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January 2021 Volume 99 | Issue 1
Refurbishment: design opportunities
Timber and carbon sequestration
Axial shortening
Thinking inside the box The design of the MultiPly pavilion explores explores the potential of timber in modular and reusable construction
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Upfront
5 Editorial 6 News 8 Climate Emergency Ta Task sk Group: End-of-year report
Climate emergency 10 An introduction to refurbishment. Part 2: Maximising 10 Maximising the opportunities at the design stage 16 Structural safety when designing lean in the climate 16 climate emergency 18 Timber and carbon sequestration 18 sequestration
Professional guidance 22 Business Practice Note No. 37: Checking the work of 22 another engineer
39
Technical 24 Technical Guidance Note Level 3, 24 No. 2: Axial shortening
Project focus 29 Design and construction of the MultiPly pavilion 29 using cross-laminated tulipwood
Opinion
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36 Viewpoint: A new approach to floor loading 36 39 Viewpoint: Aligning temporary works with the Digital 39 Plan of Work 42 Verulam 42
At the back 1 2 0 2 y r a u n a J
1 e u s s I
E V E E R D E : R E V O C
45 45 46 46 48 48 49 49 50 50
Diary dates Spotlight on Structures The Drawing Board Products & Services Services Directory
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9 9 e m u l o V
18 3 thestructuralengineer.org | January 2021
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Editorial
Upfront
PRESIDENT Don McQuillan BSc(Eng), CEng, FIStructE, FICE, FIAE, FIEI, FCIHT, FConsE, MAPM, MAE CHIEF EXECUTIVE Martin Powell EDITORIAL HEAD OF PUBLISHING Lee Baldwin MANAGING EDITOR Robin Jones t: +44 (0) 20 7201 9822 e:
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[email protected] DESIGN SENIOR DESIGNER Nicholas Daley PRODUCTION PRODUCTION DIRECTOR Jane Easterman EDITORIAL ADVISORY GROUP Will Arnold MIStructE Premma Makanji MIStructE Allan Mann FIStructE Chris O’Regan FIStructE Angus Palmer MIStructE Simon Pitchers FIStructE Eleana Savvidi MIStructE Price (2021 subscription) Institutional: £465.00 Personal (print only): £130 Personal (online only): £130 Personal (Student Member): £40 Single copies: £25 (incl. p&p) Printed by Warners Midlands plc The Maltings, Manor Lane Bourne, Lincolnshire PE10 9PH United Kingdom © The Institution of Structural Engineers. The Structural Engineer (ISSN 1466-5123) is published by IStructE Ltd, a wholly owned subsidiary of The Institution of Structural Engineers. It is available both in print and online. Contributions published in The Structural Contributions Engineer are published on the understanding that the author/s is/are solely responsible for the statements made, for the opinions expressed and/or for the accuracy of the contents. Publication does not imply that any statement or opinion expressed by the author/s reflects the views of the Institution of Structural Engineers’ Board; Council; committees; members or employees. No liability is accepted by such persons or by the Institution for any loss or damage, whether caused through reliance on any statement, opinion or omission (textual or otherwise) in The Structural Engineer , or otherwise. The Institution of Structural Engineers International Internation al HQ 47–58 Bastwick Street London EC1V 3PS United Kingdom t: +44 (0)20 7235 4535 e: mail@istructe.
[email protected] org The Institution of Structural Engineers Incorporated by Royal Charter Charity Registered in England and Wales number 233392 and in Scotland number SC038263
Robin Jones Managing Editor
Seeing the wood wood and an d the trees trees FIRST AND FOREMOST, FOREMOST, I’d I’d like to wish all members and readers a Happy New Year! May 2021 bring with it renewed optimism and opportunities after the trials of 2020. With the first Covid-19 vaccines being approved and rolled out around the world, there is certainly hope that we’ll begin to see a gradual return to ‘normality’ as the year progresses. But in the meantime, the Institution continues to off er er a range of products and services to members that can be accessed remotely. In addition to another packed issue of The Structural Engineer , there is a wide range of online CPD courses running (page 21) and 21) and a bundle discount available on purchases of manuals and guides (page 9). 9). Don’t forget that you can also make use of the e-Library resources at www.istructe.org/library .
We also publish a new Technical Guidance Note on axial shortening in tall buildings (page 24). 24). The note discusses the causes of axial shortening, how to assess and predict it, as well as potential mitigation measures. While the Business Practice Note series continues with an article off ering ering guidance on how to approach an appointment to check a design produced by another engineer (page 22). 22). Elsewhere in the issue, we present Viewpoints proposing a new approach to floor loading in the Eurocodes (page 36) and 36) and greater consideration of temporary works throughout a project’s stages (page 39), 39), another healthy selection of letters (page 42), 42), and our regular Diary dates (page 45), Spotlight on Structures (page 46) and 46) and Drawing
Returning to this issue, the Climate emergency section continues with the second part of an article on refurbishment projects. Stephen Fernandez moves on from his examination of opportunities at the feasibility stage in November/December to looking at ways to maximise opportunities at the design stage (page 10). 10). This is followed by a discussion of safety issues when designing lean (page 16), 16), and an exploration of the role of sequestration when accounting for embodied carbon in timber designs (page 18). 18). The latter article addresses an issue around which there is confusion and inconsistency, and aims to give engineers a clearer picture of how sequestration should be incorporated into calculations, along with recommendations for climate-focused timber design. The timber timber theme theme runs through through into this month’ month’s Project focus article on the design of the MultiPly pavilion – a temporary structure exploring timber’s place in the circular economy and its use in modular construction (page 29). 29). The project featured the first cross-laminated timber to be manufactured at volume in the UK, in this case from American tulipwood.
Board (page 48) features. 48) features. As ever, ever, I hope hope you enjoy the the issue.
5 thestructuralengineer.org thestructuralengineer .org | January 2021
Correction In last October’s article on the proposed SCORS rating scheme, we mistakenly named Lindsay Rasmussen of Architecture 2030 as ‘Laura’ in the Acknowledgements section on page 12. We apologise to Lindsay for the oversight.
Upfront
News
Institution news Institution news
Public vote winners announced in Structural Engineering Showcase 2020
Institution fellow appointed CHB The Institution of Structural Engineers Engineers is very pleased to announce that, on behalf of Her Majesty The Queen, The Governor-Gene Governor-General ral of Barbados has appointed Tony Gibbs to the Order of Barbados in the Grade of Companion of Honour of Barbados (CHB). The award is made in recognition of Tony’ Tony’ss ‘sterling contribution in the field of engineering in Barbados and the Caribbean’. Tony T ony,, who will now be known as The The Hon Tony T ony Gibbs CHB, FREng, FIStructE, FIStructE, generously acknowledges the encouragement and support he has had during ur his long association with the
Thousands Thousa nds of Ins Institu titution tion mem member berss and industry professionals have voted for their favourite Structural Awards submission as part of the Structural Engineering Showcase 2020 celebrations. Projects were grouped into four categories, with the public vote winners as follows: Institution as contributing in no small measure to him being recognised in this way. The Institution off ers ers Tony its sincerest congratulations and best wishes.
WINNER: Achieving architectural vision Beijing Daxing International Airport Termina Term inall Submitted by: Beijing Institute of Archit Arc hitect ectura urall Desi Design gn
Industry news
Climate Change Committee releases Sixth Carbon Budget
Industry news
Structural-Safety issues new Structural-Safety Alert on 2018 Florida Florida bridge collapse Structural-Safety has published a SCOSS Alert, Lessons Learned from the 2018 Florida Bridge Collapse During Construction, to share the key findings of the investigation with bridge owners, designers, contractors, checkers and supervisors. This was a bridge of an unusual unusual design and was being constructed in an unusual manner.. The main 53m prestressed precast manner concrete span truss was in position when cracks appeared at a node and, over a period of almost three weeks, visibly worsened until collapse occurred. All parties apparently failed to recognise recognise the bridge was in danger when inspected hours before the collapse. In hindsight, the magnitude of the cracks warranted that the road be immediately closed, and the truss supported to reduce loads, pending evaluation. The National Transportation Transportation Safety Board Board investigations focused on the design, peer review checking, site supervision and independent checking of the works.
Download the Alert at www. structural-safety.org/publications/ scoss-alerts/ .
The UK’s Climate imate Chan Change ge Comm Committe ittee e has has published its Sixth Carbon Budget to provide government ministers with advice on the volume of greenhouse gases the UK can emit during the period 2033–37. The rep report ort pre presen sents ts a blue blueprin printt for for a fully decarbonised UK. It describes the path to net-zero emissions and detail the steps the country must take to achieve this goal by 2050. The rec recomm ommend ended ed pathw pathway ay requi requires res a 78% 78% reduction in UK territorial emissions between 1990 and 2035. In eff ect, ect, it brings forward the UK’s previous 80% target by nearly 15 years. The cha challen llenging ging pat path h set out in the report creates new industrial opportunities and ensures wider gains forr the nation’s health and for nature. It envisages low-carbon investmentt scaling up to £50bn each year over the next decade, but with substantial fossil fuel savings eventually cancelling out the investment costs entirely. The Sixt Sixth h Carbon Carbon Budg Budget et rep report ort is supported by: Ò| a Methodology Report Ò| a Policy Report Ò| all the charts and data behind the report Ò| a public Call for Evidence, several new research projects, three expert advisory groups and deep dives into the roles of local
WINNER: Challenging construction Skiing Platform Submitted by: Helium Engineering
WINNER: Creative design DONUT Submitted by: Xin XinY Y Struc Structur tural al Consultants
WINNER: WINNE R: Sustai Sustainable nable leader leadership ship Hobhouse Submitted by: Thornton Tomasetti
authorities and businesses. Download the report at www.theccc. org.uk/publication/sixth-carbonbudget/ . 6 January 2021 | thestructuralengineer.org
To find out more, visit www. istructe.org/resources/news/ structural-engineeringshowcase-2020-public-vote/ .
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Opinion Upfront
Climate PlanningEmergency applicationTask procedures procedur Group es
Climate Emergency Task Group: End-of-year report The Institution’s Climate Emergency Task Group looks Group looks back at a historic year in which climate action has been front and centre for the profession. The Climate Emergency Task Task Group (CETG) has published an end-of-year report reviewing the group’ group’s s activities through the year as it endeavoured to help the profession tackle the climate emergency. This article summarises the key points from the report, which also acts as a guidance wayfinding tool, containing links to the written and recorded content that was produced by CETG contributors through the year (Figure 1). 1). Please do read and discuss the report, as well as sharing it with clients and collaborators, to highlight our role in getting to net zero, and to share the progress that our profession is making.
DOWNLOAD THE REPORT
You Y ou can download the CETG’s CETG’ s end-ofyear report at www.istructe. org/resources/ report/climateemergencytask-groupend-of-yearreport/ .
Progress in 2020 The CETG started 2020 2020 by formalising its mandate for change across four workstreams: uence: across across the Ò| Collaborate and influence: industry in the UK and abroad. Support the profession: by profession: by bringing firms together to share knowledge and progress. standards: through guidance, Ò| Raise standards: through training, webinars and conferences. standards: from education Ò| Set standards: from and chartership through CPD and awards.
carbon articles, including an introduction to the SCORS rating system. The calculation of embodied carbon continues to be a key part of the design process – informing material choices, design decisions, and circular economy and reuse strategies – and the CETG will continue to develop this important area in 2021, with the Institution due to release an Excel-based Structural Carbon Tool early in the year. Finally, we worked to implement changes across the diff erent erent stages of a structural engineer’ engineer’s s career. The Joint Board of Moderators (JBM) guidelines were updated, bringing accredited university education in the UK in line with climate emergency response principles, and two new Structural Awards categories were were introduced – for Zero Carbon and Minimal Structural Intervention.
Feedback Throughout the year, year, feedback was
Ò|
îFIGURE 1: The 1: The report acts as a wayfinding tool to guidance produced by the CETG
sought from the membership to ascertain the needs of practising engineers with respect to guidance being published. As well as asking the question, ‘What can we do more
te We refreshed the Institution’ Institution’s s Climate emergency webpage webpage (www.istructe.org/ rg/ climate-emergency),, putting all relevant climate-emergency) nt guidance in one accessible location for or members. This grew to include the 26 articles that we published through the year,, as it will continue to grow through year h 2021. In July, the Institution published How to calculate embodied carbon, carbon , providing a common set of calculationn principles for the industry to follow when hen quantifying the impact of our work. This his was accompanied by several other
of?’, at committee meetings, regional events, and the Climate Emergency Conference, we also ran two parallel online member surveys. The endof-year report outlines the resulting priorities for 2021, including materialspecific guidance and reuse articles.
Year Y ear ahead Looking forward to the coming year, the report highlights some of the task group’s key goals for 2021: uence: including including Ò| Collaborate and influence: continuing to strengthen links around the world; and advising and advocating for climate emergency response throughout governments and other institutions that we work with. Ò| Support the profession: hosting Institution conferences, helping firms to keep sharing with one another, and supporting the development of a multidisciplinary open-source sharing website. Raise standards: including the release of a free Excel-based Structural Carbon Tool, continued publication of guidance, and the creation of a Net-Zero Structural Design certificate. standards: completing our Ò| Set standards: completing work reviewing the Professional Review Interview and Membership Exams, and assisting universities in implementing the updated JBM guidelines. Ò|
Summary Su We would like to thank all of the CETG contributors who found time in 2020 to con create guidance articles, presentations cre and webinars for the rest of the an membership. We are proud of what we me collectively achieved in 2020, and are coll excited at the potential to achieve even exc more this year. We hope that you’ll join mo us in i making 2021 a year to remember!
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January 2021 | thestructuralengineer.org
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Planning application Making procedur procedures the mostes of refurbishment opportunities Climate emergency Opinion
4.Zero waste
An in An intr tro oduct ctio ion n to refurbishment. Part 2: Maximising the opportunities at the design stage Stephen Fernandez guides readers through the key design areas that offer refurbishment opportunities when working on an existing building. Introduction Part one of this article considered how to explore opportunities for refurbishment over new-build before any design work begins. The second part now describes how to maximise the potential opportunities for refurbishment during the design stage. There are many many structural considerations which will be speci fic to each existing building, and the engineer needs to be familiar with a number of topics. Although this article gives a brief introduction, the reader should familiarise themselves further with each area before starting the design (see ‘References’ and ‘Further reading’).
Structural considerat considerations ions Loading If there is no proposed change of use, a building is often initially assumed to have been designed adequately to the relevant codes of practice at the time, but this must be validated v alidated through structural inspections and surveys. If no visible signs of distress or decay are observed, it may be possible to conclude that the structure is adequately carrying the loads and that strengthening works are not necessary necessary.. Change of use can give a building a new lease of life, and ensuring it is occupied often helps guarantee that it will be maintained, although this can result in a load increase, possibly through new services or additional imposed loading. Unlike for interventions, the implications of change of use can be quickly determined. A challenge for change change of use of historic buildings is justifying floor loadings to current codes of practice, often due to the low values v alues of material
strength and stiff ness, ness, as older structures can still work to higher capacities. Many historic buildings were constructed based on experience, sometimes known as ‘empirical design’ without any structural design. This is still a reasonable structural approach and could be called ‘full-scale prototype testing’ today. It is worth highlighting that structures often have multiple load paths, so the challenge is simply to find one plausible route, although this is highly unlikely to be the actual load path. Taking T aking a blanket approach approach towards floor loading often results in the conclusion that strengthening is necessary1, but by carrying out rigorous engineering, considering possible load paths and understanding the
associated with accommodating offi ce floor loadings in older buildings and suggests that actual loading is usually significantly lower. A practical approach could be to develop a better understanding of the original design loads and ensure that the new use respects these, which may require limiting occupancy levels or setting specific uses for certain areas. When considering loading, it is important to assess realistic loadings and ask: are higher floor loadings (to give flexibility of use) actually necessary? Or can a lower value be used without compromising the flexibility or structural safety? The use of the building should should not overly restrict the loading on the floors, but equally they should not be upgraded to support loads they will never
actual loading, it is possible to avoid strengthening in the majority of cases. The 1960s Newton Building at Nottingham Trent Trent University was originally designed as a technical school and was to be adapted without change of use. Floor live loading from the original codes of practice was compared against current codes of practice. The finishes, services, ceilings and non-loadbearing walls were all to be removed. Prior to removal, detailed surveys helped determine the weight of each element and estimate the allowable new superimposed dead loading. Comparing total existing loading with total new avoided strengthening, as there was no net increase and it was not necessary to carry out extensive ‘back-justification’ calculations to try to establish theoretical capacities.
experience. Perhaps now is the time to challenge the loading stipulated in codes of practice generally?
Offi ce fl oor oor loading in historic buildings2 describes the problems
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Calculations and analysis As a starting point, structural validation checks can typically be assessed using current codes of practice. It is possible to use material strengths obtained from historic codes of practice and other referenced sources or obtained from off -site -site testing of materials obtained from site. However,, it is important to understand However how buildings were designed. Many
ARE HI GHER HIGHER FLGIVE FLOOR OOR LOADINGS (TO FLEXIBILITY OF USE) ACTUALL ACTUAL LY NECESSARY? NECE SSARY?
January 2021 | thestructuralengineer.org
Making the most of refurbishment opportunities Climate emergency
maximise the floor-to-c oor-to-ceiling eiling heights. Refurbishment works at 1–2 Stephen Street in London have transformed a previously dark and dingy 1960s building into an effi cient and stylish workplace, increasing both asset value and rental
historic codes of practice used a permissible stress design approach 3, limiting material stresses under varying loading conditions to an allowable or ‘permissible stress’. This is diff erent erent to the typical modern codified approach of ‘limit state design’, and so understanding the diff erences erences can be useful. Note that although many old codes have been superseded, they can be a useful tool when trying to work with existing structures. Accurately determining the movement of a structure can also be di ffi cult, as it is impossible to accurately understand all aspects of the structural behaviour (e.g. support conditions or connection movement). It is therefore generally beneficial to carry out comparative studies between the existing and proposed conditions, with sensitivity analyses as required.
éFIGURE 1: 1–2 1: 1–2
Where structural modelling is not helpful or meaningful, it may be possible to carry out in situ testing and prototyping, although appropriate caution should be taken to gain a reasonable amount of certainty that the loading will be achieved. Finally,, it is also important to consider Finally the entire building holistically and not focus solely on specific interventions. For example, forming large penetrations in concrete slab bays originally designed to act as continuous over multiple bays, or removing areas of floor, or major interventions for building services which can significantly alter the existing building system and aff ect ect the overall building stability. Foundations Foundations should be assessed when there is an increase in loading, e.g. due to change of use or if additional storeys
êFIGURE 2: Lowering 2: Lowering
Stephen Street in London – underground parking and loading bay converted into flexible offi ce space with maximised floorto-ceiling heights
existing foundations to increase floorto-ceiling height at George Green Library, University of Nottingham
income. The underground parking and loading bay have been converted into flexible offi ce space and floor-tooor-to-ceiling ceiling heights have been maximised by adopting a L semi-exposed services design with L Afloating bespoke metal rafts (Figure 1). 1). N S I coordination and H This required close coordination C T the transformation has increased the T Alettable area by 10%, which signi ficantly M increased the rental value. Alternatively, more extensive are to be introduced – although often old structural modifications are possible. foundations can be justified to take an The existing basement in the University increase in loading4. of Nottingham’s Nottingham’s George Green Library Reusing existing foundations would not meet modern building is becoming a key consideration, regulations for headroom, daylight and particularly in inner-city sites. This can access. The columns were therefore be due to congestion in the ground lowered to transform the basement into (which may have resulted from several a useable floor5, which required major generations of development) making it temporary works to support the existing diffi cult to install new foundations without structure. expensive foundation extraction. A pair of deep steel channels channels were Reusing the foundations can yield clamped onto the existing columns, significant cost savings and provide transferring their load onto a temporary increased programme certainty due frame using hydraulic jacks (Figure to the reduced risk associated with 2).. This process was repeated across 2) groundworks. the entire basement (while the floors above were occupied) and allowed each column to be lengthened to create a Holistic design Floor-to-ceiling Floor-to-ceili ng heights more useable level. The floor-to-ce oor-to-ceiling iling heights should be The carbon footprint of such such reviewed in an existing building at an temporary works should also be early stage as these can in fluence how assessed – at George Green Library, the it can be reused. For example, o ffi ces works were reused several times and the typically require greater heights to accommodate service requirements, but developing a truly coordinated solution can maximise the floor-tooor-to-ceiling ceiling height. It is possible to work within the constraints of an existing building to
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overall carbon savings outweighed the emissions from fabricating the temporary works. Internal layout When considering the future use of an existing building, the internal layout should be reviewed to understand how reusing the building may meet the client’s requirements. The existing structure can be viewed as a constraint, but often provides an opportunity to cost-eff ectively ectively do something transformative when considered early enough. Opening up spaces in existing buildings can transform them significantly cantly.. If alterations can be limited to removing only non-structural walls, then this can be achieved quickly and simply, but deeper interventions should be considered where the potential impact makes this worthwhile.
thestructuralengineer.org | January 2021
Planning application Making procedur procedures the mostes of refurbishment opportunities Climate emergency Opinion
íFIGURE 3: Transformation 3: Transformation
The Zeitz Museum of Contemporary Contemporary Art Africa, in Cape Town, Town, South Africa, is a radical transformation of a historic grain silo, consisting of 42 tightly packed concrete cylinders, which had been disused since 1990 due to containerised shipping. Rather than demolishing the redundant structure, spaces have been carved out of the cellular concrete
of grain silo complex at Zeitz Museum of Contemporary Art Africa Africa
features, such as underground service tunnels and basements, were also reused to distribute services. Facade upgrade Many old buildings have poor
cylinders to form galleries and an atrium, allowing natural daylight into the spaces and revealing the original intersecting geometries in an unexpected way (Figure 3). 3). The structural engineering helped give this historic structure new life li fe by reimagining the interior space while retaining the industrial character 6. The Newton Building at Nottingham Nottingham Trent T rent University contained redundant redundant engineering workshops and a loading bay,, and was subdivided with numerous bay walls. Opening up the lower levels previously blocked off by by a combination of walls, vertical bracing and shear walls stabilising the nine-storey tower has radically transformed the spaces. The bracing and shear shear walls have been totally reconfigured, with new bracing and goalpost frames providing lateral stability (Figure 4), 4), to create a new ‘communal lung’ for the campus. This has secured the long-term long-term future of this Grade II* listed building (Figure 5). 5). Upgrading building services Although the structure can can often be reused in an existing building, existing services frequently need to be replaced with new systems. However However,, existing buildings should not be treated as a ‘blank canvas’ with the new service distribution – the starting point should be to coordinate and thread through the existing fabric.
environmental performance, often suff ering ering from summer overheating and significant heat loss in winter. When contemplating reuse of an existing building, an important consideration is the envelope and there are various approaches that can be adopted. The extreme is to remove remove the existing cladding entirely, entirely, stripping back to the existing structure and replacing with modern cladding. The weight of the new cladding should be considered to either match what the structure was originally designed for or supported, or to determine the extent of strengthening required. The development of the the Arup offi ce in London at 13 Fitzroy Street involved P stripping two existing 1950s buildings U R A / back to the superstructure, allowing new E T cladding to be introduced to create a T E N single of fice complex. Recladding the U R existing buildings entirely meant that an B A innovative approach could be adopted, S S E incorporating the ventilation ductwork T distribution within the facade, resulting in shallow raised floors for air supplies and maximising the floor-to-c oor-to-ceiling eiling height (Figure 6). 6). The passive performance of the facade can be improved by incorporating measures such as secondary glazing. The Newton Building has long singleglazed facades between vertical stone 4: Reconfigured stability bracing and shear walls at çêFIGURE 4: Recon
Being proactive as a structural engineer can help guide the services team towards solutions that require minimal intervention for maximum impact. This requires detailed investigation and close coordination between the structural and services team members, but can generate significant cost and programme savings. It is often important that the mechanical and electrical strategy is developed in more detail than for a new-build. The Newton Building’s previous life as a chemistry department yielded unsuspected drainage channels in the laboratory floors and penetrations through beams. These were utilised to distribute the power, data and audiovisual connections and maximised the floor-to-c oor-to-ceiling eiling height. Existing services risers were also reused for vertical distribution, with limited new risers formed through the existing floor. Other
lower levels of Newton Building, Nottingham Trent University
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January 2021 | thestructuralengineer.org
Making the most of refurbishment opportunities Climate emergency
íFIGURE 5: Opened-
up spaces at Newton Building, Nottingham Trent University
S T C E T I H C R A S N I K P O H
íêFIGURE 6: Original
(left) and refurbished building (right) at 13 Fitzroy Street, London
fins, but the envelope was vastly improved by retrofitting secondary double glazing with electronically operated blinds within the cavity between the existing and new glazing. Note that it may be necessary to accept the performance of the existing envelope without altering the fabric in any way – often an approach taken towards listed
Practical considerations Phasing Phasing the works can transform existing buildings while still partially occupied. This can be very attractive as it may avoid the need for temporary accommodation. However, However, it does require careful consideration from the outset and can also influence the structural design and construction methodology.
or heritage buildings to retain the original fabric and character. character.
The implications for building services can also be significant, which in turn
G R E B N R E T S N O V Y E L R O M
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can impact the structure. It is therefore essential that phasing of all disciplines is incorporated into the design from the beginning. At the George Green Library Library,, a key aspect was ensuring continuous operation. A detailed phasing strategy was therefore developed to coordinate construction works with providing an operational facility (Figure 7). 7). This involved first constructing the adjacent extension and lowering the existing basement. After completion of the extension, extension, the existing library operations were ‘decanted’ and re-located into the extension, allowing the major refurbishment of the existing building to be completed. This resulted in no temporary accommodation being required and generated considerable savings on potential ‘enabling’ costs. Temporary works Structural works to existing buildings often require temporary works to be considered at an early stage, as they very often will influence the structural design and construction methodology. methodology. The temporary works can require require significant structural input and may play a major role in establishing the financial viability of a project.
thestructuralengineer.org | January 2021
Planning application Making procedur procedures the mostes of refurbishment opportunities Opinion Climate emergency
It may be necessary for the engineer to highlight the temporary works as a critical item, and careful feasibility studies should be carried out so that their viability and likely costs can be assessed. Where significant, the embodied carbon of these works should be assessed as part of understanding the impact of the work. Opening up the spaces in the Newton Building described previously could only be achieved with early consideration of the temporary works. The sequence of works was carefully planned to ensure that suitable structural support was in place at all stages, that it could physically be installed and that it was suitably priced and programmed (Figure 8). 8).
éFIGURE 7: 7: Consideration of phasing to keep existing George Green Library occupied throughout works
Stephen Fernandez MEng, CEng, FIStructE, MICE
Conclusions The two parts of this article art icle provide a brief overview of the primary areas that are necessary to consider when dealing with existing buildings. Part 1 discussed ways in which engineers can identify potential opportunities for refurbishment at the early feasibility stage. Part 2 has considered ways to
maximise these opportunities through design. In order to make best use of our existing building assets, engineers should be able to provide an informed response to the client, which includes thoroughly assessing the technical feasibility of reusing the building. This assessment can help reveal potential opportunities to add signi ficant value, so the engineer can play a pivotal role in unlocking that value. Reusing an existing building is not the answer in every case, but carrying out an early assessment can reveal exciting potential opportunities that would otherwise be missed.
êFIGURE 8: Sketches 8: Sketches illustrating early temporary works considerations at Newton Building, Nottingham Trent University
Stephen is a Conservation Accredited Engineer and Associate Director at Arup in the UK, leading leadin g the civil and structural team across Birmingham and Nottingham. He leads multidisciplinary design teams on projects locally and internationally and has extensive experience working on existing buildings of di ff erent erent ages.
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REFERENCES 1) Hume I. and Miller J. (2015) (2015) ‘Conservation compendium. Part 7: Imposed load in historic buildings: assessing what is real’, The Structural Engineer , 93 (6), pp. 40–43 2) English Heritage (1994) Of fi fice c e fl oor oor loading in historic buildings, London: English Heritage 3) Wikipedia (2020) (2020) Permissible stress design [Online] Available at: https://en.wikipedia.org/ wiki/Permissible_stress_design (Accessed: November 2020) 4) Tayler H. (2020) ‘A (2020) ‘A short guide to reusing foundations’, The Structural Engineer , 98 (11), pp. 20–23 5) Fernandez S. (2017) (2017) ‘Transformation of a 1960s concrete structure – George Green Library, Nottingham University’, T he Structural Structural Engineer, 95 (10), pp. 18–24 6) Archer F. and Brunette B runette T. (2018) ‘A silo in form only’, Arup Journal , 53 (1), pp. 14–21
FURTHER READING Addy N. (2014) (2014) ‘Making ‘Making sustainable refurbishment of existing buildings financially viable’, In: Burton S. (ed.) Sustainable retrofi tting tting of commercial buildings, Oxford:
Routledge Doran D., Douglas J. and Pratley R. (2009) Refurbishment and repair in in construction construction, Caithness: Whittles Publishing
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January 2021 | thestructuralengineer.org
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Planning application Lean but procedures procedur safe es Opinion Climate emergency
3.Lean design
Structural safety when designing lean in the climate emergency The IStructE Safety, Health and Wellbeing Panel considers Panel considers the safety implications when aspiring to a lean design.
The Institution Institution of Structural Structural Engineers’ Engineers’ Safety, Health and Wellbeing Panel finds itself in a diffi cult position when considering the relationship between safety and steps to mitigate climate change. Simplistically, if you put less material in a structure, then the level of risk increases as there is less ‘redundancy’. While no one should
percentages of over-stress (or a slight erosion of factors of safety) is rarely the primary cause of failure, and that unnecessary overdesign like this is misguided.
increase member sizesthere to guard against design inadequacies, are clearly risks with going too far the other way. In addition to the life-safety impacts of failure, emissions due to demolishing, removing and rebuilding only add to the original emissions related to the structure. Therefore, in a time where we are all being urged to minimise and optimise our designs, getting it right has never been more important. In this article, the Panel, which is made up of a disparate group of people drawn from all types of practice around the world, describes some of the topics it regularly debates – a summary of members’ shared experiences of close calls and dangerou dangerouss situations
and again to resultor in underdesig either overdesign (which is wasteful) underdesign n (which is dangerous). The importance of understanding each code clause and where it needs to be applied must not be underestimated. On the subject of codes, while we agree that industry-accepted loadings are rarely achieved in offi ce buildings2, we highlight the regularity with which loads are increased above code, often driven by the ‘added value’ perceived by clients. Work by the the SEI in the USA 2 has indicated that, in buildings examined that were designed by engineers using codes, most were ‘overdesigned’ (though it should also be noted that 25% of the buildings tested were significantly
amount materialloads used(even to resist the codified of imposed if the loads themselves have not been reduced).
– and their relationship to the climate emergency. Where possible, we also make recommendations for mitigating dangers, striving to create structures that are both safe and sustainable.
below strength). Decreasing loading below code
considered how their design might fail. Avoiding sudden sudden and brittle brittle failures failures is
Codes, loads and liability Similarly, the thoughtless application of design codes has been shown time
allowances is diffi cult if we wish to avoid allowances being liable for redesign or rebuilding our work, but clearly we should also avoid this deliberate and unnecessary overspeci fication of loads. Moving to a performance-based design approach can also allow a more accurate assessment of building performance – thus reducing the
Increasing utilisation without understanding failure Increasing utilisation is not always a safe Increasing solution. We regularly see situations where small member sizes have led to impossible connection designs. Similarly, failure of connections themselves is often overlooked, with several tower crane collapses resulting from pull-out failure of the bolts at the bottom of the mast. In such cases, the marginal carbon cost of bigger bolts would have been trivial, and this sudden failure mode could have been avoided if the engineer had
Conservativism We start with the topic of conservativism,, or ‘overeng conservativism ‘overengineering’. ineering’. Material strengths are generally wellunderstood and well-defined; and codes deal with the remaining uncertainty through partial safety factors on materials and loading. There should, should, therefore, therefore, be no need to add a further ‘factor of safety’ by increasing member capacity. However, the Get It Right Initiative1 highlights that 23% of the industry’s turnover is spent on correcting errors, which might suggest that it would be prudent to add some redundancy, ‘just in case’. Our experience indicates that a few
ëFIGURE 1: It 1: It is preferable to avoid sudden failure, such as that of the Pipers Row car park in Wolverhampton in 1997
E S H
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January 2021 | thestructuralengineer.org
Lean but safe Climate emergency
always preferable (Figure 1). 1). In beams and slabs, this means ensuring failure in bending occurs prior to shear – and then ensuring ductility by verifying that the rebar yields before the concrete crushes (incidentally, this means that the concrete can never be ‘fully utilised’). Considerationss like this make sure that if Consideration failure does occur, the risk is minimised. If an engineer is to push their designs to the limit, it is even more important to think about failure. Finally, we are aware of many dramatic failures resulting from lack of durability (a possible cause of the collapse of the Ponte Morandi in Italy in 2018). Getting the detailing right is important in terms of both safety and carbon – meaning appropriate cover, free-draining steel connections, and dry timber. Rebuilding a structure due to poor detailing is an inexcusable waste of resources.
IF WE INCREASE THE SOPHISTICATION OF OUR DESIGNS TO REDUCE OUR CLIMATE IMPACT, WILL CONTRACTORS ACQUIRE THE SKILLS TO BUILD THEM? ‘unknown unknowns’ unknowns’ might be a good place to start.
worth the risk? Similarly, if we increase the sophistication of our designs to reduce our climate impact, will contractors acquire the skills to build them? And how how willll (tradit (traditiona ionally lly conse conservati rvative) ve) insurers approach these structures? They will need need to be be reassur reassured ed that that the structural integrity and durability are not compromised, proven to a recognised standard. Not a safety risk, but a project risk nonetheless.
Summary
What should we do with the structures that we are designing now to account for the diff erent erent forces that climate change is going to impose on them? We know that global heating is making the weather more extreme: increased rainfall and snow, leading to increased loading; hotter and colder weather, leading to more thermal movement; colder weather, leading to more icing, leading to more use of salts
There is no argum There argument ent that we need need to adapt our structural designs to limit the adverse impact of the built environment on the living environment. It is a fearful crisis that we must tackle, starting now. But the solution is not as simple as designing everything to work to the maximum to minimise the upfront embodied carbon. The process is much more sophisticated, certainly in terms of safety. And if we lose one structure through minor safety mistakes (which happens), we will have thrown a lot of
reusing structures is to beothers favoured. The challenge is to persuade of the acceptability of a structure when either ‘it doesn’t meet modern codes’ or ‘there are no records’. It takes a competent engineer to look at the evidence and agree that ‘it’s good enough’, taking responsibility for the durability and structural behaviour of the reused structure. The assessment assessment techniques techniques of existing existing buildings proposed by the SEI 2 would allow an examination of an existing structure to be undertaken to ascertain its strength, and to understand whether any deterioration had taken place – allowing a confident assessment of a building’s suitability for reuse to be made.
as well as more ice accretion on lattice structures. As loads loads increa increase se in the futu future, re, we may may need to strengthen certain structures, highlighting the need to allow for safe future adaptability in our designs. One might consider increasing the capacity today, but the certain increase in carbon emissions from this needs to be balanced against the possible savings in the future. Is this ‘carbon investment’ worth it?
embodied carbon To T o exerc exercise ise any any away. influence, engineers need to understand the issues, be involved in the design process at concept stage, and take a positive lead on the solutions that minimise the overall impact on the climate. To achieve this, we need to strive to be better designers, a little more outgoing and communicative, and a little less buried in the numbers. Plenty to think about!
Upskilling in response to our new philosophy of design
REFERENCES
Allowing for future future adaptability adaptability is equally important – and designing structures in a manner that will allow them to be safely reused in the future is as important as the safe reuse of our existing building stock. Appropriate consideration should be paid to the future durability, inspectability and adaptability of the structure.
buildability, the use of novel materials, buildability, and designing to the limits of the codes are all examples of changes that require the design team to embrace this new approach. We know that there are many safety implications associated with this new philosophy of design. Perhaps this highlights the need to agree more design time (and fees) to check our work more thoroughly and avoid costly mistakes. It certainly highlights the need to avoid relying on finite element analyses with little respect for the overall structural performance or an understanding of its true behaviour. A better understanding of what makes a structure safe, or where a safety margin variation is tolerable, might be a good start. We should also remember that while some clients may see this new approach as beneficial, others will want to quantify the value it adds to the project – is it
Reuse Where possible (and where this is a lower-carbon option), adapting and
Material choices We should not lose sight of why steel and concrete have become the mainstream building materials of choice. With the use of timber increasing in response to the climate emergency, we must stress that the industry’s understanding of timber is still developing in many areas, most notably fire3–5. In fact, designing with any ‘new’ material (which is what engineere engineered d timber is) carries risks that must be considered by the engineer. Acknowledging the
Changes in loading resulting from climate change
Much of the response to the climate emergency requires a new approach to structural design. Structural arrangements optimised for carbon rather than
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1) Get it Right Initiative (2020) [Online] Available Availab le at: at: htt https:/ ps://get /getitri itright ght.uk .uk.com .com// (Accessed: November 2020) 2) Task Committee on PerformanceBased Design (2018) Advocat Advocating ing
Performance-Based Design. Report to the ASCE Structural Engineering Institute of Board of Governors [Online] Governors [Online] Availab Ava ilable le at: at: www www.as .asce. ce.org/u org/uploa ploadedF dedFiles iles/ / Technical_Areas/Structural_Engineering/ Content_Pieces/2018-sei-advocating-forperformance-based-design-report.pdf (Accessed: December 2020)
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3) Law A. and Hadden R. (2020) ‘We need
to talk about timber: fire safety design in tall buildings’, The Structural Engineer , 98 (3), pp. 10–15 4) Law A. (2019) Fire safety design:
[email protected]
we need to talk about timber [Online] Available Availab le at: at: www www.ist .istruc ructe.o te.org/re rg/resour sources ces/ / training/ fire-safety-design-we-need-totalk-about-timber/ (Accessed: November 2020) 5) Deeny S., Lane B., Hadden R. and Lawrence A. (2018) ‘Fire safety design in
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modern timber buildings’, The Structural Engineer , 96 (1), pp. 48–53
thestructuralengineer.org | January 2021
Planning application Timber procedur procedures and carbon es sequestration sequestration Climate emergency Opinion
2.Low carbon
Timber and carbon carbon sequestration
Will Hawkins discusses Hawkins discusses carbon sequestration and end-of-life processes in timber structures, and the implications for sustainable decision-making in structural design. Introduction
reported within Module A, or alongside it as a negative emission, it can create the counterintuitive impression that using timber excessively can have environmental benefits. The IStructE guide g uide therefore advises advis es that sequestration should only be aggregated with emissions when endof-life values are also included, where the stored carbon is typically cancelled
The first step towards reducing the embodied carbon of construction is calculating it reliably and repeatably, and it is therefore timely that a strong consensus has formed around a lifecycle assessment (LCA) methodology based on BS EN 15978 1. This standard underpins the recent guidance from the IStructE2, and breaks a product’s down into production (Module lifecycle A), use (B), end of life (C) and potential recovery/reuse (D). As more of these thes e stages are included within an LCA’s scope, a more complete picture of impacts is provided. However, often only Module A is included includ ed due to the considerable cons iderable uncertainty surrounding end-of-life processes. For steel and concrete, which both feature high-energy production processes, Module A dominates lifecycle emissions. The production emissions for timber products, from harvesting, drying and sawing, are also significant; however, the mass of carbon absorbed by the tree and
îFIGURE 1: Carbon stored in trees, debris/ litter and soil for typical Sitka spruce plantation with 50-year rotation period, compared with forest left unmanaged (with data from Forestry Commission4 )
out by re-emission at the of life. This articl e provides article a raend tional approach to the incorporation of sequestration in embodied carbon calculations, and provides recommendations for eff ective ective climatefocused timber design: sustainable sourcing, long lifespans and e ffi cient use of materials.
Rationalising timber sequestration Growing trees and locking away carbon in timber buildings has been proposed
stored within the material itself can be even greater. Although this thi s carbon is typically re-released at the end of life due to combustion and/or decomposition, there are climate bene fits of sequestering atmospheric carbon within long-lived timber products which act as a carbon sink 3. For example, delaying carbon emissions reduces cumulative climatic energy input, buys time for adaptation of both natural and man-made systems, reduces the possibility of reaching dangerous climate ‘tipping points’, and increases the potential for permanent storage through future technologies such as carbon capture and storage. However, accounting for sequestered carbon is often a source of debate, confusion and inconsistency. When sequestration is 18
as a potentially significant carbon sink 3. But would it be better, from a carbon perspective, to leave forests to grow naturally? Figure 1 shows 1 shows the changes in carbon storage within a typical commercially managed Sitka spruce forest with a harvesting cycle of 50 years, using data from a Forestry Commission report4, and compares this with an equivalent unmanaged forest. This reveals several important impor tant points. Carbon uptake in newly planted saplings is initially slow, but then accelerates as these become established. In an unmanaged forest, sequestration continues until the total carbon eventually tends towards a steady state. A managed forest al so achieves also a constant carbon storage, albeit cyclic between each harvesting period and lower than that of an unmanaged forest. However, it also stores carbon in the products produced from it. If these are amassed suffi ciently over time, then the total carbon sequestered accumulates and could eventually be greater than that of an unmanaged forest. Considering these observations, the approach to sequestration taken in this article is based on the following principles:
January 2021 | thestructuralengineer.org
Timber and carbon sequestration sequestration Climate emergency
Ò| Although Although
an understanding under standing of th e variation in carbon stored within a forest is informative, this carbon is not typically included in a building’s LCA scope. Instead, only the carbon in the timber product itself should be included, in line with typical product LCA methodologies5. Ò| Harvesting, processing and constructing a timber building releases a ‘spike’ of carbon into the atmosphere, whereas sequestration occurs gradually. Ò| Carbon accounting should always start at zero – credit should not be taken for a tree planted 50 years ago, even if this eventually ends up being used to build the structure under investigation. Ò
| Where trees are harvested and not replaced (deforestation), no
sequestration should be accounted for, in line with current European standards 5. This article recommends using sequestration values corresponding to the timber structure itself, such as those given in the ICE database 6 and IStructE guidance 2, rather than the entire forest from which it came. However, the assumed timing of sequestration is that of the trees which replace those harvested, starting from zero and increasing until the next harvesting cycle, assumed here to be 50 years. This ‘forward-looking’ ‘forward-l ooking’ approach is is characterised and recommended by Helin et al .7, and its implications are explored hereafter.
Comparing concrete, steel and timber building options This section sectio n compares the embodied embodi ed carbon of concrete ( flat slab), steel (composite) and timber (crosslaminated timber (CLT) with glulam frame) options for a six-storey building structure. The designs are those featured in a recent Buro Happold study8, with all options featuring a concrete core and foundations. The calculation methodology follows IStructE guidance2 and is detailed in a separate publication9. The analysis is cradle-to-grave; cradle-to-gr ave; Module D benefits (beyond the system boundary), which are reported separately in current standards1, are not included. This has the same eff ect ect as
éFIGURE 2: 2:
IT IS STILL BETTER (FOR THE CLIMATE) TO BUILD NOTHING AT ALL THAN A TIMBER BUILDING
Cumulative embodied carbon emissions for concrete, steel and timber options of example building structure8, including three scenarios for timber components
The cumulative carbon car bon emissions over a 120-year period for each structure are shown in Figure 2. 2. The concrete structure has the highest initial (Module A) emissions, followed by steel and then timber, for this structural
assuming that all material production is e ff ectively ectively decarbonised by the end of the building’s 60-year lifespan, in line with UK law, since o ff set set materials would also be zero carbon. Three carbon life cycles are considered for timber: 1) Typical sustainably sourced UK timber with replanting (sequestration) and a large carbon emission at
arrangement. For concrete andsee steel, the use and end-of-life stages only small changes in embodied carbon. For timber, however, subsequent changes are significant. In timber scenario 1, sequestration causes a small, temporary period of negative carbon emissions. This lasts only while the building is in use, ending abruptly upon demolition. If the structure is in use for 100 years, it would be carbon-negative for half its lifetime, whereas the same structure demolished after 40 years would never reach negative carbon. The dynamic climate cli mate impacts of this temporary carbon storage are considered, for a similar case study, in a
the end of life from recycling (55% by mass), incineration with energy recovery (44%) and land fill (1%)10, as given in the IStructE guidance2. 2) As above, but without replanting or sequestration, representing a worst-case scenario (non-sustainably sourced timber, uncommon in the EU). 3) An optimistic scenario which combines sustainable forest management (sequestration) with minimal emissions at the end of life. It has been suggested that up to 90% of combustion emissions could potentially be captured using bioenergy with carbon capture and storage (BECCS)11. This has been represented here by a 90% reduction in Module C3–4 emissions. Carbon capture is not permitted in a standard LCA 5, but is considered here as a hypothetical scenario.
separate publication9. Despite the large Module C emissions, the total cradleto-grave carbon is still lower than for the concrete and steel options in this scenario. In scenario 2, without sequestration, the significant release of carbon at the end of life causes the timber option to have the largest total embodied carbon. This highlights highli ghts the essential ess ential importance import ance of sourcing sustainable timber which includes replanting, as is typical in the EU. Scenario 3 shows the potential for a zero-carbon timber building if end-oflife emissions can be avoided. This is an optimistic scenario, relying on technology which does not currently exist at a meaningful scale. It would therefore be misleading to consider this in a typical embodied carbon calculation, and not permissible using today’s standards1,5. Even in this
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thestructuralengineer.org | January 2021
Planning application Timber procedur procedures and carbon es sequestration sequestration Opinion Climate emergency
REFERENCES 1) British Standards Institution (2011) BS EN 15978:2011 Sustainability of
construction works. Assessment of environmental performance of buildings. Calculation method , London: BSI 2) Gibbons O. and Orr J.J. (2020) How How to calculate embodied carbon, carbon, London: IStructE Ltd 3) Churkina G., Organschi A., Reyer C.P.O. et al. (2020) ‘Buildings as a global carbon
sink’, Nat. Sustain., Sustain., 3, pp. 269–276, doi: https://doi.org/10.1038/s41893-019-0462-4 4) Morison J., Matthews R., Miller G. et al . (2012) Forestry Commission Research Report: Understanding the
event, the large initial emission from construction is not avoided, and still contributes to global warming for several decades9. It can therefore be concluded that, even under best-case conditions, it is still better thea climate) to build nothing at all(for than timber building. Although this study shows concrete as the highest-carbon option, and timber as the lowest, these results are specific to the designs in question and do not represent a fixed hierarchy. This timber design is very v ery light, featuring 100mm thick CLT floors, and the concrete flat slabs are relatively ineffi cient at 9m spans compared with ribbed or post-tensio post-tensioned ned alternatives. Figure 3 illustrates 3 illustrates the point that wasteful or inappropriate use of timber could readily have a greater impact than a more effi cient concrete or steel alternative: it is always better to use less of any material. We cannot quickly increase total timber supply, and must therefore use this valuable resource sparingly to enable maximum potential uptake across the sector.
Conclusions This article articl e has demonstrated demonstrat ed an approach to accounting for timber carbon sequestration in line with established guidance. Through a simple case study, several conclusions can be drawn: Timber must be sustainably sustainabl y sourced, Ò| Timber with replanting, for any potential embodied carbon benefits over concrete and steel to be realised. Thankfully, sustainability sustainabilit y certi fication schemes (such as those run by the Programme for the Endorsement of Forest Certification and the Forest Stewardship Council) are well established and often a legal requirement for import.
Ò| End-of-life
carbon fluxes are significant for timber structures. The climate bene fits of timber can therefore be maximised by prolonging the life of structures, reusing timber components or
éFIGURE 3: Wasteful 3: Wasteful use of timber could have greater impact, in both short and long term, than effi ciently designed concrete and steel alternative
5) British Standards Institution (2012)
recycling into newsequestered materials, all of which keep carbon out of the atmosphere. Ò| It is hypothetically possible for timber to have a negative cumulative embodied carbon, in the long term, when it is both sustainably sourced and end-oflife emissions are also avoided, e.g. through new technologies such as BECCS. This cannot be relied upon in a typical embodied carbon analysis, however, and several decades of net positive emissions still occur. Ò| It is better to build nothing at all than a timber building. Similarly, wasteful use of timber can be
BS EN 15804:2012+A2:2019 15804:2012+A2:2019 Sustainability of construction works. Environmental product prod uct dec declara laration tions. s. Core Core rule rules s for for the the product prod uct cat category egory of const construc ruction tion produ products cts,, London: BSI 6) Jones C. and Hammond G. (2019)
Inventory of Carbon and Energy (Version (Version 3.0) [Online] Available at: https:// circularecology.com/embodied-carbonfootprint-database.html (Accessed: November 2020) 7) Helin T., Sokka L., Soimakallio S., Pingoud K. and Pajula T. (2013)
‘Approaches for inclusion of forest carbon cycle in life cycle assessment – a review’, GCB Bioenergy , 5 (5), pp. 475–486, doi: https://doi.org/10.1111/gcbb.12016 8) Roynon J. (2020) Embodied carbon:
more damaging than an e ffi cient design in concrete and steel.
structural structu ral sen sensiti sitivity vity stu study dy [Online] [Online] Availab Ava ilable le at: at: www www.ist .istruc ructe.o te.org/re rg/resour sources ces/ / case-study/embodied-carbon-structuralsensitivity-study/ (Accessed: November 2020)
Acknowledgements Acknowled gements With thanks to Aurimas Bukauskas, Sam Cooper, Steve Allen, Jonathan Roynon and Tim Ibell for their thoughts, contributions and expertise. Will Hawkins MEng, PhD Will Hawkins is a Lecturer in Structural Engineering Design at the University of Bath. His research and teaching focuses on pathways to zero-carbon building structures, through design optimisation, novel structural systems and low-carbon materials.
9) Hawkins W., Cooper S., Bukauskas A. et al. (In press) ‘Rational whole-life carbon
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carbon and greenhouse gas balance of forests in Britain [Online] Britain [Online] Available at: www. forestresearch.gov.uk/research/ understanding-the-carbon-andgreenhouse-gas-balance-of-forests-inbritain/ (Accessed: November 2020)
assessment using a dynamic climate model: Comparison of a concrete, steel and timber building structure’, Structures 10) Wood for Good (2017) Environmental Product Declaration: 1m 3 of kiln dried planed plan ed or mach machine ined d sawn sawn timb timber er used used as structu stru ctural ral timb timber er [Online] Available at: https://woodforgood.com/assets/ Downloads/EPD/BREGENEPD000124.pdf (Accessed: November 2020) 11) Committee on Climate Change (2019)
Net Zero: The UK’s contribution to stopping globa stopping globall warmi warming ng [Online] [Online] Available at: www.theccc.org.uk/publication/ net-zero-the-uks-contribution-to-stoppingglobal-warming/ (Accessed: November 2020)
January 2021 | thestructuralengineer.org
2021 CPD course programme Online training opportunities to help meet your IPD/ IPD/CPD objectives (online) Temporary works appreciation 14 Janaury 2021
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Professional guidance
Business Practice Note | No. 37
BUSINESS PRACTICE NOTES
No. 37
have been developed by the Institution’s Business Practice and Regulatory Control Committee to provide guidance on aspects of running a practice and project management.
Checking the work of
www.istructe.org/bpns
another engineer
Stephen Gregson provides Gregson provides guidance on how to approach an appointment to review a design developed by another engineer. engineer.
Introduction The aim of this this not note e is to highl highlight ight the key issues that arise when reviewing the work of another structural engineer. The review may be an inherent part of the project design process, particularly for a major project, or it may have been called because of concerns over the design. A revie review w may may also so be be requ required ired to accord with regulations or local standards. For example, BS 5975:20191 fi
either resolve the conflict or decline the appointment. A perceived conflict of interest could undermine the checking engineer’s report and findings.
Ethical considerations The che check ck and rev review iew must be undertaken professionally and factually, so as not intentionally to damage the reputation of the original designer. It is also important that the checking
Issues are likely to arise during the checking process and these should be discussed with and, if at all possible, resolved with the original engineer. However, should there be unresolvable issues, then the checking engineer should quickly make their concerns clear to both the client and the original engineer.
Remaining impartial
identi es categories of design check for temporary works, and most highways, railways and some other structures (such as wind turbine bases) require specific design checks. This not note e does does not not apply apply to: Ò| internal checks and reviews of work within the checking engineer’s organisation (e.g. routine checking as part of a quality management process within a project’s design programme) Ò| expert witness work Ò| work associated with replacing another engineer Ò| reviewing another engineer’s work during legal proceedings or where they are contemplated.
engineer works a positive, collaborative manner with theinoriginal engineer. The briefing of the checking engineer’s staff for the project should make this clear. The che checkin cking g engin engineer eer must norm normally ally ensure that the original engineer has been informed that the review will be taking place. After all, if you put yourself in the position of the engineer whose work is to be checked, you would want to know if another engineer was appointed to check your work. As note noted, d, the the che checki cking ng engin engineer eer must comply with the IStructE Code of Conduct , in particular clauses: 1) Act with integrity and fairness and in accordance with the principles of ethical behaviour.
The checki che cki ngshould ng engineer engin eer must t remai retomai n impartial and notmus seek supplant the original design engineer. If the checking engineer is later appointed to replace the original designer, it could reduce the credibility of the checking engineer’s findings and report if the checking engineer could be considered not to have been impartial in their checking work.
Before undertaking a review, engineers should read the IStructE
5) Undertake only those tasks and accept only those appointments for which they are competent. 7) Not maliciously or recklessly injure or attempt to injure the reputation of another person. 8) Avoid conflicts of interest.
the specific scope for the appointment. A partic particula ularr point point is wheth whether er the the scope scope of the check is to be technical only or is to include checking of other parts of the engineer’s appointment, e.g. their scope of work, contract documents, health and safety risk assessments, and buildability. The che checkin cking g engin engineer eer shou should ld make make recommendations to the client to extend their scope if they believe this to be necessary, and that other areas of the design or documentation need to be checked. The che checkin cking g engin engineer eer has a duty duty to warn (and discuss with) the original engineer, and then the client, of any health and safety issues that they may find. This duty exists whether or not it is explicitly stated in the scope of work. Warning other relevant parties may then also be necessary. Ideally, the scope should include
Code of Conduct and Guidance Notes2,
particularly Section 7.1 (more detail is included below). The ICE Legal Note, Reviewing the work of another Engineer and rep replac lacing ing ano anothe therr Engine Engineer er 3, is also a
very good reference.
Competence and conflicts of interest An engin engineer eer who is appr approac oached hed to undertake checking work must be satisfied that they are competent in the specific field of engineering in question, and that they have appropriate staff to to undertake the work. To T o avoid avoid conflicts of interest, they must check what other connections they may have to members of the design team. If they do have connections, they must raise them with the client and
There The re may be exce exceptio ptional nal occ occasio asions ns – usually in a legal case – when the checking has to be confidential and the original engineer is not told of the check 3. It must be stressed, however, that this should very much be the exception. The rea reason son for the confidentiality should be clear and the client should be informed that the report and conclusions are confidential. They must only be regarded as preliminary until the checking engineer has had a full dialogue with the original engineer and design team, and received all relevant information.
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Scope of work As with with all pro project jects, s, clear clearly ly de defining and agreeing the scope of work with the client is essential. In checking and review work, the scope can vary significantly. The che checki cking ng engin engineer eer,, clien clientt and and origin original al engineer, in particular, need to understand
January 2021 | thestructuralengineer.org
Business Practice Note | No. 37 37 Professional guidance
checking the designer’s risk assessments. It may also be necessary to contact the principal designer (according to the CDM 2015 Regulations4) concerning any health and safety issues that arise. The che checkin cking g engin engineer eer shou should ld not not exceed their scope of work. It is easy to get ‘drawn in’ and start proposing solutions to problems and helping to design or redesign the project; engineers are problem-solvers after all. However, unless their scope of work says otherwise, the job of the checking engineer is normally only to check the design and report on it. Notwithstanding this, a proactive, cooperative attitude always helps. If the checking engineer comes to the conclusion that the original design is seriously at fault, it is good practice to recommend to the client that a further opinion be sought.
Limit of liability The che checki cking ng engin engineer eer nee needs ds to to have have appropriate professional indemnity insurance Theirwork liability should be only for thecover. checking that they have undertaken using reasonable skill and care. In the appointment, it is important that the extent of liability in contract is limited, preferably with a clear cap, and that this has been agreed with the client. There The re may also be liabi iability lity in tort tort by the the checking engineer to the original engineer and others. The che checkin cking g engin engineer’ eer’s liabili liability ty for for ‘fitness for purpose’ should be reviewed carefully, and is normally specifically excluded; it is rarely covered by professional indemnity insurance. This is important for design-and-build projects, which can include fitness-for-purpose clauses within the contractor’s contract. In these cases, the checking engineer may inadvertently pick up this higherlevel duty of care by the reference in their appointment to the design-and-build contractor’s contract.
Contact with original engineer With the client’s agreement, the checking engineer should contact the original engineer. It is usually greatly to the bene fit of all that a dialogue be started between the engineers. Checking should, if possible, be undertaken in a collaborative manner, liaising with the engineer whose work is being checked and discussing queries and concerns before reporting. Remember that the original engineer may have devoted a great deal of time and consideration to the development of their design, the nuances of which may not be immediately recognisable to a checker initially unfamiliar with a project.
The norm normal al qual quality ity man managem agement ent processes must, of course, be followed in all dialogue. All meetings, phone calls, emails and other contacts (face-toface, Teams, Zoom, etc. with express consent), as well as any decisions agreed between the engineers, should be carefully recorded, in addition to logging information in and out, minuting of meetings, etc. The engineer neer whose whose work is being being checked should be made explicitly aware that such records are being made and circulated. If records of meetings with them are circulated, particularly to their client, they should be invited to comment on draft records before they are more widely circulated. One of the first – and key – steps for the checking engineer is to try to obtain any further information that the original engineer may have which could be relevant to the checking. It is very possible that the documents that are issued (at least initially) to the checking engineer may not contain all the information needed, and therefore
The rep report ort (whe (whether ther in draft draft or final form) should only be sent to the client, and not copied to other parties unless instructed to. This note has been prepared by Stephen Gregson MA, CEng, MIStructE, MICE on MICE on behalf of the Institution of Structural Engineers’ Business Practice and Regulatory Control Committee. Members are reminded that they should always comply with the legislation of the region in which they are working and should be aware of any jurisdi juri sdictio ctions ns spec speciific to that region. Business Practice Notes are provided as guidance to members, but do not form part of the Regulations and/or Laws of the Institution. All members are obliged to abide by the Institution’s Code of Conduct.
REFERENCES
1) British Standards Institution (2019) BS 5975:2019 Code of (2019)
may not tell the may wholehave story. Forhanded example, the documents been over by a lay person who does not understand what they contain or omit. Before starting the checking, it is essential that the checking engineer has all the facts (e.g. design brief and assumptions) about the design, and also preferably about the design process.
practice for tempor practice temporary ary works works procedures proced ures and and the permiss permissible ible stress design of falsework falsework , London:
BSI 2) Institution of Structural Engineers (2019) Code of conduct and guidance guidance notes [Online] Available Avail able at: at: www.istruc www.istructe.org/ te.org/ about-us/istructe-code-of-conduct/ (Accessed: November 2020)
Report The che checki cking ng engin engineer’ eer’s report report mus mustt be factual and professional, and not include hearsay, unnecessary and/or unverified opinion, or anything of a defamatory nature. All info informat rmation ion rec receive eived d must must be be logged and, equally importantly, information that is not available must also be recorded and stated in the report. Often these lists develop during the checking process as more information, or the lack of it, comes to light. Everything relevant in the checking process should be recorded, e.g. any events where the original engineer has not been cooperative, the cooperation of other members of the design team, contact with building control, any limitations on the engineer’s review. All issu issues es raise raised d by the che checki cking ng process must be logged, including any resolution of issues, redesign, strengthening, the checks of any strengthening or improvement works, and issues that remain unresolved. Often this is done using a spreadsheet so that the process of raising an issue, discussions about it, and its resolution can be tracked.
23
3) Metherall A. (2015) ICE Legal Note: Reviewing the work of another Engineer and replacing another Engineer [Online] [Online] Available at:
www.ice.org.uk/knowledge-andresources/best-practice/reviewingthe-work-of-another-engineer (Accessed: November 2020) 4) The Construction (Design and Management) Regulations 2015, SI 2015/51
FURTHER READING HAVE YOUR YOUR SAY SA Y
[email protected]
Royal Academy of Engineering and Engineering Council (2017) Statement of Ethical Principles [Online] Available at: www. engc.org.uk/professional-ethics (Accessed: November 2020) South African Institution of Civil Engineering. Joint Structural Division (2016) Guide to Good Practice for Structural Engineering,
@IStructE
#TheStructuralEngineer
2nd ed. [Online] Available at: www.jsd.co.za/wp-content/ uploads/2019/07/guide-togood-practice-2nd-edition.pdf (Accessed: November 2020)
thestructuralengineer.org | January 2021
Technical T Technical echnical Guidance Note | Level 3, No. 2
TECHNICAL GUIDANCE NOTES are published by The Institution of Structural Engineers to provide guidance to engineers in the early stages of their careers. This note has been prepared by AECOM on behalf of the Institution. www.istructe.org/tgns
CHRIS O’REGAN BEng(Hon BEng (Hons), s), CEn CEng, g, FIStr FIStructE uctE,, FICE FICE
Formerly Associate Director, Building Engineering, AECOM, London, UK
Glossary
Restrained movement – – axial shortening of vertical elements occurs within a state of containment and will induce stresses in the structure as a result.
Vertical movement of structures
Level 3, No. 2
Axial Ax ial sh shor orte tenin ning g Introduction
Until the late 19th century, the tallest buildings in the world were religious structures, whose spires and domes pierced the sky. This changed in 1885, with the erection of the Home Insurance Building – the world’s first tower block – in Chicago. Framed in structural steel, it stood 42m high with 10 storeys of offi ce space. The new technology demonstrated in Chicago caught on in cities across the world and the Home Insurance Building was quickly dwarfed by subsequent structures as the race to the sky began. However, it brought with it a signi ficant concern: the vertical movement of vertical loadbearing elements in multistorey structures. This Technical Guidance Note concerns the concept of axial shortening – a phenomenon that occurs in vertical loadbearing elements within all structures, but whose eects are especially pronounced in those over 15–20 storeys high. Reinforced concrete frames are most prone to axial shortening due to the impact of shrinkage and creep on composite materials. This note covers the causes of axial shortening, how it is assessed and predicted, and the mitigation measures structural engineers and building contractors employ to counter its eects, particularly in relation to fit-out and facade installation following building construction. The note also refers to ways in which current codes of practice o er a means to analyse structures for axial shrinkage. Causes of axial shortening
Terms and concepts
This Technical Technical Guidance Guidance Note describes describes axial shortening as vertical movement of a structure. To understand this concept, it is important to return to the principle that vertical elements formed from reinforced concrete concrete will move in some way as soon as they are constructed. The predominant predominant movement referred referred to in this note is a vertical contraction of the loadbearing element, which will cause an overall downward displacement. Structural engineers and building contractors should be aware that axial shortening occurs in all structures, regardless of their height. However However,, it only becomes a significant issue when a building structure is taller than 15–20 storeys, depending on its structural layout and the geometrical properties of elements that make up its frame. The causes of axial shortening relate relate to the ways in which vertical elements within a structure react to external forces that they are exposed to. These range from the more obvious application of loads, to the more diffi cult task of predicting interactions of components within a composite material, such as reinforced concrete. Time-dependen T ime-dependentt properties also need to be considered and are covered in depth later in this note.
The principal causes causes of axial shortening shortening of structures are described below. Although each cause cause is described in isolation, it is important to bear in mind that they all occur at some point in the life of the building and, therefore, have a cumulative eff ect ect on the vertical movement of the structure. Each cause should be allowed for when designing and detailing components that have a direct interaction with, or connection to, the primary structure. Axial strain
As stress is applied to a vertical or near-vertical nearvertical support element, it is accompanied by strain that is exhibited by a contraction of the element as it resists the load it has been designed to support. The magnitude of the axial strain increases incrementally as more load is applied (as more floors are built and/or as further load is added at already constructed floors, e.g. superimposed dead load and imposed load). All materials exhibit some form of axial shortening, but the behaviour is most pronounced in timber frames and reinforced concrete structures. Reinforced concrete is particularly complex to assess due to the interaction between the materials it is formed from, i.e. concrete and mild steel. For concrete 24
structures, the eff ects ects of shrinkage and creep should also be considered when calculating the magnitude of axial shortening. Shrinkage
It is important to di ff erentiate erentiate between shrinkage and creep. Both add complexity when trying to predict the amount of axial shortening that will occur in a structure, and both need to be considered to determine the overall magnitude of this movement. Shrinkage of concrete occurs as it dries and solidifies while it is curing. Many diff erent erent types of shrinkage aff ect ect reinforced concrete, concrete, but it is best described as a volumetric change due to the loss of moisture by evaporation/ drying. Shrinkage is a time-dependen time-dependentt deformation which reduces the volume of an element without any change to the externally applied forces. It is a complex phenomenon which is a function of the properties of the drying and ageing concrete, and continues for many years, or even decades, after the concrete has been cast. Shrinkage due to drying should not be confused with autogenous shrinkage, a behaviour of the concrete that occurs immediately as curing starts. Autogenous A utogenous shrinkage is quite insignificant in terms
January 2021 | thestructuralengineer.org
Technical T echnical Guidance Note | Level 3, No. 2
of magnitude when compared with shrinkage due to drying. The composite nature nature of a reinforced reinforced concrete element results in shrinkage that is dependent on the bond between the steel (which shrinks on cooling) and the concrete (which shrinks primarily with loss of moisture and on cooling). Built-in stresses are transferred from the concrete to the steel reinforcem reinforcement. ent. Figure 1 shows 1 shows a section through a reinforced concrete column with an exaggerated shrinkage of concrete, which is restrained by the steel. This results in a transfer of force into the steel. Since steel has a much greater modulus of elasticity, the overall e ff ect ect is a reduction in the amount of shrinkage that would otherwise occur in the absence of reinforcement. Construction sequence
During the construction of a building’s frame, the vertical elements undergo
íFIGURE 1: Shrinkage 1: Shrinkage within reinforced concrete element
Finally, the age at which elements are installed into the structure in fluences the magnitude of axial shortening. As precast concrete columns are cast in advance and stored before delivery to site, they will have undergone initial shrinkage by the time they are erected. In contrast, shrinkage of adjacent in situ concrete shear walls in cores occurs
Technical
concrete, the relative humidity of the environment the concrete was cast in, and the geometrical properties of the element. Creep eff ects ects should be added to the elastic axial shortening eff ects ects and the shrinkage component of the shortening, and – where necessary – temperature eff ects. ects. Creep is a very complex behaviour aff ected ected by time-depende time-dependent nt concrete properties and a full discussion is beyond the scope of this note. Designers evaluating axial shortening should ensure they are fully conversant with this behaviour. Detailing for axial shortening
In the design of taller buildings, axial shortening is a particular concern when non-structural elements, such as partitions, finishes, ceilings and facade components, are subsequently added or connected to the structure. Some of
two forms of axial shortening that Figure are related to the application of load. 2a shows the first shortening to occur, which is caused by the permanent self-weight of the floor structure as this load is applied directly to the vertical supporting elements, causing them to contract as a result of elastic shortening. This is known as pre-installation pre-installation shortening. Each subsequent floor structure applying load to the initial supporting elements will cause additional vertical movements to occur in those elements. This is known as post-installation post-installation shortening (Fig. shortening (Fig. 2b). 2b). It is important to note that these phenomena aff ect ect all subsequent vertical
during construction. Even after the bulk of the shrinkage eff ects ects and the initial elastic shortening have occurred, a reinforced concrete structure will continue to shorten during the life of the building. Structures wrapped in an enclosed facade will not typically be aff ected ected by temperature, so the predominant factor causing this ongoing shortening is creep*. Creep is defined as the shortening of an element over time due to the application of a sustained load. With respect to axial shortening, creep occurs within vertical support elements such as walls and columns and will continue
these elements are designed to achieve very tight construction tolerances, and facade or curtain wall systems must also accommodate lateral movement of the building due to external factors such as wind. Greater understanding of taller structures and research into concrete behaviour means that structural engineers can design buildings to mitigate some of the eff ects ects of axial shortening. However, However, taller towers with complex structural systems make the task of designing for these eff ects ects evermore complex, so a holistic solution is required. Delaying the installation of brittle, non-structural components, notably
supporting elements at each floor in the structure and are additive to other factors, such as shrinkage and creep. The act of construction construction itself impacts impacts on the magnitude of axial shortening. Methods and types of material used are variables that alter the way in which the concrete structure moves during its lifetime. Creep movements form a significant part of axial shortening and are related to the period during which loading is applied and the sequence of construction for each storey of the building. The way way in which the concre concrete te is poured has an influence on the shortterm shrinkage of the vertical concrete elements. Typically, Typically, the level at which the concrete is poured is a theoretical one and thus some shortening is recovered. This become becomess part of the the expected expected relative location of the floor slab, which will continue to move after it has been cast.
to cause shortening for the life of the building. Many external and internal sources impact on the magnitude of creep. These include the the strength grade grade of the
cladding, will alleviate some of the eff ect ect of post-installation frame shortening on these elements, as some axial movement will already have occurred. However, engineers should take care
Creep
a) Pre-installation shortening
êFIGURE 2: Pre- and post-installation shortening
b) Post-installation shortening
* Nonetheless, shrinkage does still occur in enclosed structures. In environments with a low relative humidity, shrinkage can be the governing component of axial shortening in extreme cases.
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Technical T Technical echnical Guidance Note | Level 3, No. 2
íFIGURE 3: Modifying 3: Modifying arrangement of structure to counter differential axial shortening
when preparing movement reports to highlight the creep component that is likely to occur after these elements are attached. A further issue that that should be considered is the potential for diff erential erential axial shortening between adjacent vertical elements that have significantly diff erent erent geometry. For example, the vertical movement of a wall in a reinforced concrete concrete shear core will be diff erent erent to that of an isolated column. Therefore,, if they are relatively Therefore relatively close to one another, a horizontal element fixed between them could experience significant rotation. To T o counter this problem, problem, the layout of the structure should be developed to mitigate the impact of diff erential erential axial shortening by placing vertical elements suffi ciently far apart from one another (Figure 3). 3). In a tall building, the largest diff erential erential axial shortening between columns and core walls will typically occur at around two-thirds to three-quarters of the height of the structure (Figure 4). 4). This is primarily because the higher up the tower a vertical element occurs, be it a wall or column, the smaller the imposed gravity load to cause shortening. In addition, higher elements will experience less shortening due to creep eff ects ects as they are subjected to a lower sustained load. The same is true for column-tocolumn diff erential erential movement. This is especially crucial for edge and corner columns, where excessive diff erential erential settlement arising due to diff erently erently loaded columns (e.g. typical columns with typical loads and either columns with significantly heavier loads due to transfers or columns which extend
ìFIGURE 4: Differential shortening between core walls and columns in tall building
further up a building than adjacent ones) can impact cladding design. Cladding panels typically have limited capability to accommodate ‘racking’ deformation unless they have either opening joints or wider and specially detailed joints. It is also possible to mitigate the impact of axial shortening by altering the material composition of vertical elements. By increasing the amount of reinforcement reinforce ment in a column, provided the section size is the same throughout its length, it is possible to reduce the creep that occurs between the concrete and the two materials. This is not always practical, practical, however, however, given the strict requirements of BS EN 1992-1-1 with respect to reinforcement within columns, as well as other issues such as cost, material effi ciency and structural embodied carbon. Baker et al . (2007) proposed an analytical method to account for reinforcement reinforce ment by consideration of strain compatibility between the concrete and the steel. This formulation is often
It is also possible to predict the amount of axial shortening that will occur so that the placement of horizontal elements that are supported by vertical can be pre-set at a higher relative level. This is especially useful useful when vertical supporting elements have a large cross-section and are therefore more susceptible to eff ects ects that lead to axial shortening. However, due to lack of certainty when calculating predicted vertical movements of structures, this method is not often adopted. The adoption of precast elements elements eff ectively ectively removes some of the shrinkage and reduces creep due to the age of the concrete at first loading. This is especially effi cient when used in combination with pre-setting in situ concrete elements. Another approach approach is to install horizontal members with rigid connections between columns and core walls that are likely to be subject to significant axial shortening. These elements reduce the impact of di ff erential erential
incorporated structural packages thatinpredict the analysis magnitude of axial shortening of building structures. One approach to reduce the impact of axial shortening on the structure is to limit the applied stress onto vertical support elements by optimising their size, location and number, thereby reducing the amount of axial shortening they exhibit.
settlement; to be designed tohowever, withstandthey the need high shear forces that will be generated as the movement occurs. Any modifications to the structure will have an impact on its overall sti ff ness ness and will attract more load, which could become problematic in terms of element and connection design. It is also possible to relieve the stress
N O I T C U R T S N O C & G N I R E E N I G N E O O W E A D
26 January 2021 | thestructuralengineer.org
Technical T echnical Guidance Note | Level 3, No. 2
Technical
Analysis of axial shortening shortening is quite complex due to the number of variables that need to be considered. It requires a sound understanding of how a vertical reinforced concrete element contracts during its lifetime within a building structure. This note has explained explained the causes causes of axial shortening and mitigation measures
Most structural analysis packages permit a ‘staged construction analysis’, whereby a structure can be ‘built’ virtually in time and ‘loaded’ sequentially according to an anticipated construction sequence, allowing for the eff ects ects of elastic deformation, creep and shrinkage. The analysis will allow for the development of these eff ects ects over time, as well as those of material properties (e.g. elastic modulus) where relevant. While prediction of movements related to reinforced concrete is much more diffi cult than in steel construction, such models can provide insight into the magnitude of movements which will occur during construction, at the time of component installation, and after installation during the lifetime of the structure. However, even with the most sophisticated modelling eff orts, orts, movements observed in practice may vary from those predicted. It is therefore advisable to validate such models and
that are on used to address the issue, with a focus creep and shrinkage. The nature and behaviour of the composite materials that form reinforced concrete structures require engineers to carefully plan layouts to allow for for,, or mitigate, the eff ects ects of axial shortening. A common design example that an engineer may need to consider is a steel-framed superstructure fixed to a reinforced concrete concrete core. The core will exhibit axial shortening that is diff erent erent from that of the steel frame, requiring allowances to the erection of the steelwork to accommodate the resulting diff erential erential movement. BS EN 1992-1-1 provides guidance on how to calculate the eff ects ects of creep
bracket properties betweenbehaviours upper and and lower bound attributes, so that the engineer can understand the full range of movement. To T o assess the validity of models models used to determine the impact axial shortening has on a structure, based on the method contained in BS EN 1992-1-1, it is possible to apply simple rules of thumb and compare them to the results: Ò| Vertical Vertical concrete elements elements move by approx. 1mm/m. This takes into account both shrinkage and creep eff ects ects that have been described in this note. Ò| The The larger the cross-sectional cross-sectional area area of the element, the less axial vertical
that builds up between columns and core walls by installing a storey-height perimeter truss around the frame of the building (Figure 5). 5). This is known as a ‘belt truss’ and has the e ff ect ect of minimising uplift as the columns in the perimeter must resist tension as well as compression due to the presence of the truss. The primary use of of a belt truss, when working in combination with a shear core and outriggers, is to stiff en en the building structure, but an added bene fit is a reduction in the impact of axial shortening.
Outrigger
Belt truss
Assessment of axial shortening
and shrinkage on reinforce reinforced d concrete elements in Section 3.1.4 and Annex B. These methods rely on sophisticated computational tools to provide a reasonably accurate model of the magnitude of axial shortening. This approach considers three variables that impact on the creep and shrinkage of a reinforce reinforced d concrete element – the relative humidity of the ambient environment the concrete was cast in, the size of the element, and the material properties of the concrete with respect to its mix design – and can be summarised as follows: Ò| Consider the key variables that impact creep and shrinkage in reinforced concrete, i.e. humidity, element section geometry, and the composition of the concrete. Ò| Determine the creep coe ffi cient, φ(t ,t 0), which is related to E c, which can be taken to be approx. 1.05E cm,
where E cm is the secant modulus of elasticity of concrete, which varies depending on the strength grade of concrete. The value of the creep coeffi cient can be read from Figure 3.1 of BS EN 1992-1-1 for concrete that is exposed to a stress 0.45f ck , where f ck is the characteristic compressive cylinder strength of concrete at 28 days. It is typically in the order of 1.5–2.0 for most elements. Ò| Calculate
the creep deformation of the concrete based on the age of the structure as the load is applied. Ò| If the compressive stress of the concrete exceeds 0.45f ck (t 0), where (t 0) is the age of the concrete at the time of loading, then nonlinear analysis needs to be used to determine the creep and shrinkage of the concrete structure. Ò| Consider humidity as the values given in Figure 3.1 of BS EN 1992-1-1 are valid for ambient temperatures between 40°C and +40°C and a mean relative humidity between 40% and 100% (typically, RH = 50% and RH = 80% are used for internal and external elements, respectively). Ò| Calculate the shrinkage strain based on drying and autogenous shrinkage strain. Each occurs at diff erent erent rates as the concrete ages.
ëFIGURE 5: Belt 5: Belt truss within structure relieving impact of axial shortening (columns omitted for clarity)
movement there is likely to be. the amount of reinforcement reinforcem ent in an element, while maintaining the cross-section size, can reduce axial shortening. Ò| Increasing concrete strength can reduce axial shortening. Ò| Using elements of the same size, but with diff erent erent concrete strengths and reinforcement, alters the magnitude of axial shortening. Ò| Increasing
ANALYSIS OF AXIAL ANALYSIS SHORTENING IS QUITE COMPLEX DUE TO THE NUMBER OF VARIABLES THAT NEED TO BE CONSIDERED
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Technical T Technical echnical Guidance Note | Level 3, No. 2
Applied practi practice ce BS EN 1992-1-1:2004+A1:2014 Eurocode 2: 2: Design of concrete structures. General rules and rules for buildings NA+A2:14 to BS EN 1992-1 1992-1-1:2004+A1:2014 1:2004+A1:201 4 UK National Nation al Annex to Eurocode 2: 2: Design of concrete structures. General rules and rules for buildings
Further reading
Concrete Society (2008) TR67: Movement, restraint and cracking in concrete structures, Camberley, Concrete Society
Resources
The Concrete Society www.concrete.org.uk/
Fintel M., Ghosh S.K. and Iyengar H. (2005) Column shortening in tall structures – prediction and compensation, Skokie, Il: Portland Cement Association Fu F. (2018) Design and analysis of tall and complex structures, Oxford: Butterworth–Heinemann Kim H. and Shin S. (2014) ‘Reduction of dierential column shortening in tall buildings’, Int. J. Civil Environ. Struct. Construct. Arch. Eng., 8 (2), pp. 145–148
Baker W.F., Korista D.S., Novak L.C., Pawlikowski J. and Young B. (2007) ‘Creep and shrinkage and the design of supertall buildings – a case study: the Burj Dubai Tower’, ACI SP-246: SP-246: Structural Implications of Shrinkage and Creep of Concrete, 246, pp.
Working group of The Concrete Centre and fi b Task Group 1.6 (2014) Tall buildings: Structural design of concrete buildings up to 300m tall , London: MPA The Concrete Centre and Lausanne:
133–146
Fédération internationale du béton ( b )
fi
The Concrete Centre: RC Spreadsheets to BS EN 1992-1-1 (TCC55: Axial Column Shortening and TCC55X: Axial Column Shortening:24 storeys) www.concretecentre.com/PublicationsSoftware/RC-Spreadsheets-v4C/RCSpreadsheets-v4B-2-Contents.aspx
AECOM is AECOM is built built to deliv deliver er a better better worl world. d. We We design design,, build, finance and operate infrastructure assets for governments, businesses and organisations in more than 150 countries. As a fully integrated firm, we connect knowledge and experience across our global network of experts to help clients solve their most complex challenges.
28 January 2021 | thestructuralengineer.org
MultiPly timber pavilion Project focus
Design and construction of the MultiPly pavilion using cross-laminated tulipwood SIMON BATEMAN îFIGURE 1: MultiPly pavilion at original V&A site site for for London London Design Festival
MEng,, CEng, MEng CEng, MISt MIStruct ructE, E, MICE MICE
Senior Structural Engineer, Arup, London, UK
CAROLINA BARTRAM MA, MDes MDesS, S, CEng, CEng, FISt FIStruct ructE E
Associate Assoc iate Direc Director tor,, Arup, Arup, London, London, UK UK
SYNOPSIS This paper describes the design, fabrication and erection of the MultiPly timber pavilion,
E V E E R D E
which was made from the first cross-laminated timber (CLT) to be manufactured at volume in the UK. Fabricated from Americ Ame rican an tulip tulipwood wood,, it was originally displayed at London Design Festival 2018 and has since been reconfigured and exhibited at design festivals in Milan and Madrid. The pavilion comprises a ‘vertical maze’ of stacked boxes and staircases allowing the public to reach a height of over 9m from the ground. Designed to be completely demountable and reusable, it is intended to be an exploration of the issues surrounding timber’s place in the circular economy and its use in modular construction. This paper details the development of the pavilion’s concept, its analysis and design through use of both physical and computer models, the manufacture of the tulipwood CLT, and the fabrication and erection of the pavilion on a highly constrained site – all taking place within a timescale of less than seven months.
29 thestructuralengineer.org | January 2021
Project focus
MultiPly timber pavilion
Project credits Sponsor
American Amer ican Hard Hardwood wood Expo Export rt Counci Councill
Client
London Design Festival
Architec Arc hitectt
Waugh Thistleton Architects
Structural engineer Structural
Arup
Fabricator and contractor
Stage One Creative Services
Timber processing
Glenalmond Timber Company
CLT manufacture
Construction Scotland Innovation Centre
Testing and technical advice to CLT manufacture
Edinburgh Napier University
Adhesives and delam Adhesives delaminati ination on testing
Henkel
Lighting design
SEAM
Introduction The Mult MultiPly iPly pavilion lion (Figure 1) is the latest in a series of collaborations between Arup, the American Amer ican Har Hardwo dwood od Exp Export ort Cou Council ncil (AH (AHEC) EC) and London Design Festival (LDF) to promote the use of structural hardwoods in construction and explore issues relating to design in timber. Each is delivered with a diff erent erent architect, using donated American timber, to a landmark site in London. Arup delivered vered the Timb Timber er Wave Wave in 2011 20111 (with 2,3 AL_A), AL_A ), the End Endless less Stair in 2013 (with dRMM Archite Arc hitects) cts) and the Smile4 in 2015 (with Alison Brooks). The architect selected for LDF 2018 was Waugh Thistleton Architects (WTA), a practice specialising in timber construction. A challen challenging ging brie brieff was was devel developed, oped, exploring oring diff erent erent aspects from previous years. The pavilion had to: Ò| visibly demonstrate the application of CLT to housing Ò| be erected as quickly as possible in the V&A museum’s busy Sackler Courtyard, a site with
Once the desired configuration had been agreed, the extremely tight programme meant that the order had to be placed with the contractor, and panel production begun, while the structural design was ongoing. Initial hand calculations were carried out on the overall stability of the structure, and of key elements, to demonstrate that the structure was feasible and to allow an initial order for panels to be released, allowing production to start before the final numbers of each panel type were known. Initial calculations were also carried out on connections to allow these to be sketched up for pricing.
Structural analysis
architecturally. After setting up rules to begin with, the team decided to allow a large cantilever at one end to add drama and demonstrate the strength of the hardwood.
A numbe numberr of app approa roaches ches wer were e used used to to analys analyse e and and design the structure. The process started with basic hand calculations to verify basic stability in response to wind loads, as well as estimate the load in key connections. Hand calculations were also used to design the floor and roof panels. As more sophisticated modelling methods progressed, these hand calculations were used as checks against computer analysis results. For more detailed element and connection design, finite element (FE) modelling in Oasys GSA 5 was used (Figure 4). As CLT is an orthotropic composite material, eff ective ective properties were
The cons constrai traints nts of the site also had to be be considered, as the courtyard is a long-span roof over a gallery below, with strict loading limits. Preliminary calculations were used to check overall loading limits, which informed the maximum height of the structure, the plan area and the occupancy. Initial checks against stresses and the amount of timber available meant that two types of panels were proposed: 100mm and 60mm thick ( five and three layers of 20mm board respectively). These would be assembled into boxes using computer numerical controlled (CNC)-cut castellated edges (Figure 3).
calculated for in-plane and bending behaviour in each direction, for application to the FE shell elements. Figures published by AHEC6, determined on previous LDF projects, were used, with three sets of properties depending on the panel build-up and orientation. Particular attention had to be given to in-plane shear, the eff ects ects of which can be significant in CLT. Connections between elements on MultiPly generally act as hinges and were modelled as such. In order to ensure that loads in connections were not underestimated, it was important that they were modelled in the correct locations, so they
very limited loading capacity and access be completely demountable, and ideally reconfigurable (all connections would therefore need to be reusable) Ò| be made from a limited amount of donated tulipwood, processed and manufactured into CLT in the UK, unlike previous pavilions Ò| be made from low-grade tulipwood not suitable for joinery use Ò| follow a programme of less than seven months between initial sketches and realisation on site. Ò|
Design concept and initial design A numbe numberr of conc concept epts s were were explored ored by by WT WTA and and Arup befo before re the team sett settled led on an arran arrangeme gement nt of of standardised ‘boxes’ stacked to create a vertical maze. During this stage of design evolution, the engineering devised simple engineering (Figure 2) toteam aid the architect in arranging therules boxes in a way that would be stable. This enabled WTA to explore many di ff erent erent box configurations. With engineer and architect collaborating, a final form was reached that worked both structurally and
P U R A
ëFIGURE 2: Initial ‘rules’ for stacking boxes
30 January 2021 | thestructuralengineer.org
MultiPly timber pavilion Project focus
acted with the correct lever arm in any overturning behaviour. Load distribution through the structure is sensitive to the panel type and orientation, and connection position. Therefore, as the design developed, and panels changed and connections moved, this information had to be fed back into the model for re-analysis. Once analysis was complete, panel stresses were checked against eff ective ective capacities for each panel type in each direction, taking into account buckling for compression members, which was critical especially at free edges of the thinner panels. Connection forces were also extracted for connection verification.
Statical indeterminacy The arra arrangem ngement ent of pane panels ls and and boxe boxes s formin forming g MultiPly is highly indeterminate. Combined with the brittleness of timber, this presents an unusual challenge. There are a number of uncertainties in timber elements and connections (such as timber stiff ness ness variability, bolt fit and connection friction) which aff ect ect the relative stiff ness ness of possible load paths; therefore, the load paths that develop in reality may not be those in the analysis model. Furthermore, these can vary over time due to eff ects ects such as creep and drying movement or swelling. With a brittle material such as timber, it is not suffi cient to simply carry out an elastic analysis and design for the forces, as, unlike with steel and reinforced concrete, we cannot rely on the structure to plastically deform and redistribute load if the actual load distribution does not re flect that designed for. This could lead to connections or elements being overloaded and the structure would simply fail. Accordingly, a methodology had to be developed to ensure that this highly irregular structure could be designed safely. The meth methodo odolog logyy used used was was based based on altern alternativ ative e load paths. The most indeterminate part of the structure was considered to be the large cantilever, which requires a horizontal ‘pull’ reaction at the top to prevent it from tipping over (Figure 5). A physical model was assembled, and engineering judgement was used to assess the primary credible paths for this to be traced through the structure (Figures 6 and 7). Four load paths were identified in all.
îFIGURE 4: Typical plot of panel stresses in GSA
P U R A
P U R A
ìFIGURE 3: Key features of MultiPly ‘box’
LOAD DISTRIBUTION THROUGH THE STRUCTURE IS SENSITIVE TO THE PANEL TYPE AND ORIENTATION Depending on relative stiff ness, ness, the cantilever could be supported by just one of these, or multiple load paths in combination. The unde understa rstandin nding g gaine gained d from from the phy physica sicall model was used to tune the behaviour of the FE model. By using GSA’s analysis stages feature and selectively adjusting element properties, the model was configured to analyse the key load paths in turn and combine the results for element and
connection design. This ensured that whichever distribution of loads occurred within the finished structure, the team could have confidence that it would be within the design capacity of the elements and connections.
Connections As prev previous iously ly note noted, d, the the conne connectio ctions ns neede needed d to to be be demountable and reusable. The challenge the team set itself was therefore to have no glue, screws, nails or anything else irreversible between elements anywhere on the structure. The team did, however, have the benefit of CNC manufacture to employ in creating a suite of connections. To T o form form the box boxes, es, a CNC-cu CNC-cutt castel castellated lated edge edge was applied to each panel allowing them to slot together. Large-scale furniture-type connectors, with a fabricated threaded steel barrel, were used to hold the panels tightly together (Figure 8). Between boxes, vertical bearing connections were achieved as often as possible by aligning the walls on plan so that walls overlapped. These were held in position with a tie connection comprising a threaded rod embedded within the panels above and below, with plates held in a CNC-cut pocket (Figure 9). These connections were also used to transmit tension and shear between boxes above and below. Around some connections which appeared susceptible to out-of-plane forces, screws were added through the thickness of the panel as reinforcement. In a small number of places, brackets were introduced, secured back to embedded barrels within the walls using long bolts. These were used where high tension or shear was concentrated at a corner, which key to stability the structure. During initialwas fabrication, panels of were manufactured to include all connections required for the London configuration, which had been incorporated into a global 3D model and programmed into the CNC machine. When
31 thestructuralengineer.org | January 2021
Project focus
MultiPly timber pavilion
îFIGURE 5: Cantilever box showing push, pull and bearing supports required
P U R A
additional connections were needed for the Milan and Madrid installations due to the change in configuration, logistical constraints prevented all of these being CNC-cut in the same way; therefore, limited screw connections between boxes were introduced.
Timber sorting, grading and testing While developing the Endless Stair with dRMM in 2011, Arup had identified that tulipwood was one of the few hardwoods suitable for CLT thanks to its combination of high strength but still relatively low density, enabling ease of machining, and also its ease of gluing. The tulipwood pwood used for Mult MultiPly iPly was timb timber er which had been rejected for joinery use and would ordinarily have gone to waste. As grading for joinery is purely aesthetic (e.g. by colour), there was a high chance that much of the timber could achieve a high grade structurally, provided it met requirements including grain direction and knot size. The donated wood was initially received at Glenalmond Timber Company, near Perth in Scotland, which carried out grading and processing of the boards. The UK stan standar dard d for for stru structur ctural al gradi grading ng of 7 hardwoods is BS 5756 , and the TH1 visual grade was used as a basis to sort the wood. To maximise
ìFIGURE 7: Illustration of two possible alternate load paths which would each have different effect on panel and connection loads
ìFIGURE 6: Card model used to P manually visualise U R possible load paths A
the yield from the limited raw material, boards were subdivided where necessary, with large knots cut out of boards which otherwise met the TH1 criteria, and the boards finger-jointed back together (Figure 10). AHEC AHE C had had already ready carr carried ied out stru structur ctural al testi testing ng on TH1 graded tulipwood, and therefore panels made of this wood could be used for primary loadcarrying elements of the structure with confidence in the design stresses and other structural properties.
S T C E T I H C R A N O T E L T S I H T H G U A W
This first grading left behind a large quantity of wood which did not meet the TH1 grade, but could still be useful in fulfilling functions not structurally integral to the pavilion, such as non-accessible roofs. Structural criteria were defined for a lesser grade and a yield of this wood was obtained. This minimised the amount of wood ultimately going to waste, which was typically wood with very large knots, very irregular grain, or which was split or warped so that it could not be laminated. Following grading, the boards were finger-jointed and re-cut into boards 2.7m long for fabrication of 2.7m square panels. They were then planed to a regular size of 20mm thick by 75 or 100mm wide. Samples of these boards were taken for testing, which was carried out by Edinburgh Napier University (Figure 11). Material properties were tested to find mean and characteristic values, including density, to ensure that published values for tulipwood were valid for this design. The strength of the typical finger joint produced at Glenalmond was also measured, to determine a characteristic tensile stress for the boards.
CLT CL T panel production MultiPly featured the first CLT manufactured at volume in the UK. It was produced at the Construction Scotland Innovation Centre (CSIC) near Glasgow. CSIC operates a vacuum CLT press measuring × 3.5m,simultaneously. in which four 2.7m square panels were13m fabricated One hundred and two panels were fabricated in total for the structure over a period of five weeks, as well as a number of test pieces. Fifty 100mm thick (five-layer) panels and twenty-eight 60mm thick
32 January 2021 | thestructuralengineer.org
MultiPly timber pavilion Project focus
îFIGURE 8: Furniture-style connection with recess and embedded barrel
S T C E T I H C R A N O T E L T S I H T H G U A W
îFIGURE 9: Box–box pocket connection
FOLLOWING GRADING, THE BOARDS WERE FINGER-JOINTED AND RE-CUT R E-CUT INTO BOARDS 2.7M LONG
I C J E R K R T E P
íFIGURE 10: Timber boards following sorting, grading and finger jointing
I C J E R K R T E P
(three-layer) panels were made from the structurally designated TH1 timber. A further twenty-four 60mm panels were made from the lower-grade timber. The stan standar dard d for for prod productio uction n of CL CLT T, BS EN 163518, states that bonding shall take place not more than 24 hours after planing. This is partly to ensure that, once the boards have been planed to a fine level of precision, there is not time for a change in moisture level to occur, which would significantly alter the board thickness and aff ect ect bonding. With the production process being split over two sites, and the panel production process being more time-consuming than the planing, this presented a logistical challenge requiring close coordination and ‘just-in-time’ delivery. Early each morning, the boards for the day’s panels were selected at Glenalmond, planed and loaded onto pallets. Afterr a 90 Afte 90-minu minute te journey ourney to to CSIC, CSIC, the boa boards rds were sorted, primed and then hand-laid within the CLT press, to three or five layers as necessary (Figure 12). Adhesiv Adh esive e was was applie applied d and and the the rubb rubber er-lik -like e membrane was then used to cover the panels, and a vacuum was applied for five hours as per the adhesive manufacturer’s recommendations for hardwoods and for the chosen adhesive. The panels were then andlacquer. sprayed on each side with layers of unloaded a protective Each batch of four panels took approx. seven hours of press time. This ran in a ‘daytime’ and ‘night-time’ shift, allowing manufacture of eight panels per day.
33 thestructuralengineer.org | January 2021
Project focus
MultiPly timber pavilion
Because of the ambitious nature of the structure, the team needed full confidence in the quality of the panels. A representative from the adhesive manufacturer, Henkel, visited CSIC early in the process to confirm that the panel manufacture was taking place in accordance with its recommendations. Henkel also kindly off ered ered to carry out delamination tests on small samples. These Thes e were were sent sent to Ger Germany many for test testing, ing, whic which h also so confirmed the quality of production. Arup engineers neers also so monit monitore ored d the the glued glued join jointt thickness of the finished panels, which was not allowed to exceed 0.3mm to ensure that bonding was eff ective; ective; this meant that adjacent boards needed to be machined to a very close tolerance. The use of the the CSIC CSIC vacu vacuum um press press allo allowed wed high high-quality CLT to be produced using bespoke layups, in a wood not traditionally used for CLT, producing a product not currently available from the major CLT suppliers in Europe.
I C J E R K R T E P
ìFIGURE 11: Testing of wood samples at Edinburgh Napier University
Fabrication and assembly The cont contract ractor or appo appointe inted d to to fabri fabricate cate and assemble the pavilion was Stage One Creative Services. A five-axis CNC machine which had been programmed from the 3D model was used to cut each individual panel and its connections to less than 1mm tolerance (Figure 13). The ‘flatpack’ panel components were then assembled into the 17 boxes of MultiPly (Figure 14), and a section of the structure was erected within Stage One’s warehouse to trial the procedures for assembling the structure. The boxes were prepared with the stairs pre-installed as well as lighting to door openings. Following disassembly of the trial, the boxes were loaded onto flatbed trucks for the journey to the V&A Museum in London. The courtyard itself presented the final challenge: it would not accommodate a full-sized mobile crane, so the structure had to be assembled using a smaller spider crane, progressing from the back of the courtyard to the front, using temporary propping as necessary (Figure 15). Each of the boxes at base level was placed on a custom-made platform, CNC-cut to accommodate the fall of the courtyard, to create a level surface. The prim primary ary stru structur cture e was was comp complete leted d withi within just just four days due to the accuracy of fabrication and of the preformed connections. A further three days was required to complete the balustrades and lighting. Following completion on site, the pavilion was open for approximately two and a half weeks before disassembly. Over 160 000 people visited the V&A during the festival.
I C J E R K R T E P
ìFIGURE 12: Laying-up of panels at CSIC
Reuse A key key theme theme of Mult MultiPly iPly was the exploration oration of disassembly and reuse, and the team had the opportunity to test this potential immediately following the end of LDF. Following disassembly at the V&A, six boxes were transported three miles to Bloomsbury and reassembled as part of an exhibition for New London Architecture at the Building Centre. MultiPly has also been shown in new configurations at the Salone del Mobile, Milan (April 2019; Figure 16) and Madrid Design Festival (February 2020; Figure 17).
I C J E R K ìFIGURE 13: CNC R T cutting of panels at E P Stage One
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MultiPly timber pavilion Project focus
as a pavilion, it is anticipated that the panels will be downcycled to a new, longer-term use. This could involve, for instance, cutting up the panels to fabricate new elements (such as stairs) which could be installed in a new building. This is preferable to resorting to biomass or landfill, which would result in return of the carbon, originally sequestered in the timber, to the atmosphere.
Conclusion MultiPly has been a challenging and rewarding project for architect, engineer, manufacturer and contractor alike. It is a significant achievement in the use of engineered timber in the UK. It has included: Ò| analysis and design of a highly irregular structure, requiring a subtle mix of ‘first principles’ engineering and advanced analysis to ensure confidence in a brittle material PÒ| further examples of how hardwood CLT can be U R A used, building on knowledge and experience gained on previous LDF projects such as the Endless Stair and the Smile Ò| the first CLT manufactured at volume in the UK, using a vacuum press to create high-quality CLT. This is parti particularly cularly suit suited ed to to unusual unusual woo woods ds and bespoke panel layups Ò| use of 3D modelling and CNC fabrication of all connections to create finished panels, ready for
ìFIGURE 14: Assembly of box using castellated corner connections
íFIGURE 15: Assembly on site Assembly at V&A, showing temporary propping in place
Ò|
C E H A
ìFIGURE 16: MultiPly at Salone del Mobile, Milan 2019
I C J E R K R T E P
The orig original inal design gn for MultiPl tiPly was was only only for the configuration in London, but the standardisation of geometry was intended to ensure that future arrangements would be possible without substantial modification. Both the Milan and Madrid configurations presented diff erent erent challenges: Milan was a much more slender construction, while Madrid had more cantilevers and was at a much more exposed site. Each configuration required some re-analysis and re-engineering of connections. Once MultiPly has reached the end of its life
íFIGURE 17: MultiPly at Madrid Design Festival 2020
assembly creation of a timber structure which can be fully disassembled and reassembled by using only reversible, bolted connections.
REFERENCES 1) American Hardwood Export Council (2020 ) ) Timber Wave [Online] Available at: www.americanhardwood.org/index.php/ en/examples/our-projects/timber-wave (Accessed: November 2020) 2) American Hardwood Export Council (2020) The Endless Stair [Online] www. americanhardwood.org/index.php/en/ examples/our-projects/the-endless-stair (Accessed: November 2020) 3) Campbell A., Groat H. and Lawrence A. (2014) ‘Engineering (2014) ‘Engineering the Endless Stair’, The Structural Engineer , 92 (9), pp. 16–21 4) American Hardwood Export Council (2020) The Smile [Online] Available at: www. (2020) americanhardwood.org/index.php/en/ examples/our-projects/the-smile (Accessed: November 2020) 5) Oasys Ltd (2020) (2020) Oasys GSA 10 [Online] Available at: Available at: www.oasys www.oasys-softw -software.c are.com/ om/ products/structural/gsa/ (Accessed: November 2020) 6) American Hardwood Export Council (2005) (2005) Structural Design in American Hardwoods, London: AHEC 7) British Standards Institution (2017) (2017) BS 5756:2007+A2:2017 Visual strength grading of temperat grading temperate e hardwood. hardwood. Speci fi fication c ation, London: BSI
8) British Standards Institution (2015) BS EN 16351:2015 Timber structures. Cross laminated laminat ed timber timber.. Requirement Requirements s, London: BSI
35 thestructuralengineer.org | January 2021
procedures es Opinion OpinionPlanning A newapplication approach procedur to floor loading
Viewpoint
A ne new w ap appr proa oach ch to floor
loading
Alastai Alas tairr Hugh Hughes es sets sets out a proposal to simplify the Eurocode approach to floor loads by moving from an occupancy-based categorisation to one based on loading. Introduction For most buildings of more than one storey, the important variable action – the payload, so to speak – is gravity loading imposed on floors. Currently,, in the Eurocodes, values for design Currently are derived from the intended occupancy of the space in question, according to a table in EN 1991-1-11 – or strictly speaking its National Annex (NA), because nations do not always see eye to eye. These assumptions aff ect ect safety, so EN 1991-1-1, while off ering ering RVs (Recommended q- and Q-values), has to tolerate a credibilitystretching range of national choice for its 12 categories or subcategories of occupancy occupancy.. The UK NA 2 generates multiple sub-subcategories, each with its own pair of values. The motive may have been to ease the transition from predecessor standards, but the result is complexity and disharmony. In 2021, with firstgeneration Eurocodes up for revision, we have an opportunity to do better better.. This article explores the idea of categorising categorising the opposite way round: not by occupancy but by loading. Four categories, with q = 2, 3, 4 and 5kPa (kN/m2 if preferred), will cover most normal building floors. As now, each of these equivalent uniformly distributed loads is partnered by an alternative singly applied roving point load, Q, Q, of of 2, 4, 4 and 5kN respectively (with one outlier: car parking floors at 10kN, to allow for a wheel change). Instead of quoting two values (in kPa and kN) for every variant of occupancy, the table can simply list the occupancies that each category is good for. This new approach is not revolutionary in any other sense. There is no intention to overturn the traditional loadings or their modus operandi. It does, nevertheless, provide opportunities to tidy up, to refresh and to promote borderless structural safety safety..
Why make this change? There are three motives. First, it reflects the spirit of the age we live in. None too soon, fi
designed, sometimes even to diff erent erent loadings on diff erent erent areas of the same floor. In future, a building (or floor) ought to be reusable for other occupancies listed against its category category,, or any lower one. Second, it advances the ease-of-use agenda and reduces error potential. Just four standard categories, with memorable load values. Bespoke design will still have a place for occupancy categorised as ‘5+’, mainly
govern, but rounding up by 0.5kPa in the area loadings, q, will (at least for residential that was only 1.5kPa to start with) have a perceptible impact. Or will it? Within living memory in the UK, CP35 brought the loading down from 40psf (1.92kPa) for residential (other than small houses) to 1.5kPa. If we didn’t notice buildings getting cheaper then, we won’t notice them getting more expensive now!
storage and industrial. Above 5kPa, loadings change character and lose any probabilistic pretensions. Essentially, such loadings are ‘superstandard’ and project-specific; values to match operational needs must be declared, then respected, by the client. Third, with a modicum of rounding rounding up, it can lubricate the process of European consensus. The categories are neutral; shouldn’t the nations be able to agree on which is the minimum for each occupancy? Table 1 includes suggestions as a starting point. These are informed by the work of PraxisRegelnBau (PRB)4 – Practice-oriented Rules in Building Construction,, a 2011 initiative by a number Construction of German construction industry associations to promote Eurocode improvements, with a focus on ease of use – which is gratefully acknowledged. Tentatively, Tentatively, the table has been extended to cover roofs as well as floors.
It is sobering to reflect that 1.5kPa accommodates less than two people per square metre; a third will exhaust the load factor.. Since many residents host social events factor from time to time, some of which get really crowded, there must be floors around that owe their survival either to the forgiving nature of timber in the short term or the diluting e ff ect ect of substantial permanent concrete. So, 2kPa for all residential, back into line with many other nations, could be viewed as a rectification. It would also spare us some head-scratching. For certain residential subcategories, the current UK NA demands 2kPa, so users may already be adopting this value. Nobody wants to design against a patchwork of diff erent erent loading. Since we have been cheerfully designing offi ces for 4 or even 5kPa for years, rounding up from 2.5 to 3 should not startle the quantity surveyor,, and consistency between ‘upstairs’ surveyor and ‘downstairs’ offi ce levels is a bonus. For parking, the same increase might be viewed diff erently: erently: as overdue acknowledgement that cars have been putting on weight over the years. Explicit recognition that live load reduction (LLR) may apply to parking structures, for the vertical structure at least, would sugar the pill.
Rounding up Any slight increase in design loading, though though safety-enhancing, is bound to result in some loss of economy. This is not so much the result of higher point loads, Q, which rarely
IN FUTURE, A BUILDING (OR FLOOR) OUGHT TO BE REUSABLE
Tidying up In retail, EN 1991-1-1 retains an obsolete distinction between department stores (5kPa) and the rest (4kPa), while supermarkets go unmentioned. Although many would
3
the ‘long life/low energy/loose ethos is being embraced wholeheartedly.t’. Well-made wholeheartedly buildings are outliving their original occupancy and being repurposed, not demolished. Time is running out for the kind that was tightly
FOR OTHER OCCUPANCIES LISTED AGAINST ITS CATEGORY
regard 5kPa as today’s facto ‘industry standard’, 4kPa remainsdetenable as a minimum requirement. As part of this exercise, staircases staircases deserve a fresh look. Escape routes can experience
36 January 2021 | thestructuralengineer.org
A new approach to floor loading
Opinion
TABLE 1: Loading categories for floors (and roofs) C a te g o r y
UDL q (kPa)
PL Q (kN)
Occupancy
0
0
0
Roof of a private or agricultural glasshouse or polytunnel (not accessible to the public) with maintenance, snow clearance and repair conducted from the ground. NOTE Loads from wind, snow and any suspended planting or equipment need to be accounted for.
1
0.6
1
Roof with access only for the purposes of maintenance and repair. NOTE q acts on the vertically projected projected area of the roof (may optionally be disregarded where the slope exceeds 60°). Snow load substitutes for q, if higher. An occup occupied ied roof is treate treated d as a floor of the appropriate category (2 or above).
2
2
2
Residential (including hospital wards and hotel bedrooms, their bathrooms and circulation areas). Stairway within private house or residential unit. Catwalk for technical and maintenance staff (including associated stairway or ladder).
3
3
4
Offi ce. Non-residential healthcare, education. Hospitality with fixed seating or tables. Laboratory with no heavy equipment. Assembly Asse mbly are area a with with fixed seating as in a church, cinema or theatre. Stairway (or horizontal escape passage) serving no more than four levels of Category 2 or non-assembly Category 3.
10
Park rkin ing g (of (of ca cars rs and li ligh ghtt va vans in typ ypic ical al arrangements arrangemen ts with each space accessible) including ramps.
4
4
4
Retail, library, gallery, hospitality/assembly without fixed seating. Balcony cantilevering from a floor in Category 2, 3 or 4. Stairway (or horizontal escape passage) other than those included under Category Category 2, 3 or 5.
5
5
5
Public space, sports hall, concourse, station platform, footbridge. Any are area a that that may may be be subjec subjectt to inten intense se crow crowd d loading including ramps, stairways and circulation areas. Light storage.
5+
As requi required red (>5)
As requi required red (
