered systems were put in place at ‘dead-end’ tunnels which would stop trains automatically. Other train crashes have been linked to drivers missing red signals, e.g. Ladbroke Grove A.67, A.68 (1999).  The significance of this case is that it may not be adequate to rely on one person taking the right action when the consequence of failure is high or there is a risk of malicious action, e.g. terrorism. If the hazard cannot be engineered out, the management arrangements may need to include supervision.

 A.15 Design change  Two aerial walkways collapsed at the Kansas City  Two Hyatt Regency A.69 (1981), killing 110 people and injuring injur ing more than 200. The immediate cause of the collapse was a poorly fabricated connection between the walkway’s supporting steel rod and the walkway beams (see Figure A.2). The fabricator had changed the design to simplify assembly, inadvertently doubling the load on the conne connector ctor.. In addition, addition, the cross beams where the hanger bars anchored should have been made of two rolled steel channels (RSCs) back to back  (with the hanger rod passing between them). However, for aes aesthe thetic tic rea reason sons, s, two RSCs wer were e wel welded ded to for form ma box section, introducing large local bending stresses in the flanges of the RSCs. Ultimate load tests of a similar assembly assem bly showed that the detail was only strong enough to take 10% of the design load. Responsibility was disputed because of design changes and uncertain communications.  The collapse of a concrete canopy during alteration works occurred at Albert House, Aberdeen, Hong Kong A.70 (1994) resulting in one death and several injuries. Evidence pointed to lack of maintenance, plus unauthorised works resulting in extra loadings on the canopy.

 A.16 Temporary works and construction failures  There have been many temporary works and

information given to the demolition contractor and the planning of the site work (for which insufficient time had been allowed).

construction failures. The Barton High Level Bridge collapses A.71, A.72 (1959) were failures of temporary works and stability.

 The collapse of an industrial building at Ya Yau u Tong, Hong Kong A.64 (2001) resulted in six people being

 The 68m high falsework collapse at Almun˜ e´ ´ car car in Spain A.73 (2005) is an example of a temporary works

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failure with serious consequences. According to a report from the judicial inquiry (April 2007), the collapse was caused by a defective welding seam fracturing due to excessive stress caused by a defective bolt. For such a small defect to be so critical, however, suggests that robustness may have been lacking as well.

 A.17 Inadequate procedures Disasters as diverse as the Kings Cross Underground Fire A.74 (1987), Piper Alpha A.75, A.76 (1988) and Hillsborough A.77 (1989) all highlighted the need to have adequate management procedures for safety, as in each case loss of life was made worse by failures to plan in advance a response in the event of a disaster.  The football stadium incident at Hillsborough led to a renewed edition of  Guide  Guide to safety at sports  grounds A.78. This was first published in 1975 and now collec col lectiv tively ely tak takes es acc accoun ountt of a numb number er of spo sports rts gro ground und incidents.

 A.18 Systems failures Civil engineering structures are commonly required to interact with other engineered systems. They are frequently freq uently called called upon to enhanc enhance e safety by actin acting g as engineered barriers to assist in providing plant safety.  The apparent failure of the control system at Buncefield A.79 (2005) led to the biggest deflagration and fire in Western Europe since the Second World War (see Figure A.3). Good examples of the chain of  events that may initiate a disaster can be taken from other engineering disciplines. The failure to check that the bow door was closed, plus a lack of safety management, on the Herald of Free Enterprise A.80 (1987) led to the car deck being flooded in a manner that made the ship unstable and led to it capsizing. The failure of a tyre on Concorde A.81 (2000) generated debris that penetrated the plane’s fuel tanks which, coupled with the design of the tanks, led to the crash and subsequent fleet abandonment. Lessons from such failures are used to develop safety thinking in areas of complex interaction such as nuclear and chemical plant design.

Figure A.2   Damaged channels channels at Hyatt Regency

originally designed for low-rise construction in a country without a tradition of piped gas supplies. The transposition to the UK took no account of this. Moreover, quality control was poor and the investigation discovered that even the nominal connections were incomplete. These issues might have been addressed addressed if the severity of the potential consequences had been considered. Nobody seems to have asked ‘‘what’s the worst thing that can happen?’’  The attack on the Pentagon in Washington DC A.82 (2001), using a hijacked airliner, caused the death of  125 people on the ground; this was, however, far fewer than might have been the case. The airliner impacted impacted the lower floors of the Pentagon building, causing extens ext ensive ive dam damage age,, but the con contin tinuou uous s spi spiral ral lin links ks to the columns confined the core concrete and allowed it to maintain some strength. In combination with catenary action in the continuously reinforced floor beams, this limited the area of collapse of the upper floors and allowed their occupants to escape (See Figure A.4; note that use of military imagery does not imply or constitute endorsement of the Institution of Structural Engineers, its products, or services by the U.S. Department of Defense.)

 A.19 Robustness  The collapse of the 22-storey Ronan Point A.28 apartment tower, London (1968) was initiated by a relatively small explosion. Only two months after completion an accidental gas explosion on the 18th floor blew out an external wall panel, triggering progressive collapse of the whole corner of the building. The incident was the main driver for the introduction of robustness rules into the UK Building Regulations.  The building was built using a ‘large panel system’ of  precast concrete panels for walls and floors. These relied on gravity for stability, with only nominal connections between them. It was a Danish system,

Buncefield Figure A.3   Warehouse damaged by blast at Buncefield The Institution of Structural Engineers   Risk in structural engineering

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Pentagon Figure A.4   Damage to the Pentagon

 A.20 Mobile structures

 A.21 Fail Failure ure to lear learn n from previous previous incidents incidents

 The Severn Bridge gantry accident A.83 (1990) occurred while moving the gantry from its station to its position of work. It was designed concurrently with the bridge deck to allow inspection and maintenance of the permanent works (steel box girder), and was being used by a contractor to paint the box girder. The gantry geometry was intended to have ha ve be been en lo lock cked ed bu butt pr prev evio ious us mi misu suse se ha had d le left ft it in an unlocked mode.

In 1994, tunnels were being driven at Heathrow  Airport, UK, using the ‘New Austrian Tunnelling ling Method’ A.86 (NATM). This is an ‘observational’ method, in which feedback from the construction process is used to modify the approach, if necessary, as work  progresses. NATM takes advantage of the ability of the ground to support itself, at least partly, if cut in an appropriate way. Sprayed concrete is then used to form a permanent support. The process is dependent on the desig designer ner having a good understandin understanding g of the ground conditions and a skilled, experienced and appropriately motivated site team.

 The Avonmouth Bridge accident A.84 (1999) involved a construction gantry travelling on runway beams designed for a permanent works inspection gantry. Strengthening works to the bridge superstructure and replacement of the existing permanent gantry runway beams were combined into a single operation. The front hangers were running on old runway beams with the rear hangers suspended from sections of newly installed runway beams. At the time of the accident there was a gap of  approximately 3.5m between the ends of the new and the end of one of the old runway beams. Wind blew the gantry along the rails and it rolled through the gap, falling down and killing four construction workers. The cause of the accident was unsatisfactory hardware and unsafe working practices, ineffective restraints on beams to prevent longitudinal movement of gantries and no provision of end stops as a ‘fail-safe’ in the event of  restraints not working effectively. Above all, there was failure to anticipate the mode of failure; to think  ‘what can possibly possibly go wrong?’ wrong?’ The underlying underlying cause was documented as a general failure to plan, organise, control, monitor and review the operations; in particular, the lessons should have been learned from the earlier incident. Further advice on the design of bridge access gantries is contained in the Institution of Structural Engineers’ report A.85. 58

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 The face of the Heathrow tunnel collapsed during a night shift, resulting in further collapses over three days and subsidence subsidence of the airp airport ort above the tunnel A.87, A.88. There were no fatalities but the airport suffered severe disruption and the construction programme was seriously delayed. An adjacent rail tunnel, in use, came close to being affected, and work  was stopped on tunnels in central London which used a similar method. The principal contractor was subsequently fined £1.2 million.  At the time of the collapse, information existed on 116 previous collapses related to NATM. Kevin Myers, the HSE’s chief inspector of construction, said that the collapses could have been prevented but for a cultural mindset which focused attention on the apparent economies and the need for production rather than the particular risks.

 A.22 Safety culture  The space shuttle, Challenger Challenger,, exploded shortly after launch A.89 (1986). The primary cause was the failure of 

 

Case studies

an O-ring seal made brittle by cold weather, which allowed hot gas to impinge on the external fuel tank  and its supports, leading to rupture of the fuel tank, a fireb fir eball all and dis disint integr egrati ation on of the cra craft. ft. Thi This s was a kno known wn problem and a workaround existed, related to the temperature before launch, but those who decided to launch the shuttle did not fully understand the danger.  The lessons for safety culture in NASA were not fully learned, leading to the loss of Columbia from a different physical cause 17 years later A.90.

In this respect, the safety of all structures is underpinned by the quality assurance (QA) regimes required for assurance of product quality. Long supply chains provide an opportunity for low quality products with forged certification to be supplied; in many cases the final purchaser can only distinguish these from the specified product by carrying out tests. While counterfeit components have not, so far as the authors are aware, resulted in serious structural failures, they have been responsible for loss of aircraft A.95.

 The physical root cause of the loss of Columbia (2003) was impact of a piece of insulating foam onto a heat resisting tile. The official enquiry A.91 concluded, ‘‘that NASA’s organizational culture had as much to do with this accident as foam did’’. NASA’s formal safety policies, although outwardly making safety the top priority, were not fully effective in practice. The independent safety reviewers were funded by the project they reviewed, resulting in a conflict of interest. Lessons from the loss of Challenger had been addressed but not absorbed into the culture. When engineers raised concerns which had schedule or cost implications implications,, they were put down or igno ignored red by managers. A number of necessary improvements to safety were identified, but were later cancelled on cost grounds. grounds. There was a view that normal flight should shoul d continue, unless the shuttl shuttle e was prov proved ed to be unsafe, rather than an expectation that every activity should firstabe shownoftomissions, be safe. Foam struck the shuttle on number but it survived with limited damage, which led to a belief that it would always survive, irrespective of the engineering issues and safety margins. All these factors created an environment within which potentially serious engineering issues could grow and become actual life threatening problems, culminating in the loss of the shuttle.

 A.24 Failure to understand the structure Navier’s suspension bridge A.96 crossing the Seine at Les Invalides, Paris (1826), was almost completed when cracks in the foundations provoked the need for remedial work which had significant financial, political and engineering consequences. The case was high profile; the project was declared to be a matter of  national pride with no economic motivation but, surprisingly, was financed privately.  At the time the mathematics of the hanging chain were being elegantly developed to the satisfaction of  those involved. Navier developed and applied these equations to complement the empirical experience of  Brunel, in designed Bristol, UK. The superstructure was therefore from theoretical considerations.  The masonry foundations, however, were designed by traditional empirical methods.

 This is also an example of the value of studying ‘near misses’. It was not the first time foam had become detached; on Concorde it was not the first time tyre damage had occurred and the Kings Cross escalator was not the first wooden escalator to have caught fire.

 The bridge experienced unexpected cracking in the foundations which anchored the main suspension cables. The cracking first occurred as the permanent load of the deck was applied to the ties, then more extensively when a broken water main flooded one foundation. The combination of the increased pore pressure, the strain compatibility in the curved masonry compression pier and the lower tie segment and friction restraining slipping on the saddle will all have contributed in different ways to the cracking.

 A.23 Competence and quality

Interfaces, such as between the iron superstructure and the masonry infrastructure (one theoretical, one empirical) are often imperfectly resolved.

In Coco Beach, Florida, a four-storey building collapsed during construction A.92. Investigation disclosed that the building had not been designed for shear capacity at the slab to column connection. The severely overloaded connection collapsed, killing several workers; it was subsequently determined that the engineers formally responsible for the design were aeronautical rather than structural engineers. In Christchurch, New Zealand, the Canterbury  Television  T elevision building collapsed in an earthquake (2011).  The Royal Commission of Inquiry into the Building Failure Caused by the Canterbury Earthquakes has not yet issued its final report on this building, but the inquiry gave significant consideration to whether the designers of the building had appropriate expertise A.93 and it has been alleged that the engineer who supervised its construction had fraudulent credentials A.94.  There are increasing concerns over sub-standard building components such as cements and bolts.

 A.25 Novel design  The failure of the roof of a departure hall at Roissy  Terminal  T erminal 2E airport (Charles de Gaulle) A.97 by punching shear, a year after commissioning, caused several deaths. Although tragic, the failure is interesting in te term rms s of ri risk sk be beca caus use e it ap appe pear ars s to ha have ve be been en a re resu sult lt of both engineering errors and organisational faults.  A fundamental question is ‘how could such a prestige project with such a skilled design team get it wrong?’  There are numerous n umerous lessons.  There was an initial error in the design concept, leading to cracking which was known about but not fully responded to. Failure occurred by punching shear of the radial steel spacer struts, through the concrete shell, which had insufficient steel reinforcement. A prototype had demonstrated that the design worked, that the detai details ls could be made, the concrete could be placed and that it stood up. The Institution of Structural Engineers   Risk in structural engineering

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 The inadequately reinforced concrete resisted the punching shear in the prototypes, and in the first year of use, by a combination of arching and tension in the concrete, but the cyclical temperature stresses, time and progressive cracking reduced the resistance to punching. The structural concept was not intrinsically robust, but individual modules were nominally linked together to obtain some mutual support. The linking, however, not only failed to prevent collapse but allowed one module to drag down the adjacent one.

 A.9

 

Shirley-Smith, H. H. Report  Report on collapse of Milford Haven  Bridge: fatal accident, 2nd June 1970 . [s.l.]: [s.n.], 1970

 A.10   ‘Koblenz report pinpoints collapse cause’. cause’. New  New Civil  Engineer , May 1972, p10 theory blamed for Koblenz Koblenz bridge  A.11   Cottrill, A. ‘Stability theory collapse.. New Civil Engineer , 23 November 1972, collapse pp10-11 Engineers. Steel  Steel box girder bridges:   A.12   Institution of Civil Engineers.

Finally, although there were three layers of procedures in place for checking, these seem to have been too procedural, stifling the creative thought that is required for true assessment of risk. One issue which may need to be addressed is that much of the post-accident analysis has been carried out for the benefit of the insurers, and has not been made accessible for the engineering community to learn the lessons.

 A.13   Department of the Environment et al. al. Inquiry  Inquiry into the  basis of design and method of erection of steel box  girder bridges: report of the  [Merrison] Committee    [Merrison]  Committee . London: HMSO, 1973

 The lack of redundancy in the operation of the building was more expensive for the airport than rebuilding the structure; the failure of one bay closed the whole terminal for several years. The extensive repetition of a flawed design amplified the consequences of the error. The judicial inquiry as to the causes and responsibilities of the tragedy of  Roissy remained ongoing in 2013.

Kielland accident: accident: report of a   A.15   The Alexander L. Kielland Norwegian public commission appointed by royal  decree of March 28, 1980, presented to the Ministry  of Justice and Police . [s.l.]: [s.n.], 1981

proceedings of the international conference, London, 13-14 February 1973. London: 1973.  London: ICE, 1973

 A.14   Ministry of Power Power..  Report of the Inquiry into the causes  of the accident to the drilling rig Sea Gem . London: HMSO, 1967

 A.16   Withey, P.A. ‘Fatigue failure ure of the De Havilland Comet Comet 1 Engineering’. Engineering’. Engineering  Engineering Failure Analysis , 4(2), June 1997, pp147-154

 A.26 References  A.1

 

Martin, T. T. and MacLeod, I.A. ‘The Tay Bridge disaster: a reappraisal based on modern methods of analysis’. ICE Proceedings, Civil Engineering , 108(2), May 1995, pp77-83

‘Manchesterr B of the Bang Bang sculpture sculpture core sold sold for   A.17   ‘Mancheste scrap’. BBC News website, 4 July 2012. Available at: http://www.bbc.co.uk/news/uk-england-manchester18703854 [Accessed: 25 February 2013]  A.18   British Geological ogical Survey. Survey. Folkestone Earthquake 28  April 2007 07:18 UTC (08:18 BST) 4.2 ML. Available at: http://www.ea http://www.earthquakes.bgs.ac.uk/education/  rthquakes.bgs.ac.uk/education/  reports/folkestone/folkestone_28_april_2007.htm [Accessed: 25 February 2013]

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Thomas, J. J. The  The Tay Bridge disaster: new light on the  1879 tragedy . Newton Abbot: David & Charles, 1972

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CEGB.  Report of the Committee of Inquiry into collapse  CEGB. Report of cooling towers at Ferrybridge, Monday, 1 November, 1965 . London: CEGB, 1966

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Institution of Civil Engineers. Engineers. Natural  Natural draught cooling  towers – Ferrybridge and after: proceedings of a  conference, London, 12 June 1967 . London: ICE, 1967

S.C.C. Report  Report on the failure of roof beams at Sir   A.20   Bate, S.C.C. John Cass School Stepney. BRE Current Paper 58/74 . Garston: BRE, 1974

‘Tornado injures 19 in Birmingham’. BBC News  A.19   ‘Tornado website, 28 July 2005. Available at: http://news.bbc. co.uk/1/hi/england/west_midlands/4725279.stm [Accessed: 25 February 2013]

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‘Severn Bridge and Wye Bridge Strengthening Work  1985-1991’. Available at: http://www.severnbridge.co. uk/history_strengthening.shtml [Accessed: 25 February 2013]

 A.21   Failure Failure Knowledge Knowledge Database/100 Database/100 selected selected cases: cases: Brittle  e  fracture of Liberty Ships . Available at: http://www. sozogaku.com/fkd/en/hfen/HB1011020.pdf [Accessed: 25 February 2013]

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Walshe, D.E. and Wyatt, T.A. ‘Bridge aerodynamics: 50 years after Tacoma Narrows’. Journal Narrows’.  Journal of Wind  Engineering and Industrial Aerodynamics , 40(3), 1992, pp317-336

 A.22   Carino, N.J. et al. ‘Investigation of East Chicago ramp collapse’. ASCE collapse’.  ASCE Journal of Construction Engineering  and Management , 110(1), March 1984, pp1-18

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Winter, S. and Kreuzinger Kreuzinger,, H. ‘The Bad Reichenhall icearena collapse and the necessary consequences for  wide span timber structures’.  World conference on  timber engineering. Miyazaki, Japan, 2008 , paper  271. Available at: http://www.ewpa.com/Archive/2008/   june/Paper_271.pdf [Accessed: 25 February 2013]

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Carino, N.J. et al. al. Investigation  Investigation of construction failure   A.23   Carino, of the Riley Road interchange ramp, East Chicago, Indiana. NBSIR 82-2583 . Available at: http://www.nist. gov/manuscript-publication-search.cfm?pub_id= 908826 [Accessed: 25 February 2013] Dam failure’. failure’. Engineering   Engineering   A.24   Londe, P. ‘The Malpasset Dam Geology , 24(1-4), 1987, pp295-329 Maurenbrecher, P.M. P.M. ‘The Malpasset Dam failure  A.25   Maurenbrecher, investigation and analysis examined’. In Neale, B.S. ed.

 

Case studies Forensic engineering: from failure to understanding . London: Thomas Telford, 2009, pp229-238 A.W. and Vaughan, Vaughan, P.R. P.R. ‘The Failure Failure of  A.26   Skempton, A.W. Carsington Dam’. Ge  Dam’.  Ge ´ ´ otechnique  otechnique , 43(1), March 1993, pp151-173 R.E. Failure  Failure of Carsington embankment: a   A.27   Coxon, R.E. report to the Secretary of State for the Environment . London: HMSO, 1986  A.28   Ministry of of Housing Housing and and Local Local Government. Government. Report  Report of  the Inquiry into the colla collapse pse of flats at Ronan Point, Point, Canning Town . London: HMSO. 1968 Executive. The  The Abbeystead   A.29   Health and Safety Executive. explosion: a report of the investigation by the  Health and Safety Executive into the explosion on  23 May 1984 at the valve house of the Lune/Wyre  water transfer scheme at Abbeystead . London: HMSO, 1985

B.W. ‘Case studies: Emley Moor mast’. In  A.39   Smith, B.W. Campbell, P. ed. Learning ed.  Learning from construction failures:  applied forensic engineering . Latheronwheel: Whittles, 2001, pp274-277  A.40   Arup. Arup. The  The Millennium Bridge . Available at: http://www. arup.com/MillenniumBridge [Accessed: 25 February 2013] al. ‘The London London Millennium um footbridge’.  A.41   Dallard, P. et al. The Structural Engineer , 79(22), 20 November 2001, pp17-33 Penman, D. ‘Pink ‘Pink Floyd Floyd ‘very ‘very angry and and upset’ upset’ over   A.42   Penman, accident: human error could have caused temporary stand’s collapse at rock concert attended by 15000 fans’. The fans’.  The Independent , 14 October 1994. Available at: http://www.independent.co.uk/news/uk/pink-floyd-veryangry-and-upset-over-accident-human-error-couldhave-caused-temporary-stands-collapse-at-rockconcert-attended-by-15000-fans-danny-penmanreports-1442784.html [Accessed: 25 February 2013]

  & Others v  Others  v .  Binnie  &   & Others. Court of Appeal  A.30   Eckersley  & (Civ Div). Fox, Bingha Bingham m and Russell L.JJ http://  oxcheps.new.ox.ac.uk/new/casebook/cases/Cases Chapter 13/Eckersley v Binnie.doc [Accessed: 25 February 2013]

Grainger,, G.D. G.D. Report  Report on the   A.43   Menzies, J.B. and Grainger collapse of the sports hall at Rock Ferry  Comprehensive School Birkenhead. BRE Current Paper  69/76 . Garston: BRE, 1976

 A.31   Hudson, S. S. Bridge  Bridge Strikes – Special Topic Report . London: Railway Safety, 2002. Available at:

Tribunal appointed appointed to inquire inquire into the  the   A.44   Report of the Tribunal disaster at Aberfan in October 1966 . London: HMSO,

http://www.rssb.co.uk/sitecollectiondocuments/pdf/  reports/Bridge Strikes – Special Topic Report.pdf [Accessed: 25 February 2013]  A.32   Institution of Structural Engineers. Engineers. Guide  Guide to  inspection of underwater structures . London: IStructE, 2001 Unit. Malahide  Malahide Viaduct   A.33   Railway Accident Investigation Unit. Collapse on the Dublin to Belfast Line, on the  21st August 2009. Investigation report no. R2010  004 . Blackrock: RAIU, 2010. Available at: http://www. raiu.ie/download/pdf/accident_malahide.pdf raiu.ie/download/p df/accident_malahide.pdf [Accessed: 25 February 2013] Transport. t. Report  Report on the collapse of   A.34   Department of Transpor Glanrhyd Bridge on 19th October 1987 . London: HMSO, 1990. http://www.railwaysarchive.co.uk/  documents/DoT_Glanrhyd1987.pdf [Accessed: 25 February 2013]  A.35   National Transpor Transportation tation Safety Board. Board. Derailment  Derailment of   Amtrak Train No. 2 on the CSXT Big Bayou Canot  Bridge near Mobile, Alabama, September 22, 1993. RAR-94/01.. Summary available at: http://www.ntsb. RAR-94/01 gov/publictn/1994/RAR9401.htm [Accessed: 25 February 2013] gone: the severed artery’. artery’. Available at: http://   A.36   ‘Bridge gone: www.parliament.tas.gov.au/history/brd1.htm. [Accessed: 25 February 2013]

1967 J. Aberfan:  Aberfan: the story of disaster . London:  A.45   Austin, J. Hutchinson, 1967 Executive. Walkway  Walkway collapse at Port   A.46   Health and Safety Executive. Ramsgate: a report on the investigation into the  walkway collapse at Port Ramsgate on 14 September  1994. Sudbury: 1994.  Sudbury: HSE Books, 2000  A.47   Barber, Barber, J. ‘Ramsgate walkway collapse: legal ramifications’. In Neale, B.S. ed. Forensic ed.  Forensic engineering:  a professional approach to investigation . London: Thomas Telford, 1998, pp39-60  A.48   Fleming, Fleming, D. ‘Design ‘Design doubts: doubts: why the Ramsgate walkway collapsed’. New collapsed’.  New Civil Engineer , 22 September 1994 ‘Collapse of the Ramsgate Ramsgate Walkway’. kway’.  A.49   Chapman, J.C. ‘Collapse The Structural Engineer , 76(1), 6 January 1998, pp1-10  A.50   Ministry of Tr Transport. ansport. Enquiry  Enquiry report on the failure of  an overbridge overbridge that occur occurred red on 10th May 1965 at  Clapham Junction in the Southern Region, British  Railways . London: HMSO, 1965. Available at: http://www.railwaysarchive.co.uk/documents/MoT_ Clapham1965.pdf [Accessed: 25 February 2013]

 A.37   Court of Marine Enquiry. Enquiry. S.S.  S.S. Lake Illawara. Report  157 , Canberra: Australian Government Publications Service, 1975

J.G.M. Pipers  Pipers Row Car Park, Wolverhampton:   A.51   Wood, J.G.M. quantitative study of the causes of the partial collapse  on 20th March 1997 . Chiddingfold: Structural Studies & Design Ltd, 2002. Available at: http://www.hse.gov. uk/research/misc/pipersrow.htm [Accessed: 25 February 2013]

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 A.52   ‘Waitress killed by falling masonry ’. ’. BBC News

after oil-rig collision towing accident ends in severe damage’. The damage’.  The Herald , 5 August 1996 1996.. Avail Available able at: http://www.heraldscotland.com/sport/spl/aberdeen/  erskine-bridge-is-closed-after-oil-rig-collisiontowing-accident-ends-in-severe-damage-1.442658 [Accessed: 25 February 2013]

website, 29 June 2000. Available at: http://news.bbc. co.uk/1/hi/scotland/811988.stm [Accessed: 25 February 2013] Council for Scotland. Scotland. Risks  Risks to   A.53   Construction Industry Council public safety from falling masonry and other materials:  The Institution of Structural Engineers   Risk in structural engineering

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