CBDG Guidance on the Assessment of Concrete Bridges
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Acknowledgements All members of theTask Group contributed significantlyto the publication but special mention must be made of the joint editors, Ceorge SomeM'lle OBE, a past Chairman of CBDC, and Bob Lark at Cardiff University. Both expended much time and energy in transformingthe information and findings into the finished article and CBDC would like to express its gratitude to these two and all the other membersof the Group.
Publishedfor and on behalf of the Concrete Bridge Development Group by The Concrete Society Riverside House, 4 Meadows BusinessPark,Station Approach, Blackwater,Camberley,Surrey CUI7 9AB Tel: +44 (0)1276 607140 Fax: +44 (0)1276 607141 www.concrete.org.uk CCIP-024 PublishedJune 2007 ISBN 1-904482-35-X 0 Concrete Bridge Development Group Order reference: CBDG/TG9 CClP publications are produced by The Concrete Society on behalf of the Cement and Concrete Industry Publications Forum -an industry initiativeto publish technicalguidance in support of concrete design and construction. CClP publications are available from the Concrete Bookshop at www.concretebookshop.com Tel: +44 (0)7004 607777 All rights reserved. Except as permitted under current legislation no part of this work may be photocopied, stored in a retrieval system, published, performed in public, adapted, broadcast, transmitted, recorded or reproduced in any form or by any means, without the prior permission of the copyright owner. Enquiries should be addressed to the Concrete Bridge Development Group.
Although the Concrete Bridge DevelopmentGroup (limited by guarantee)does its best to ensure that any advice, recommendations or information it may give either in this publication or elsewhere is accurate, no liability or responsibility of any kind (including liability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Group, its servants or agents.
Printed by Cromwell Press, Trowbridge, UK
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Guidance on t h e Assessment of Concrete Bridges Contents Foreword
X
Abbreviations and acronyms
xii
Notation
xiv
1.
The assessment process
1
1.1
The need for assessment
1
1.1.1 Asset management
1
1.1.2 Effectiveness of current programme
1
1.1.3 Network management
2
1.1.4 Sub-standard bridges
2
1.1.5 Assessment documentation
2
Management of the assessment process
2
1.2.1 Experience
2
1.2.2 The brief
3
1.2.3 Existing information
3
1.2
~
1.3
1.2.4 Access
4
1.2.5 Site work
4
1.2.6 Level of assessment and analysis
5
1.2.7 Management of resuks
5
Staged assessments
5
1.3.1 BA79: The management of sub-standard highway structures
6
1.3.2 BA79: Advice on assessment
6
~
1.4
1.3.3 Levels of assessment
7
1.3.4 Conclusions
8
Assessment standards
8
1.4.1 Highways Agency documentation
8
1.4.2 DeDartures from standards
10
~~
1.4.3 Other bridge owners
10
1.5.1 Whole life assessment 1.5.2 Asset management
11
1.5.3 Consequences for assessment
13
11
~
1.5.4 A continuous assessment Droeramme
2.
1.6 Assessment of the serviceability limit state 1.6.1 Serviceability assessment
13 14
1.7 Historical aspects of assessment 1.8 European developments
15 17
1.8.1 The BRlME proiect 1.8.2 Structural assessment
17 17
1.8.3 Deterioration
18
1.8.4 Chloride ingress
18
1.9 References
18
Technical approval and documentation
19
2.1 2.2
General Drocedures
19 19
2.3 2.4
Obiectives Key stages
20 21
2.5
‘Departures’ and ‘Aspects not covered by Standards’
25
2.6
Reference
26
General principles
Inspection for assessment
3. ~
27
~
3.1
Process and eeneral Drocedures
27
3.1.1 Background and introduction
27
3.1.2 Reauirements for insDection
27
3.1.3 Future requirements for inspection
28 29
3.1.4 The inspection process 3.1.5 Risk assessment and safety issues 3.2
Condition assessment
3.2.1 BD2l approach 3.2.2 CSS bridge condition indicator
~~
3.3 3.4 3.5 3.6
ii
13
Network Rail’s Structures Condition Markine Index (SCMI) Other work Preferred approach References
30 30 30 30 31 32 33 33
4.
In-situ and laboratory testing
34
4.1
Background and Introduction
34
4.2
Current testing practice
34
4.3
In-situ sampling and testing
35
4.4
Laboratory testing
35
4.5
Concrete parameters for assessment
35
4.6
Reinforcement parameters for assessment
37
4.7
In-situ stress measurement - concrete
37
4.8
In-situ stress measurement - steel reinforcement
38
4.8.1
38
4.9
5.
4.8.2 Cut bar method
38
Geophysical Techniques
39
4.10 References
39
Loading
40
5.1
General principles
40
5.1.1 Scope of live loading
40
5.1.2
41
5.2
Background t o assessment live loading
Bridge specific Loading
42
5.2.1 Bridge specific probabilistic loading model
42
5.3
Highway surfacing effects
43
5.4
Road traffic and abnormal loads
44
5.4.1 Purposes of highway structure assessment
46
5.4.2 Design and assessment
46
5.4.3 BD86 - Assessment of highway structures for the effects of Special Types General Order (STGO) and Special Order (SO) Vehicles 5.4.4 BD86 Load Models - SV Vehicles
46
5.4.5 BD86 - HB Conversion Charts
47
5.4.6 Assessment of Abnormal Load Movements using BD86
47
References
48
5.5 6.
Blind hole drilling
47
Analysis for assessment
49
6.1
Assessment principles
49
6.1.1 General principles
49
6.1.2 Analytical procedure
49
6.1.3 Lower bound and upper bound methods
50
6.1.4 Ductility
50
6.1 .S
50
Differences from design
6.2
6.3
6.1.6 Structural condition
51
6.1.7 Global and local analvsis
52
Simple methods
52
6.2.1 Strip method
52
6.2.2
52
BA16 method
6.2.3 Upper bound limit and check
52
Elastic methods
53
6.3.1 Elastic grillage
53
6.3.2
54
Elastic finite elements
~~
6.4
6.5
6.3.3 Westergaard and Charts
56
Plastic equilibrium methods
57
6.4.1 Plastic redistribution
58
6.4.2 Hillerborg strip method
58
Yield-line analysis
59
6.5.1 Principles
59
6.5.2 Upper bound solutions
59
6.5.3 Slab decks
60
6.5.4 Beam and slab decks
62
6.5.5
~
7.
Box culverts and retaining walls
62
6.5.6 Limitations of yield-line methods
63
6.5.7 Computer analysis
64
6.5.8 Shear
65
6.6
Strut and tie action
66
6.7
Serviceability limit state
66
6.7.1 Inspections
66
6.7.2 Plastic methods of analysis
67
6.7.3 Special inspections
67
6.8
Soil structure interaction
67
6.9
Conclusions
68
~~~
6.10 References
68
Hidden strengths
69
7.1
Reinforcement anchorage and bond
69
7.1.1 First principles
69
7.1.2 Reinforced concrete beams
69
7.1.3 Sub-standard cover and deteriorated concrete
71
7.1.4 Bond a t laps
72
7.2
7.3
7.1.5 Haunched sections
72
7.1.6 Bent-up reinforcement
72
7.1.7 Inclined links
74
7.1.8 Bearing clamping of reinforcement
75
Shear
76
7.2.1 Shear in reinforced concrete slabs
76
7.2.2 Effect of varying section size
77
7.2.3 Shear enhancement a t supports
77
7.2.4 Shear in mestressed flawed beams
79
7.2.5 Shear in webs of post-tensioned prestressed beams
79
Deck surfacing
79
7.3.1 Properties of surfacing materials
79
~
7.4
7.5
7.3.2 Discussion
80
Moment field analysis of reinforced concrete slabs
80
7.4.1 Moment fields
80
7.4.2 Wood-Armer equations
81
7.4.3 Moment field approach
82
7.4.4 Torsionless grillages
82
ComDressive membrane action
82
~
7.6
~
~~
~
7.5.1 Local strength of deck slabs and restraint
83
7.5.2 Global behaviour of beam and slab decks
84
7.5.3 External restraint
84
7.5.4 Non- Iinear nu mer ical a nalvs is
85
Piers
85
7.6.1 Assessment of piers
85
~~
8.
7.6.2 Slender piers
86
7.6.3 Global restraints
87
7.6.4 Buckling of multiple piers
88
7.7
Redundancy of elements
89
7.8
Parapet edge stiffening
89
7.9
Width of sumorts
90
7.10 Foundations
90
7.1 1 References
91
Specific structural forms
92
8.1
Reinforced concrete slabs
92
8.2
Reinforced concrete beams
93
V
9.
8.3
Half joints
94
8.4
Hinge joints
94
8.5
Box culverts
95
8.6
Precast pre-tensioned beams
96
8.7
Reinforced concrete arches
97
8.7.1 Elastic analysis
97
8.7.2 Mechanism analvsis
98
8.8
Post-tensioned structures
99
8.9
References
99
Soecific material and assessment factors
100
9.1
Introduction and general principles
100
9.2
Alkali-Silica Reaction (ASR)
101
9.2.1 Background
101
9.2.2 Structural assessment: current practice and reference documents 9.2.3 Kev Doints in structural assessment
102 103
9.2.4 Structural sensitivity: elements most a t risk
103
9.2.5 Indicative modifications to design models for strength assessment 9.2.6 Assessment of Highway Structures - BD44/95 and BA44/96; BA52/94; BD21/01
104
Aggressive chemical attack
105
9.3.1 Background
105
9.3.2 ‘Conventional’ form of sulfate attack
107
9 . 3 . F Thaumasite form of sulfate attack (TSA)
108
9.4
Freeze-thaw action
109
9.5
High Alumina Cement (HAC)
110
9.6
Supersulfated Cement (SSC)
111
9.7
Chlorides
112
9.8
Carbonation
113
Steel corrosion
115
9.9.1 Introduction and background
115
9.9.2 Effects of corrosion
115
9.9.3 Corrosion rate
116
9.9.4 Prestressed concrete
116
9.3
105
~~
9.9 ~
vi
9.10 Fatigue
117
9.1 1 Sub-standard reinforcement detailing
118
9.12 Deteriorated reinforced concrete structures
120
9.12.1 Background and introduction
120
9.12.2 Effects of deterioration, and principles for assessment
121
~
~
9.12.3 Sources of detailed information
122
9.12.4 Procedures in accordance with Highways Agency documentation
122
9.13 References
123
10. Load testing
126
10.1 General principles
126
10.1.1 Background
126
10.1.2 Reasons for presence of reserves of strength
126
10.1.3 Types of load test
127
10.1.4 The aims of load testing
127
10.1.5 Documents in the Design Manual for Roads and Bridges
128
10.1.6 Safety
128
~~
~
10.2 Supplementary load testing
129
10.2.1 Status
129
10.2.2 Methodology
129
10.2.3 Further guidance
129
10.3 Proof load testing
129
10.3.1 Background
129
10.3.2 Methodology
130
10.3.3 Application
130
10.3.4 Current status
131
10.3.5 Safety
131
10.4 Full-scale or model-scale testing to failure
131
10.4.1 Background
131
10.4.2 Broad principles
132
~
10.4.3 Extent of testing
133
10.4.4 Application
134
10.5 Hybrid and other forms of load testing
134
10.5.1 Examples
134
10.5.2 Load testing for monitoring
134
10.5.3 Reliability and updating
135
10.6 Dvnamic load testing
135
10.6.1 Structures where dynamic response is critical
135
10.6.2 Dynamic tests for health monitoring
136
vii
10.6.3 Applications for assessment
136
10.6.4 Case studies and research in the literature
137 138
~~
10.7 References 11. Reliability and risk-based techniques
139
1 1 .I Background
139
I 1.2 The appropriate application of reliability and risk-based assessments 1 1.3 Overview of reliability-based assessments 11.3.1 General
140 141
11.3.2 Uncertainties in bridge assessment 11.3.3 Reliability Index
141 141 142
11.5 Bridge specific risk-based assessments
143 144 146
11.6 References
147
1 1.3.4 Interpretation of Reliability Analysis Results 11.4 AcceDtance criteria ~~
12. Bridge management and assessment
12.1 General principles
148
148
12.1.I Managing sub-standard bridges
148
12.1.2 The BA79 Advice Note
148
12.1.3 Managing the results of assessments 12.1.4 Immediate risk structures
149 149
12.1.5 Interim measures during assessment 12.1.6 Interim measures on completion of assessment 12.1.7 Assessment and maintenance
150
12.2 Monitoring 12.2.1 Appropriate monitoring
150 151 151 152
12.2.2 Classes and frequency of monitoring 12.2.3 Application of monitoring
153 153
12.2.4 Permanent monitoring 12.2.5 Monitoring specifications
154 154
~~
12.2.6 Methods and took for monitoring 12.3 Cracking and crack widths 12.3.1 Crack widths
156
12.3.2 Types of crack and diagnosis
157 158 158
12.4 Deterioration rates 12.4.1 The causes of deterioration 12.4.2 Timescales and rates of deterioration
viii
155 156
158
12.4.3 Structural assessment
161
12.4.4 Structural amraisal
162
~
12.5 Legal matters
163
12.5.1 Historical background
163
12.5.2 Current situation
164
12.6 The assessment of bridges for abnormal loads
167
12.6.1 Abnormal load categories
167
12.6.2 Management of abnormal indivisible load movements
167
12.7 References
Appendix Relevant historical references to design and materials specifications and standards used in concrete bridge construction
170 171
ix
Foreword This publication contains guidance on a variety of topics related t o the assessment of concrete bridges. It was prepared by the Assessment Task Group (ATG) of the Concrete Bridge Development
Group and is a development of the report of the ATG published in 1997. Each Chapter is based on an early draft prepared by a member of ATG, which was subsequently reviewed and extended by other members. The whole publication was then edited by Professor Ceorge Somerville and the Convenor of the group, Dr R. J. Lark (Cardiff University). At the time of publication, the Assessment Task Group consisted of the following members: Bob Lark (Convenor, 2001 t o date) Graham Cole (Convenor, 1997 t o 2001) David Bone Mike Chubb Robin Church Steve Denton lan Frostick Paul Jackson Martin Lynch George Somerville
Cardiff University Surrey County Council Royal Haskoning Atkins Highways & Transportation Essex County Council
Parsons Brinckerhoff Network Rail Gifford and Partners Highways Agency Consultant
The input of Brian Barton of the Highways Agency and David Cullington, formerly of the Transport Research Laboratory, is also acknowledged. The current assessment programme effectively started in 1984 with the publication of Departmental Standard BD2l. Substantial progress has been made since then and it might appear that there was little need for guidance on assessment. However, substantial numbers of sub-standard structures still exist, many of which may benefit from further assessment. There is a growing realisation that a continuous assessment process forms an essential part of the sustainable maintenance of the national bridge stock. The purpose of this Guide is t o both record the experiences of past years and t o give examples of best practice for the future. It should be used with due care and attention, as they may not be applicable t o a specific project. The intention is that they should be used as supporting documentation in conjunction with the appropriate standards and, therefore, have been written based on the assumption that the reader will have some experience and a basic knowledge of bridge assessment. The Concrete Bridge Development Group (CBDG) and the members of the ATG assume no responsibility for the adequacy of the advice given, nor for the legal, contractual or financial consequences of its use. The intention of this Guide is t o address the assessment of all types of concrete bridge, irrespective of their use or ownership. It is recognised nevertheless that a significant
X
proportion of such structures are reinforced concrete highway structures, and that many of the standard procedures that are currently used are founded in the requirements of the Highways Agency and/or, when road over rail, Railway Group Standards. As such, much of the guidance that is given here is also based on these requirements, but wherever possible reference is also made t o new research and alternative points of view, which it is believed will both aid and advance the assessment process. References and discussion of the advice and requirements contained in other publications and Standards, particularly the Highways Agency’s Design Manual for Roads
andBridges, will inevitably become out of date, as these documents are themselves updated from time to time In drafting this guidance, the task group has endeavoured t o focus on the principles that underpin best practices in the assessment of concrete bridges, and therefore t o provide guidance which, whilst set in the context of current requirements, should hopefully remain useful even i f these requirements change Comments and feedback on this Guide would be welcomed. They should be sent t o the Secretary, Concrete Bridge Development Group, at the address given inside the front cover. R. J . Lark July 2006
Abbreviations and acronyms AIL AIP
ALL AS R
AW AWR BCA BCI BRIME
BSALL BSLL C&U CBDG CEB
CDM COBRAS CONTECVET
css DAF DfT DMRB
fib FIP FTA HA HAC HACC HCV HRM HSE IAN N DT NLFE NSCLTB
Assessment Live Load Alkali-Silica Reaction Authorised Weight Authorised Weight Regulations British Cement Association Bridge Condition Indicator Bridge Management in Europe research project Bridge Specific Assessment Live Loading The Basic Static Live Load effect Construction and Use Concrete Bridge Development Group Comite Euro-International du Beton (Euro-International Concrete Committee) Construction, Design and Management Concrete Bridge Assessment Package EU project on the Service Life of Concrete Structures (BE 4062) CSS (Formerly the County Surveyors Society) A Dynamic Amplification Factor The Department for Transport Design Manual for Roads and Bridges Federation internationale du beton (The International Federation for Structural Concrete) - Formed by the merger of CEB & FIP Federation lnternationale de la Precontrainte (International Federation for Prestressing) Freeze-Thaw action The Highways Agency High Alumina Cement High Alumina Cement Concrete Heavy Goods Vehicle High Speed Road Monitor The Health and Safety Executive Interim Advice Note Non-destructive testing Non-linear finite element analysis National Steering Committee for the Load Testing of Bridges
Pfa REHABCON
National Vocational Qualification Pulverised fuel ash EU project on Strategies for the Maintenance and Rehabilitation of
RH SCC SCMI
Concrete Structures Relative Humidity Stress corrosion cracking Structure Condition Marking Index
NVQ
xii
Abnormal Indivisible Load Approval in Principle
sls
so SRPC
ssc
Sewiceability limit state. Special Order Sulfate-Resisting Portland Cement
UDL U Is VSE
Supersulfated cement Special Types General Order Technical Approval Technical Approval Authority Technical Appraisal Form Technical Approval Schedule Transport Research Laboratory Thaumasite form of sulfate attack Uniformly distributed load Ultimate limit state Vehicle Standards and Engineering
wcs
Worst Credible Strength
STG 0 TA TAA TA F TAS TRL TSA
xiii
Notation Ash ASV
B C C
D
6 E F", fCl
fC" fsh
ft fY fsv
H
h I0
I,,,,. 1, K
I,1, 4 M, m M* P
Q R Rtt
Area o f bent-up bars Cross-sectional area of vertical stirrup legs at each stirrup location Force in bent-up reinforcement Live load capacity factor/Compression force Diameter of loaded area Width of support Deflection Elastic modulus The total ultimate anchorage force in the longitudinal reinforcement Concrete strength at transfer Concrete cube strength Stress in bent-up bars Concrete tensile strength Yield stress Yield strength of reinforcement Depth of section Slab thickness The performance level of a structure at the time of construction The critical performance level of a structure The performance level of a structure at the time of an assessment Load reduction factor Linear measures/span Values of moment Resistance moment Value o f point load Applied force Reaction force A random multiplier t o model the uncertainty in a load effect due t o the static weight of vehicles Radius Stirrup spacing along a beam Slab span Live load effect (e.g. bending moment or shear force) Tensile force Wall thickness Shear force The fully anchored shear capacity Shear capacity Concentrated point load Uniformly distributed load Linear measure Effective depth Angular measures Factor t o allow for enhancement of bond due t o transverse pressure
xiv
P Yi
Ym
A P
P
r
Factor used in the calculation of bond stress influenced by bearing clamping action Loading partial safety factor Material partial safety factor Joint displacement Coefficient of friction Transverse bearing clamping pressure Reduction factor
1. The assessment process 1.1 The need for assessment
The highway bridge assessment programme in the United Kingdom effectively began with the publication of Departmental Standard BD21 in 1984. (Frequent reference will be made throughout this Guidance document to relevant Standards (BDs) and Advice Notes (BAs) produced by the Highways Agency. A full list of these is given in Section 1.4,) This Standard replaced the previous document, BE1/73, and introduced the limit state concept based on the Bridge Code, BS 5400. The requirement to permit the passage of vehicles of up to 40 tonnes and with 11.5 tonne axles was introduced by directive 85/3/EEC. The United Kingdom and Ireland obtained derogation from the directive until 31 December 1998. Despite having 14 years to complete the assessment programme, work was not completed by the due date and risk management measures had to be introduced. Current estimates are that the initial programme will take a further 1 to 10 years to complete, depending on the bridge owner. Nevertheless,the main area for future work, under current vehicle regulations, is with re-assessment.
1.1.1 Asset tTlanagement
The emphasis today is very much on asset management. This means moving away from reactive maintenance (simply spending roll forward budgets), and towards targeted schemes which increase the overall value of the bridge stock (the asset), but at lowest whole life cost. Key activities in asset management are the inspection and assessment process. It is necessary to determine the general condition and structural safety of each item in the overall asset, i.e. each bridge. Having established the current state, it is also necessary to estimate deterioration rates, so that some idea of the residual life of the structure can be determined. This work should be done to a consistent standard, so that investment decisions can be made with some degree of confidence across the network. Use of this Guide will assist in the effective management of an important national asset.
1 . I .2Effectiveness O f Current programme
The assessment programme has been carried out by a very large number of organisations to varying standards. The actual capacity of structures which were given a 40 tonne rating is not known. This approach was satisfactory a t the time but does not lend itself to determining the residual life of a structure and, ultimately, the value of the asset. Many bridges that just passed assessment in 1984 may now have become sub-standard.
Previous page is blank
1
A t the beginning of the assessment programme it was anticipated that the majority of the problems would occur with shear capacity in bridges built before 1973 when design rules changed. Although this did occur there were a significant number of theoretical
failures in flexure. The assessed levels of loading were also found, in many cases, to be well below 32 tonnes, which was the previous loading standard. A significant amount of experience has been acquired during the assessment
programme, much of which has been highlighted in a review of the programme undertaken by Parsons Brinckerhoff on behalf of the Highways Agency’ ’. This Guide draws on this experience and summarises good practice.
1. I .3 Network management
1. I .4 Sub-standard bridges
1.1.5 Assessment documentation
2
The Highways Agency, in particular, is looking a t route management strategies and the management of abnormal load vehicles. For this, it will be necessary to know the assessed capacity and condition of a bridge to a consistent standard.
Bridges that fail assessment are described as sub-standard. In a simplistic approach, these ‘failures’ then become the strengthening programme for future years. There are thousands of these bridges across the country. It may be that a careful application of further assessment techniques could remove these structures from the programme saving millions of pounds in both direct and indirect costs. Use of this Guide will assist in the maintenance of a safe highway network.
There is information on assessments contained in Highways Agency publications, text books, conference proceedings, learned journals, research papers, etc. This Guide provides easy reference to much of this source material.
1.2 Management of the assessment process
This Section sets out some basic concepts that need to be taken into account in the management of the assessment process and serves as an introduction to later, more detailed, guidance. It is based on the conclusions of the work of the first Concrete Bridge Development Group (CBDG) Assessment Task Group, which was completed in 199712.
I .2.1Experience
Bridge assessment requires the use of experienced engineers and inspectors who can interpret defects present in a bridge and who understand the background to Codes of Practice. The Highways Agency and the CSS (formerly the County Surveyors Society) are considering the implementation of an accreditation scheme for bridge inspectors. Other bridge owners already run NVQ schemes. The client should ensure that the staff proposed for the project have sufficient experience.
I
1.2.2The brief
The initial stages of the assessment programme were primarily used t o identify those bridges which, when using conservative assumptions, could be shown t o be satisfactory. More sophisticated, and expensive, techniques could then be concentrated on the 30% or
so of the bridge stock that failed this initial assessment. This concept of staged assessment has now been incorporated in HA Advice Note BA79. More recently, the need t o carry out assessments t o a consistent minimum standard, say Level 3 of BA79, has been proposed t o assist with asset management. Therefore, the requirements for a brief have changed since the assessment programme began with the first publication of BD2l in 1984. In inviting tenders for assessment, the bridge owner is required t o produce contract
documents that include the scope of the work and available existing information. A key objective should be t o allow the assessing engineer t o concentrate on the investigation and analysis of the bridge concerned by removing, as far as possible, the uncertainties of the process. Typical examples of uncertainties are:
0 Existing information 0 Statutory undertakers
0 Access 0 Possessions 0 Testing 0 Analysis 0 HB, C&U and abnormal load assessments 0 Health and Safety. For a lump sum bid, the assessment engineer requires a clear brief, free of requirements t o make guesses at the tender stage on the extent of work required. The high level of risk, which exists during the desk study and site stage, tends t o reduce money available for assessment calculations with the inevitable consequences. The approach which should give best value is t o use a mixture of lump sum and time charge work. The lump sum should be restricted t o work that can be defined clearly. Time charge work could be set against a pre-agreed budget that should not be exceeded without the client’s permission. However, this requires a professionally qualified client who can enter into technical discussion with the assessment engineer. It does allow more of a team approach and the clients are much more able t o ensure that the end product meets their requirements.
1.2.3 Existing information
The existence of good records of design, alterations and previous assessments, as well as drawings, considerably assists the assessment engineer. The procurement of drawings is sometimes left t o the assessment engineer. This may be an easy task or may involve considerable time and effort. This is a risk item in a lump sum bid. The client is generally in the best position t o obtain this information. The assessment engineer is often requested t o contact the statutory authorities t o determine the approximate location of existing services. The client should obtain this
3
information. If it is necessary to determine precisely the position of services by constructing trial pits, then this should be incorporated in a comprehensive site testing and investigation programme.
1.2.4 Access
Frequently, assessment engineers are requested to make their own arrangements for access across private land. This is a risk item since the difficulty of making such arrangements cannot generally be determined at tender stage. Local Authorities have powers under the Highways Act 1980 to obtain access for the purpose of inspecting, surveying and maintaining highway structures. They are also responsible for making compensatory payment if damage is caused to private property. Details of adjacent land owners and access arrangements should be provided by the client leaving the assessment engineer only to obtain access. The need to arrange for road closures can be assessed and undertaken by the assessment engineer, but adequate time should be allowed for the process when setting a contract programme. A further complication is introduced when dealing with railway tracks. Obtaining track possession can be a protracted and time-consuming business, which is a risk item for the
assessment engineer. Track possessions should be booked and paid for by the client, with the assessment engineer being responsible for undertaking the work within the possession and for the cost of any overrun. This cost must be stated clearly in the tender documents. The possessions must also be a reasonable period within which to undertake the work required. The costs associated with cancelled possessions should be recoverable. Similar considerations can apply to lane closures on motorways. Health and Safety is becoming ever more important and the Construction, Design and Management (CDM) regulations impose certain obligations on the client. Bridge inspection work often falls outside the regulations. However, the client should ensure that adequate Health and Safety measures are proposed by the assessment engineer. Providing adequate Health and Safety is expensive and in the past, attempting to minimise the tender price, assessment engineers have sometimes taken unacceptable risks. The client should carry out risk analysis of each bridge in a commission so that access arrangements and methods of working are common to all tenderers.
1.2.5 Site work
Having carried out a desk study, collected existing information, obtained the necessary approvals and established a safe method of working, the inspection and possible testing of the structure can be carried out. It is important to realise that an inspection for assessment differs from other types of
inspection.Given limited possession times, it is essential that effort is concentrated on areas that will be critical to the assessment calculations.It may be useful to carry out a preliminary analysis of the structure prior to site works, so that these critical areas can be identified.
4
The extent of testing required is often left t o the discretion of the assessment engineer. This is an unacceptable practice in a lump sum bid as it leaves the assessment engineer with the dilemma of how much t o include and the client with lump sum bids which may vary widely depending upon the assumptions made by the assessment engineer. The requirements for testing can generally be best determined after or during the inspection and should be paid for based upon a schedule of rates t o be expanded after discussion between the client and assessment engineer on the testing required t o achieve the clients objectives. Alternatively, the client must define precisely what testing is t o be undertaken in the contract documents.
1.2.6 Level O f aSSeSSment and analysis
The expectations of the client on the degree of sophistication of analysis must be defined clearly. The client should not, as was frequently the practice, leave the assessment engineer with the requirement t o include for more sophisticated analysis if the bridge fails under simple methods. This approach requires the assessment engineer t o determine at tender stage whether a particular bridge is going t o fail a simple assessment and, therefore, t o include for further work. In view of the introduction of BA79, and the growing interest in asset management, the client should state the minimum level of assessment. Therefore a clearly defined approach t o analysis is required with payments for the work undertaken. Non linear computer work, special modelling and reliability analysis in accordance with a Level 4 or 5 assessment in BA79 should be paid for o n a time basis, or by an agreed lump sum when the extent of the work can be defined clearly. Generally, an analysis of the assessment of a bridge t o carry abnormal load vehicles should be carried out if a 40 tonne Assessment Live Loading is obtained. There may be bridges on certain minor roads where this process cannot be justified. The assessment live loading model has changed from HB loading t o Special Types General Order (STGO) loading (see Section 5.4).
1.2.7 Management
Of
reSU[tS
1.3 Staged aSSeSSmentS
Once an assessment has been completed t o an appropriate level of sophistication, which has been agreed with the client and Technical Approval Authority (TAA), see Chapter 2, then the results need to be managed. If the result is less than 40 tonnes then the bridge is defined as sub-standard and should be managed in accordance with BA79. If the bridge has achieved 40 tonne Assessment Live Load (ALL) t o an agreed minimum level of analysis then the result can be used t o assist in determining a long-term maintenance regime for the structure. Finally, it is necessary t o determine a date by when the bridge is re-assessed.
The traditional approach t o assessments has been t o use the simplest method of analysis in order t o achieve a 40 tonne Assessment Live Loading. Indeed, this technique is still referred t o in paragraph 6.2 of the 2001 version of BD2l as follows:
5
0 me assessment process
the choice of the appropriate method (of analysis) will depend upon the structural form of the bridge and the required degree of accuracy. The simple methods, although conservative, are quick to use and should be tried initially where appropriate, before ‘I.
..
progressing to more accurate but more complex methods.
’I
A staged approach t o assessment has been defined in BA79
1.3.1 BA79: The management of substandard highway structures
This Highways Agency (HA) Advice Note was published t o provide guidance on the implementation of interim measures for structures that were assessed to be substandard, or provisionally sub-standard, in order t o maintain the safety of the highway network. It provided further advice on assessment methods and, for the first time, set out formal
definitions of levels of assessment in its Appendix B. It was also intended that the Advice Note would provide guidance applicable t o “future
cyclical assessments within maintenance management programmes”. The Advice Note was drafted by a working group consisting of representatives from a large number of bridge owners, consulting engineers, safety experts and the Transport Research Laboratory. I t s use is applicable t o virtually all highway bridges and, for the first time, gives the bridge owner a sound method for dealing with the problem of managing bridge and other highway structures that have failed assessments.
1.3.2 BA79: Advice O n assessment
The details given in paragraph 2.1 of the BA79 are important and are repeated here in full. “The process of assessment and subsequent action is of crucial importance for ensuring that all highway structures remain in a safe and serviceable state. The assessment rules and criteria need to be applied rigorously and in a consistent manner. Ifassessments are unduly conservative, structures will be unnecessarilystrengthened, using up scarce resources and causing traffic disruptions. A t the same time, if the rules are lax, or applied unevenly, some structures where the margins of safety are unacceptable may be left without appropriate measuresbeing implemented. I’
The Advice Note provides a flow chart t o illustrate the assessment process and related measures. It also recognises the importance of recording the various deliberations associated with the management of sub-standard structures and provides a typical proforma. This Advice Note has more references t o the role and requirements of the Technical Appraisal Authority than probably any other HA Standard or Advice Note. These topics are discussed further in Chapter 12.
6
Paragraph B1.3 of Appendix B of the Advice Note states: “Structural failure is not acceptable t o the public hence the order of probability of failure inherent in the assessment criteria is very small. When a structure is assessed to be substandard, it does n o t mean therefore that it will necessarilyfail or collapse. However, if such structures were left in large numbers without remedial action, there may be an unacceptable risk that a collapse in service would occur. The assessments are based on probabilities and therefore it is impossible to know beforehand which bridges would actually fail in practice. ”
1.3.3Levels O f aSSeSSment
It is possible t o carry o u t assessments at five distinct levels, as defined by BA79. Existing assessment Standards and Advice Notes are only applicable t o Levels 1, 2 and 3. Any assessment carried out t o Levels 4 and 5 will require the detailed approval of the TAA concerned. Each level of assessment has been summarised below. Reference should be made t o Appendix B of BA79 for full details.
Level 1 - Simple The simplest level of assessment, using simple analytical methods, which will give a conservative estimate of load capacity.
Level 2 - Refined This level of assessment involves the use of more refined analysis and better structural idealisation. More refined analysis may include grillage analyses and finite element analyses. Non-linear and plastic methods of analysis may also be used. This level also includes the determination of characteristic strength for materials based on existing available data, but not specific site testing.
Level 3 - Bridge Specific A bridge specific assessment live loading may be derived rather than the use of values from BD21 or BD50 (see Section 1.4). This approach is unlikely t o be justified for short span bridges. This level allows the use of specific material testing t o determine Worst Credible Strengths.
Level 4 - Modified Criteria This level involves the use of partial safety factors that would be specific t o a particular bridge. It should be noted that the derivation of structure specific partial safety factors involves applying the fundamental principles of reliability theory. The Advice Note offers the following warning: “The background knowledge and engineeringjudgement requiredfor this level of assessment is of a high order and hence, the TAA needs to be closely involved.” Draft rules have been prepared but not issued by the HA although advice can be sought from the HA if required.
7
Level 5 - Reliability This level involves reliability analyses that require probability data for all the variables defined in the loading and resistance equations. The Advice Note states: “This type of assessment requires specialist knowledge and expertise and is only likely to be worthwhile and possible in exceptional cases.”
1.3.4COnChSlOnS
For bridge owners with a small number of structures on mainly local roads it would be sensible to continue to use the simplest level of assessment that is appropriate. However, for larger owners with a more complex stock of bridges on primary routes or motorways, there are good economic reasons for developing whole life costing maintenance strategies. In these circumstances it is necessary to carry out assessments to a consistent standard, say Level 3, rather than just determine ‘pass’ or ‘fail’.
1.4 Assessment standards
The Highways Agency provides the principal source of Standards applicable to the design and assessment of highway bridges in the United Kingdom. These documents are contained within the Design Manual for Roads andBridges. For completeness, the documents current at the time of writing are listed here; the Reader should check that he/she is using the current versions and any standards issued subsequently. The requirements of other bridge owners are also considered.
1.4.1 Highways Agency documentation
The documents listed below form part of the Design ManualforRoadsandBridges (DMRB) in Volumes 1 and 3. They are logged on website:-
http://www.standardsforhighways.co.uk/dmrb/index.htm, which should be consulted for revisions and updates. Throughout they are referenced as Highways Agency publications but in this context this normally includes the other overseeing departments - The Scottish Executive, The Welsh Assembly and The Department of the Environment for Northern Ireland. For details refer to the publications themselves.
Volume 1 Section 1 Approval Procedures BD2/05 Technical Approval of Highway Structures on Motorways and Other Trunk Roads Part 1 : General Procedures
Volume 3 Section 1 Inspection Part 2 BD54/93 Post-tensioned Concrete Bridges. Prioritisation of Special Inspections Part 3 BA50/93 Post-tensioned Concrete Bridges. Planning, Organisation and Methods for Carrying Out Special Inspections BD63/07 Inspection of Highway Structures Part 4
a
Section 2 Maintenance Part 1
BD62/07
As Built, Operational and Maintenance Records for Highway
Structures Section 3 Repair
Part 1
BA30/94
Strengthening of Concrete Highway Structures Using Externally Bonded Plates
Part 2
BA43/94 BD27/86 BA35/90
Part 3
BA83/02
Strengthening, Repair and Monitoring of Post-tensioned Concrete Bridge Decks Materials for the Repair of Concrete Highway Structures The Inspection and Repair of Concrete Highway Structures Cathodic Protection for use in Reinforced Concrete in Highway Structures
Section 4 Assessment Part 1
BD46/92
Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 2 - Modern Short Span Bridges
Part 2
BD50/92
Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 3 - Long Span Bridges
Part 3
BD21/01
Part 4
BA16/97
The Assessment of Highway Bridges and Structures The Assessment of Highway Bridges and Structures (Incorporating Amendment No. 1 dated November 1997 and Amendment No. 2 dated November 2001)
I
Part 5
BA38/93
Assessment of the Fatigue Life of Corroded or Damaged Reinforcing Bars
Part 6 Part 7
BA39/93
Assessment of Reinforced Concrete Half-joints The Assessment and Strengthening of Highway Bridge Supports
Part 8
BD48/93 BA54/94
Part 9
BA55/06
Part 10
BA52/94
Part 1
BD56/96
The Assessment of Steel Highway Bridges and Structures
Part 2 Part 3
BA56/96 BA51/95
The Assessment of Steel Highway Bridges and Structures The Assessment of Concrete Structures Affected by Steel Corrosion
Part 4
BD44/95
The Assessment of Concrete Highway Bridges and Structures
Part 5 Part 16
BA44/96 BD61/96
The Assessment of Concrete Highway Bridges and Structures The Assessment of Composite Concrete Highway Bridges and
Part 17
BA61/96
Structures The Assessment of Composite Concrete Highway Bridges and Structures
BD34/90
Technical Requirements for the Assessment and Strengthening
BA34/90
Programme for Highway Structures. Stage 1 - Older Short Span Bridges and Retaining Structures Technical Requirements for the Assessment and Strengthening Programme for Highway Structures. Stage 1 - Older Short Span Bridges and Retaining Structures
Load Testing for Bridge Assessment The Assessment of Bridge Substructures and Foundations, Retaining Walls and Buried Structures The Assessment of Concrete Structures Affected by Alkali Silica Reaction
9
1 Uhe assessment process
Part 18 BA79/06 Part 19
Part 20
1.4.2 Departures from Standards
The Management of Sub-Standard Highway Structures (Incorporating Amendment No. 1 dated August 2001) BD86/04 The Assessment of Highway Bridges & Structures For the Effects of Special Types General Orders (STGO) and Special Order (SO) Ve hicles BD81/02 Use of Compressive Membrane Action in Bridge Decks
The relevant Highway Authority’s procedure for agreeing structural ‘Departures from Standards’ and ‘Aspects not covered by Standards’ is an important part of the Technical Approval procedure but will vary between different Authorities. It is usual for the procedure to require the assessor/designer to recommend a Departure and, with the agreement of the checker, submit this with a justification to the Technical Approval Authority (TAA). The TAA is required to consider the proposal and should seek advice from an appropriate specialist, who has a particular expertise in the subject of the Departure. Usually, final approval and endorsement requires a positive recommendation from both the TAA and the relevant specialist. See Chapter 2 for further information on the Technical Approval process.
1.4.3Other bridge Owner8
This Guide is written primarily for highway bridges. They are equally applicable to motorway bridges and bridges carrying local roads. Therefore, there will be a large number of bridge owners involved besides the Highways Agency.
~
Another major national bridge owner, Network Rail, is responsible for a significant number of road over rail bridges although these are mostly of metal or masonry construction. Network Rail issues Railway Group Standards that are similar to, and make extensive reference to, Highways Agency Standards. Network Rail also issues a number of Current Information sheets, which effectively provide information on seeking Departures from Standards. There are a large number of autonomous local authorities, some of which have specialist bridge teams. The Bridges Group of the Engineering Committee of the CSS (formerly the County Surveyors Society) provides a network support group for local authority bridge owners. CSS Bridges Group also co-ordinates local authority input into Highways Agency Technical Project Boards. The Department for Transport (DfT) is embarking currently on a programme of providing national advice to local authority engineers - including those responsible for the bridge stock. This is particularly important as the HA Standards have generally been focussed towards the high-speed motorway and trunk road network. Normally, local authorities would follow Highways Agency Standards and Advice Notes, particularly with regard to assessment and design Standards. However, few Standards are written specifically with local roads in mind and fully justified Departures from Standards may be necessary, particularly for geometric problems.
10
1.5 Assessment and asset management
It has been acknowledged by the Highways Agency (HA) that the assessment of the bridge stock is likely t o become a ‘continuous’ process. This is so that bridge maintenance strategies can become safety related and based on whole life performance techniques rather than solely based on condition. It is interesting t o note that Network Rail has a domestic assessment programme based o n an 1 8 year cycle. This Section is based substantially on a paper entitled Whole life Performance BasedAssessment Rules BackgroundandPrinciples’ It provides useful background although some of the details have changed since publication of the paper.
’.
1.5.1 Whole life assessment
The procedures proposed by the Highways Agency are intended t o ensure that present levels of reliability o f existing bridge stock should be maintained at levels considered t o be socially and economically acceptable, without the economic consequences of undue conservatism. To provide marginally excessive reserves of safety in a design adds little t o first cost, but the provision of unnecessary margins in assessing existing bridges for increased loading or deterioration can be disproportionately expensive. For ease of use in conjunction with current practice it was proposed t o relate reliability t o the load capacity factor, K,on HA loading as defined in BD21, which a structure in service may be assessed t o achieve. That factor takes into account any deterioration in structural resistance. ‘Profiles’ of diminution of ‘available’ K factors with time may be derived from surveys of representative bridges and empirical predictions of deterioration. The procedure proposed by Flint and Das’ was developed into a draft Highways Agency Advice Note BA81 entitled Whole life assessment of highway bridges andstructures. This was trialled by a number of HA Maintaining Agents during 2000. The requirement was t o determine the performance level of the structure at the time of assessment, I,, and compare this t o the Critical Performance level, I,, ,. The present assessment process (BD21) is such that I , has t o be greater than I,, , t o achieve an assessment ‘pass’. If I , is less than I,, , then the structure has t o be managed in accordance with the advice given in BA79 (see Figure 1.I). The next stage in the development of any asset management plan is t o attempt t o determine when structures, where I , is greater than I,,,,, will reach a Critical Performance level. In order t o achieve this goal i t is necessary t o have an understanding of the deterioration rate. Unfortunately, published deterioration models need t o be treated with care and considerable judgement is required in their use.
1.5.2 Asset management
If I , and I,,,, can be reliably determined, together with a reasonable approximation of future deterioration, then it is possible t o evaluate various maintenance options using whole life costing techniques (see Figure 1.2). If the maintenance options for individual structures are combined in a comprehensive bridge management system then it is possible t o determine a strategy t o maximise the overall value of the asset.
11
No. of years from construction t Performance level a t time of construction I, Performance level of the structure being assessed a t time t I, lCri Critical performance level Whole life
- -
- ...
performance profile shown dashed \
\
- - - - - - - - - - > - - -\ \
0
t
No. of years from construction
Structure (a)
Option 1 2
Maintenancestrategy strengthen the structure now weight restrict structure/implement interim measures and monitor
Structure (b) Option 1 2 3
Maintenancestrategy strengthen the structure now undertake minor repairs to abate deteriorationhonitor the structure do minimum work now and replace at reduced life t,
0
t,
t2
No. of years from construction
The simplistic annual review of maintenance budgets is now outdated. Bridge owners need to be able to demonstrate the effects of various levels of funding on the value of the asset. The CSS produced a useful paper’ on recommended levels of maintenance spending and this approach is probably satisfactory for owners of small numbers of bridges. However, a substantial amount of work is now under way to link the production
12
of performance indicators, asset management plans and asset valuation models, and although the discussion of these topics is beyond the scope of this Guide, i t is clear that there will be a substantial role for effective bridge assessment techniques t o play in the overall process.
1.5.3COnSeqUenCeS for assessment
1.5.4A
COntinUOUS
a s s e s s m e n t programme
1.6 Assessment of the serviceability limit state
If comprehensive asset management plans are t o be developed based on whole life costing techniques and asset management plans, then it will be necessary t o accurately determine I,, I,,,, and deterioration rates. Use of this Guide should assist in this process.
Currently, the Highways Agency is considering proposals t o re-assess a proportion of its bridge stock every year t o ascertain their adequacy t o support imposed loads. Such reviews would be undertaken when significant events occur that could increase the imposed loads above those previously assessed and/or reduce the load-bearing capacity of structures. Typically, a re-assessment every 12 years might be anticipated. It will be for other users t o determine whether t o follow a similar approach.
An alternative approach t o the notion of a continuous assessment programme is the development of procedures for assessment at the serviceability limit state (sls). Although currently some checks are required at the sls, particularly when plastic methods of analysis are used (see Section 6.7),the UK approach t o assessment has safety and the ultimate limit state (uls) capacity of a structure as its focus. Proposed by Lark and Mawson’ 5! the aim of assessment at the sls would be t o create a more positive link between assessment procedures and the sls criteria that drive maintenance decisions, and t o provide a framework for recording and incorporating the information that has allowed engineers in the past t o assess a structure on the basis of engineering judgement. Although this concept is closer t o what is currently continental practice, there is evidence that the latter is moving more towards the contemporary UK approach of ultimate checks and so the demand for a serviceability approach has yet t o be proven. Nevertheless, it is also recognised in the UK that many of the structures that have been assessed as sub-standard have been so classified, n o t because of deterioration but because of the enhanced requirements of the technical standards. The response of Collins16 of the National Assembly of Wales has been therefore t o call for the development of asset management policies based on the whole life cost of maintaining the function of highway structures rather than just their strength, an approach that is more compatible with the concept of assessment at the sls. However, if such an approach is t o be used in the UK, further work is required t o support its general implementation and therefore this Section is provided for information rather than direct guidance, as is the case in Chapters 2 t o 12.
13
1.6.1 Serviceability assessment
The justification for proposing an alternative approach t o assessment based on serviceability limit state criteria is that certification of a structure today is no guarantee of its performance tomorrow. To address this, what has been suggested is a procedure in which the everyday response of a structure is monitored and then projected forward, and a ‘change management’ procedure is adopted whereby it is the variation of the reliability of the in-service structural and environmental response of the structure with time that is of interest. It has been shown that the reliability of a structure with respect t o an environmental
serviceability limit state, such as the onset of corrosion, can be expressed as a reliability index, the time-dependent variation of which can then be derived by assuming a deterioration model. This, typically, is a function of the chloride levels in the concrete’ The latter is a measurable parameter. Therefore by monitoring chloride levels, the likelihood of corrosion being a problem can be anticipated and, by comparing the development of this likelihood with time, with that predicted on the basis of the design parameters, a rational decision on the need for preventative action can be made.
’.
Likewise, but not yet explored t o the same extent, it is also possible to calculate the reliability of a bridge with respect t o structural serviceability limit states. The variation of this reliability with either the applied loads or the material resistance can be determined therefore, and if these in turn are related t o time, (for example due t o changes in loading regimes and the perceived likelihood of extreme loading events, or t o changes in material resistance due to the effects of creep, shrinkage and/or deterioration, etc.), then the variation of this reliability with either these changes themselves or with time can also be examined. As with the environmental example given above, the significant benefit of adopting a sls approach is that the analytical procedure and limit state models adopted in this procedure can be verified by monitoring the response of the structure. Then, if found t o be inappropriate, they can be modified t o reflect the actual behaviour of the structure and, when satisfactory, can be updated in response t o the past performance of the structure. In other words, the actual service of the structure can be used as a ‘proof’ load and by monitoring the structure’s behaviour the uncertainties associated with its assessment can be continually refined and reduced. The challenge of this approach will be t o identify appropriate criteria that can express capacity a t the sls. This is not something that will be easy to do with confidence, because existing definitions of sls behaviour are generally stiffness related whereas uls behaviour is strength related and currently there are no uls criteria for effects such as those due t o shear. Nevertheless, what is being proposed is a procedure which seeks to make use of an understanding of the reliability of bridge structures applied in such a way that it can support bridge managers’ engineering judgement of the response of their stock, and can be enhanced by both traditional and future bridge monitoring techniques. For example, strain, deflection and/or the nature of cracking patterns may be used currently to characterise the response of a structure to both loads and movements from which its structural reliability can be identified. Likewise chloride levels, moisture content, porosity and degree of corrosion, etc. are measures of environmental reliability. In the future therefore, it can easily be envisaged that defect, vibrational and other NDT monitoring techniques may also provide suitable methods for gauging the ‘in-service’ reliability of a structure.
14
i
Whichever ‘measure’ is adopted, it is the variation of the resulting reliability from that anticipated a t the time of design which is significant; with improvements representing an enhanced understanding of the structure’s behaviour and reductions typically being due to changes in loading and deterioration, although they can also be due to changes in understanding of the performance of the structure. What matters is that these variations in reliability are managed. Thus, if the in-service reliability of a structure is reduced because of deterioration or an increase in loading, the question that must be answered is can it be restored through a knowledge of the actual behaviour of the bridge or by tighter control of the traffic using it, or are remedial works required. Unlike assessment a t the uls, the adequacy of the response to this question can be monitored and the bridge manager can be assured that a margin of safety against total collapse still exists. The suggestion is that this provides an opportunity to manage the risk posed by the structure and, in each period, to address the changes which have occurred in both the response and loading of the structure, whether due to traffic, environmental or deterioration effects; to assess the consequences of these changes and, on the basis of this assessment, to implement the necessary monitoring, controls or remedial action. Further research is required to identify exactly how the above will be achieved and the data that is needed to implement it.
1.7 Historical aspects O f assessment
This Section is a summary of a paper entitled A review ofbridge assessments through time, presented by Barry Mawson, Gwent Consultancy, to the South Wales branch of the Institution of Highways and Transportation on 10 February 2000, and has been reproduced by kind permission of the author. To review developments in bridge assessment it is necessary to consider the history o vehicular loading and the link with design standards. The French appear to have been the first to produce a design loading code for highway bridges in about 1850. In 1894 the Dutch produced a similar loading standard consisting of two alternative carts, a lightweight and a heavyweight cart, both drawn by a single heavy horse. Nineteenth century bridge designers in the UK were still allowed some freedom. A uniformly distributed load of Icwtlft’ together with the local effect of a 10 tonne wheel load was commonly applied. A typical British bridge design towards the end of the 19th century would accommodate a single 30 tonne vehicle on four wheels. In 1922 the Ministry of Transport produced its first standard load train for highway
bridges. It consisted of an engine pulling three heavy trailers. In 1931 the Ministry of Transport devised a standard loading for highway bridges, referred to as the equivalent loading curve. For any particular loaded length, this curve represented an equivalent uniformly distributed load, which, when combined with a specific point load located within the loaded length, would produce the same bending moment and shear force as the original loading train. This simplified quick analysis of smaller and standard bridges in the design process.
15
The assessment process
Axle weights and gross vehicle weights continued to increase and by 1973 a 10 tonne
axle and 32 tonne maximum gross vehicle weight was permitted. The Ministry of Transport published a loading standard BE1/77, which was in parallel with the revised loading standard B S I 53. The loading consisted of two types, HA loading which was similar to the earlier equivalent loading curve, and an additional HB loading model to represent abnormal vehicles. Overloading of vehicles tended to increase, traffic densities increased, and in May 1983 gross vehicle weights were permitted under ‘Construction and Use Regulations’ to increase from 32.5 tonnes to 38 tonnes provided they were on 5 or 6 axles. Then EC Directive 85/3 required a minimum standard to be allowed in all EC countries, i.e. 40 tonnes on 5 axles, and that axle weights should be permitted to a level of 11.5 tonnes. A derogation allowed the UK to delay implementation until 1 January 1999. In 1984 the Department of Transport embarked on a 15 year programme for rehabilitation of highway structures. The assessment programme was organised in three
stages. Stage 1 covered older short span bridges which were not designed for, or known to be designed for, present day loading requirements, i.e. pre-1922. Stage 2 of the programme covered modern short span bridges designed prior to changes in design rules introduced in 1973, notably for shear in concrete. Stage 3 included long span bridges where increase in long length loading was important, due to the effects of a larger HCV proportion in traffic queues. The history of bridge assessment goes back well into the 19th century for railway bridges. The London North Western Railway commenced a programme of assessment in the 1880s. This was followed by a more general assessment of railway bridges in the 1920s and led to the Bridge Stress Committee during the period 1923 to 1928, which specifically covered the dynamic effects on railway bridges. The need to assess road bridges was first properly considered in the 1960s with the move towards the 32 tonne vehicle. This was implemented with the Assessment Standard BE4 and was aimed a t the pre-1922 bridge stock. The further increase in lorry weight to 38t was again covered in 1984 by the Department of Transport Standard BD21/84. This was amended with the introduction of the potential for the 40t lorry in 1987 and consolidated into the standard in 1993 (BD21/93 - now BD21/01). A considerable number of highway bridges were in the ownership of the railways and their duty was limited to a design capacity of ‘the traffic of the day’. This was defined clearly in the Statutory Instrument 1705 in 1972 which set the railway’s loading obligation as that defined by assessment standard BE4. No allowance was made for the reference in BE4 to the fact that it covered bridges:
0 Not designed to HA loading and so considered as sub-standard and having a limited life. 0 That their replacement should not be unduly delayed. 0 That structures should be critically examined a t such intervals as their conditions require but a t least once every three years.
16
The standard loading curves used in assessment had been developed from the design standard and allowed for the likelihood of impact, bunching and potential overload of vehicles. Research showed that this was unduly conservative for assessment and the introduction of assessment standard BD21/97 gave reductions of up to 35% of load effect. Following the new vehicular standards of January 1999, modifications required to the assessment standard were introduced to cover the weight limits of 18 tonnes to 26 tonnes, which allow a 11.5 tonne axle. Further changes may be necessary if the general use of 44 tonne, or heavier, vehicles is permitted.
1.8 European develop ment s
It is beneficial to study the process of bridge assessments overseas, particularly in Europe. A European research project, known as Bridge Management in Europe (BRIME), has been
undertaken to develop an outline framework for a bridge management system for the European road network. This Section summarises five papers, which were presented at the Fourth International Conference on Bridge Management a t the University of Surrey in April 2000. The original papers are detailed in the References below.
1.8.1 The BRIME project
The project was co-ordinated by the Transport Research Laboratory in the UK in collaboration with five European partners. The project was divided into eight work packages as follows: 1. Classifying the condition of a structure. 2. Assessing the load carrying capacity of existing bridges, including the use of risk-
based methods. 3. Modelling of deteriorated structures and affect of deterioration on load carrying capacity. 4. Modelling of deterioration rates. 5. Deciding whether a sub-standard or deteriorated structure should be repaired, strengthened or replaced. 6. Prioritising bridges in terms of their need or repair, rehabilitation or improvement. 7. Reviewing systems for bridge management and development of a framework for a bridge management system. 8. Project co-ordination. The deliverables from the BRIME project may be viewed on the TRL website, a t http://www.trl.co.uk/brime.This Section summarises the contents of work packages 1 to 5 1 8 ' 0 ' 1 2
1.8.2 Structural assessment
In most of the partner countries, assessment is only carried out on specific structures when there is a need to carry an exceptional load or where the bridge has been subjected to deterioration, mechanical damage, repair or change of use. The UK is
17
unusual in that it has carried out a comprehensive programme of assessment where bridges have been built t o outmoded design standards and have not been assessed t o current standards. The rules used in bridge assessment are provided mainly by design standards with additional standards relating t o testing methods including load testing. The UK is unusual in that it has a comprehensive set of assessment standards. Generally, limit state methods are used, although Germany still uses allowable stress techniques.
1.8.3 Deterioration
The current practice is for all of the partner countries t o take account of deterioration in some way. Except for the UK, taking account of deterioration is, in general, carried out on an adhoc basis and depends on the knowledge and experience of the assessment engineer. The UK documentation tends t o be general in nature and contains little quantitative guidance. At present there are few procedures available for taking deterioration into account in a structural assessment.
1.8.4 Chloride ingress
In order t o estimate the remaining service life of a bridge, it is necessary t o predict deterioration rates once the existing condition of the bridge has been assessed. The objective of this work package was t o consolidate and improve existing knowledge of chloride penetration in concrete, which is the principle mechanism of deterioration. The work package was not complete a t the time the paper was written. However the initial conclusion was that the technique was not suitable for general use and that only experienced engineers or corrosion experts should be allowed t o act upon the results generated by a chloride ingress model. The authors stated that although chloride ingress models could be used for determining future maintenance, they should not be used for assessing structural capacity or deterioration.
I .9References
1.1
PARSONSBRINCKERHOFF, TechnicaiAuditoftbeappiication ofBA79 and A rewewofbridgeassessmentfailureson the Motorway and TrunkRoadnetwork, Final Reports HBR80616 - Highways Agency Contract 2/419, Parsons Brinckerhoff, December 2003
1.2 CONCRETE BRIDGE DEVELOPMENT GROUP, ibeassessment ofconcrete bridges, 1, CBDG, Camberley, 1997 1.3 FLINT, AR and DAS, PC. Whole life performance based assessment rules - Background and principles, SafetyofBndges Conference, Thomas Telford, London, 1996.
1.4 CSS, FundingforBridge Maintenance, CSS Bridges Croup, February 2000. 1.5 LARK, RJ and MAWSON, BR, Assessment at the serviceability limit state. BridgeManagement 4, (Ryall, MJ, Parke, CAR and Harding, JE, eds), Thomas Telford, 2000, pp 426-433.
1.6 COLLINS, TJA, "Highway Function" approach t o bridge maintenance and a more flexible approach t o design loading and dimensional requirements, Surveyors'Annuai Bridge Conference, March 2002
1.7 CEHLEN, C and SCHIESSL, P, Probability-based durability design for the Western Scheldt Tunnel, Structura,'Concrete, Vol P1, No 2, 2000. References 1 8 to 1 12 below were all in BndgeManagementl (Ryall, MJ, Parke, CAR and Harding, JE, eds), published by Thomas Telford Ltd in 2000.
1.8 WOODWARD, RJ, VASSIE, PR and CODART, MB, Bridge Management in Europe (BRIME) overwew of project and review of bridge management systems.
1.9 BLANKROLL, A eta/, BRIME -Chloride ingress and bridge management 1.10 KASCHNER, R, HAARDT, P, CREMONA, C, CULLINCTON, D and DALY, AF, Bridge Management in Europe (BRIME) structural assessment
1.11 DALY, AF, Bridge Management in Europe (BRIME). modelling of deteriorated structures 1.12 BERC, L eta/, Bridge Management in Europe (BRIME) condition assessment of bridge structures
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2. Technical approval and documentation 2.1 h l e r a l procedures
This Chapter is intended t o introduce the Technical Approval procedure for the assessment, design of strengthening and refurbishment of highway structures t o engineers with little previous experience of the subject. It includes: 0 General principles of the procedure
0 An explanation of its objectives 0 A description of the key stages of the procedure 0 Flow charts and figures.
The Chapter is not intended t o give the full details of every aspect of the procedure. Nor can i t cover in detail how i t is implemented by different Highway Authorities, and the many different types of contract and procurement methods currently in use. However, i t should give assessing and checking engineers basic information about the procedure and, most importantly, explain the need t o involve the Technical Approval Authority (TAA) right from the start of the assessment process. The details included refer specifically t o the Highways Agency procedures. Local Authority and other bridge owners (e.g. British Waterways and Network Rail) have similar Technical Approval procedures using the principles given in this Section. One key difference is that normally the TAA function is not separated from the in-house engineers.
2.2 General principles
The Technical Approval procedure is a form of Quality Assurance for Highway Authorities and the owners of highway structures used by the general public. The procedure was first introduced into the UK following a number of bridge failures in the early 1970s. These failures were blamed in part on the use of inappropriate design Standards and also because designers of the time were adapting the Standards for use on structures that were outside the limits for which they could be used safely. The procedure was introduced following an official inquiry into these failures and has been modified and adapted over time. The procedure is a mandatory requirement for all Department for Transport (DfT) funded highway structural design work of any significance* '. This includes designs for new bridges and other highway structures, bridge assessments and structural modifications, refurbishments and repairs; the standard procedure for the assessment and strengthening of highway structures and its relationship with the design approval procedure is illustrated in Figure 2.1. The procedure is also applied t o any temporary structural works on the highway that could affect the safety of the public. Some form of Technical Approval or Appraisal Procedure is a requirement of all other Highway Authorities and rail structure owners in the United Kingdom. The detailed requirements of the procedure in use by the Highways Agency are set out in the standard BD2/02 (see Section 1.4). Technical Approval of highway structures is an ongoing process, which starts at the concept or feasibility stage. It continues through design, construction and on through the
19
Design standards
Assessment standards
Departures from standard to be used in assessment agreed with TAA
E
i
Assessment report Non-compliance with standard revealed in assessment
Departures to be retained in the structure agreed with Highway Authority
1
I Strengthening/ refurbishment AIP
Implement agreed action refer to BA 79/98 Management of Substandard Structures
I
a
I
I
Departuresfrom Standard (ULS & SLS) proposed for strengthening / refurbishment works agreed with Highways Authority
New works to current design standards plus ULS departures in existing elements agreed with the Highway Authority
I
1
Strengthening / refurbishment design
1
-.. .. +-.
nt
Figure 2.1 Standardsfor assessment and strengthening of highway structures.
-
life of the structure, including steady-state assessment and any strengthening, repairs or modifications. The process is only concluded finally with the demolition of the structure. The various documents, which are a product of the process throughout the structure’s life, form a very important part of the structure records and should be carefully retained.
2.3 Objectives
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The original, and still the primary, objective of the procedure is to ensure the safety of highway structures by ensuring adequacy of structural design, assessment, construction and of steady state maintenance. Recent CDM legislation and court findings have made it even more important for Highway Authorities and other structure owners to maintain a robust procedure for ensuring structural safety.
The procedure for assessment has had to be developed and extended to incorporate resulting strengthening, modification and refurbishment of the structures. Highway Authorities and bridge owners have introduced additional objectives with the aim of ensuring good value for money, minimising environmental impact, improving sustainability and aesthetics. The recent introduction of new forms of contract for the maintenance and procurement of highway works has led to the need to further modify how the procedures are applied.
2.4 Key Stages
The detailed application of the Technical Approval (TA) procedures for the assessment of highway structures depends on the particular requirements of the Highway Authority or structure owner. However, this does not alter the general principles of the procedure, which can be adapted to apply, in most instances, to all forms of structure. Technical Approval or TA should start a t the concept planning or feasibility stage of any assessment, strengthening, modification or refurbishment work involving highway structures. The earlier the assessment engineer involves the Technical Approval Authority (TAA), usually the better. The procedure requires that the TAA have a responsibility to ensure that only competent and experienced engineers carry out the assessment of highway structures and the check of the structural work. The precise nature of the check and the relationship between assessor and checker is dependent on the category of the structure. As soon as an assessor is designated or appointed, it is usual for discussions to take place between them and the TAA on the various assumptions to be made and options available for inspection and testing for the assessment. The assessor will also agree the category of the structure with the TAA, based on the complexity, size, value or risk of the project. BD2 defines four categories of structure: 0, 1, 2 or 3
0 Category 0 structures are typically buried structures of less than 3 m clear span and having more than 1 m cover, multi-cell buried structures where the cumulative span is less than 5 m and having more than 1 m cover or single span simply supported structures with a span less than 5 m. 0 Category 1 structures are those with a single simply supported span of less than 20 m and having less than 25" skew or buried concrete box structures with a clear span of less than 8 m. 0 Category 2 structures are those that are not otherwise defined as Category 0, 1 or 3. 0 Category 3 structures are typically complex structures, which require sophisticated analysis or with features such as high structural redundancy, unconventional, novel or esoteric design aspects, a span exceeding 50 m, skew exceeding 45", moveable bridges, bridges with suspension systems, post-tensioned concrete structures with internal grouted tendons.
21
2
However, BD2 does note that the category boundaries are not rigid. If in doubt, the assessor is required t o make the decision in consultation with the TAA having regard t o the wider issues of the value of the structure and the potential consequences of failure. In the case of Category 3 structures, an independent consultant is required t o carry out a check of the assessment. For Category 2 structures, this check may be carried out by a checking team, which may be from the same organisation but must be independent of the assessment team, while for Category 0 and 1 structures, although an independent check is still required, it may simply be carried out by another engineer from the original assessment team. The procedure for obtaining Technical Approval for an assessment is given in figure 2.2 and can be described as follows. The assessor completes an Approval in Principle document, which is generally called an AIP, or Technical Appraisal Form (TAF) depending on the procurement route being adopted, the latter typically being more used on design and build or related projects. This sets out the technical proposals for the assessment or design of strengthening and includes all the structure geometry, ground conditions and nature of the existing foundations. It also includes assumptions on material properties or proposed testing, methods of analysis and assessment loading criteria, etc. Any non-compliances or ‘Departures from Standard’ or ‘Aspects not covered by Standards’, which are being proposed at this stage should be identified and recorded on the AIP. Advice should be sought from the Highways Agency when considering Departures from their Standards or Codes of Practice. Usually, it will be necessary for the assessor t o discuss the proposed ‘Departures’ with the Highway Authority designated representative and/or TAA, who normally are required t o accept any such proposals. A description of the usual procedure for agreeing ‘Departures’ and ‘Aspects not covered by Standards’ is given below. A list of relevant approved Standards that are t o be used in the assessment is attached in the form of the Technical Approval Schedule (TAS). It is important to note that the AIP or TAF is a ‘living’ document subject t o revision and amendment during the assessment or design process. These are important record documents, which need t o be retained for future reference. The signed AIP is submitted t o the TAA for acceptance and to the checker for comment and agreement. The significance of a personal signature by the individual design or assessment team leader is to ensure ‘ownership’ and responsibility for the accuracy of the information in the AIP. Acceptance by the TAA will be dependent on their confidence in the assumptions, the method of analysis and material properties t o be adopted in the assessment or design. Usually, the TAA will be required t o seek approval for any ‘Departures’ from the Highway Authority’s designated representative if distinct from the TAA a t this stage. The detailed calculations required for the assessment or the design of the structure can then start, followed shortly by the check. At any stage the assessor/designer, with the agreement of the checker, may consider that further ‘Departures from Standards’ could safely be applied t o the particular structure t o improve the assessment. The introduction of any material changes or the introduction of further ‘Departures’ would require an
22
-----
Figure 2.2 Technical approval procedure for assessment.
4-
Assessment AIP submitted toTAA
I0
I
Proposed departures from standards* agreed by Highway Authority’s designated representative? Initial or higher level of assessment agreed?
I
1 I
L L TAA sign AIP/Addendum
Addendum t o AIP identify departures and/or higher level of assessment
Carry out assessment and check
tyes
Amendment t o AIP identified? Assessment required at next level? no Acceptable non-compliances with standards revealed by assessment?
Assessment and check certificates
Assessment and check certificates with reference t o assessment report (where non-compliances with standards revealed by the assessment are recorded)
+
+
Accepted criteria for departures from standard* entered in the AIP with an addendum endorsed by the TAA and the Highway Authority’s designated representative (departures and endorsement could be recorded on certificates i f required by the Highway Authority)
+ I
Assessment and check certificates accepted by TAA
TAA return a facsimile of the assessment and check certificates and retain the originals
and aspects not covered by standards
addendum to the AIP, recording all the changes to the original. This applies equally to any significant changes that might occur during the construction of strengthening works, etc. The approval procedure for strengthening and improvement works is similar to that above and is detailed in Figure 2.3. It is quite common for it to become necessary for a strengthening design to be modified during construction. This can result from a number of causes; the most common being unforeseen conditions necessitating a change to the
23
Non-compliance with standards* revealed by assessment of the existing structure?
1
.
Engineer considers each non-compliance with standards* and proposes whether it can be retained in the structure, or upgrading (to current design standards) t o be carried out
no
I
I
I
1
4-
I
Non-compliance with standards* revealed by the assessment t o be retained in the structure agreed by the Highway Authority’s designated representative?
I
I
Strengthening / improvement AIP
I Proposed departures from standards* t o be used in the design agreed by the Highway Authority
TAA sign AIP
Carry out the design and check
Design and check certificates
Accepted criteria for departures from standards* entered on the assessment and check certificates and endorsed by the Highway Authority’s designated representative
I Assessment and check certificates accepted by the TAA
I TAA return a facsimile of the assessment and check certificates and retain the originals for their records
24
proposed strengthening details. It is most important that any changes affecting the structure are subjected t o the same rigorous examination of the TA procedure as the original proposals. The final part of this stage of the TA procedure is the submission by the assessor/ designer and checker of the design and check certificates and their acceptance by the TAA. In the case of an assessment the assessor would normally submit a detailed Assessment Report, which will include a comprehensive extract of the Inspection Report and any test results. The Report will contain full details of the assessment findings and the assessed capacity of the structure. The assessment certificate and assessment check certificate should state the assessed capacity of the structure. As with the AIP and reports, these are important documents and need t o be carefully retained with the structure records.
2.5 'Departures' and 'Aspects not covered by Standards'
It has been suggested that the application of the TA procedure will inevitably stifle innovation. This can happen with the rigid application or misinterpretation of assessment Standards. In contrast, careful application of the procedures by experienced bridge engineers has been demonstrated t o encourage innovation by the use of 'Departures' or 'Aspects n o t covered by Standards'. The acceptance of non-compliances t o Standards, which can be retained safely, has been shown t o avoid unnecessary restrictions and expensive strengthening. It should be noted that most Highway Authority bridge engineers, who are designated as or by the TAA, are involved with the approval of the assessment of many structures. They are very well placed therefore t o promulgate best practice and safely consider 'Departures from Standards'. The full effect of any 'Departure from Standard' or 'Aspect not covered by Standards' is considered and formally notified to, and has t o be accepted by, the TAA. This includes any 'Departures', which can be justified by the use of higher levels of assessment. The procedure for the management of standards is detailed in Figure 2.1. The full effect of any 'Departure from Standard' or 'Aspect not covered by Standards' has t o be considered by the TAA including any 'Departures' arising from the adoption of different levels of assessment. The Highways Agency's Advice Note BA79/98 sets out the philosophical approach t o assessment and the principle of different levels of assessment. It identifies where 'Departures' are required for the higher, more complex levels of assessment. The assessment process can be carried out in 5 distinct levels with Level 1 being the simplest t o Level 5, the most sophisticated:
0 Level 1 assessments are carried out using simple analysis methods, in accordance with the assessment Standard BD2l and the accompanying Advice Note BA1 6. 0 Level 2 assessments involve more refined analysis and the use of characteristic strengths of materials. 0 Level 3 introduces the concept of Bridge Specific Assessment Live Loading and makes use of material test results t o determine worst credible strength values. 0 Level 4 assessments include a reduction in partial safety factors if these can be justified particularly where dead load or superimposed dead loads are high. 0 Level 5 assessments involve complex reliability analysis for particular structures.
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The relevant Highway Authority’s procedure for agreeing structural ‘Departures from Standard’ and ‘Aspects not covered by Standards’ is an important part o f the TA procedure but will vary between different authorities. It is usual for the procedure t o require the assessor/designer t o recommend a departure and, with the agreement of the checker, submit this with a justification t o the TAA. The TAA is required t o consider the proposal and should seek advice from an appropriate specialist, who has a particular expertise in the subject of the departure. Final approval and endorsement usually requires a positive recommendation from both the TAA and the relevant specialist. In the case of most Highway Authorities, the designated representative usually only approves ‘Departures’ and ‘Aspects not covered by Standards’. It is the responsibility of the TAA t o resolve any differences of opinion between the assessor and checker and this includes the use o f specific ‘Departures’ or ‘Aspects not covered by Standards’. This is particularly important in relation t o any ‘Departures from Standard’, which may be accepted by the Highway Authority, as all parties are required t o agree.
2.6 Reference
26
2.1
HIGHWAYS AGENCY, Highways structures Technical Approval, Easy Guide, Advice and Technical Appraisals Group, May 2002
3. Inspection for assessment 3.1 Process and general procedures
3.1 . I Background and introduction
Inspection is the first step generally in the assessment process, with the double objectives of establishing the condition and dimensions of the structural components and of verifying the form of construction. The extent of the work will depend on what is already known from archive and maintenance records, and therefore physical inspection is integrated with desktop studies. It may also be necessary to carry out in-situ and laboratory testing to establishkonfirm assessment parameters, and this is covered in Chapter 4. Even a t this early stage of the process, it must be remembered that the purpose of assessment is t o evaluate current and future structural capacity. Generally, this is done via analytical methods (Chapter 6), while taking advantage of any hidden strengths (Chapter 7). A major objective of inspection and testing is therefore t o determine input parameters for this analysis, particularly with regard t o geometrical and mechanical properties; where these properties are affected by different forms of deterioration, then the guidance given in Chapters 3 and 4 is supplemented by that in Chapter 9. For concrete bridges, the process and general procedures involved are those given in relevant Standards and Advice Notes (see Section 1.4), with BD21, BA50, BD and BA44, BD and BA63 being particularly relevant. Chapter 3 follows these procedures, although it should also be recognised that the situation is undergoing continuous change, as represented by the references at the end of this Chapter. This Chapter therefore deals with inspection for assessment, the management of the inspection process and the application of inspection results t o both one-off and continuous assessments. Section 3.2 covers condition assessment and specific test methods and techniques are discussed in more detail in Chapter 4. The concept of inspection, or in an ongoing context, condition monitoring, is to make best use of past, present and future performance-based data and observations to verify the current integrity, and more ambitiously t o predict the future performance, of a structure.
3.1.2 Requirements for inspection
BD21 states that inspection for assessment: . . is necessary to verfy the form of construction, the dimensions of the structure and the nature and condition of the structural components. Inspection should cover not only the condition of individual components but also the condition of the structure as an entity and especially noting any signs of distress andits cause.” ‘I.
The standard also gives specific requirements for Inspection for Loading and for Inspection for Resistance.
27
BD2l refers t o Volume 3, Section 1 of the Design Manua[forRoadsandBridges, which essentially means BD63 and BA63 for concrete bridges. BD63 defines the following four main categories of inspection: 1. Superficial involving a cursory check for obvious deficiencies. 2 . General involving a visual inspection of representative parts of the structure t o
ascertain its condition and note any items requiring special attention. 3. Principal involving a close examination of all parts of the bridge at six-yearly, or
exceptionally up to ten-yearly, intervals. Originally, Principal Inspections solely comprised visual examinations, but more recently limited testing has also been included. 4. Special Inspections involving a close inspection or testing of a particular area or defect. They are carried out for specific reasons such as: following up on a defect identified during an earlier inspection; checking for any effects caused by particular events (e.g. flooding, vehicle impact, etc.); and before and after the passage of abnormal loads. BD2l notes that General Inspections are unlikely t o be adequate for assessment purposes and also states that “all constituent parts ofthe superstructure shall be inspected to determine their respective strengths”. As a consequence, inspections for assessment are normally carried out in conjunction with Principal Inspections, but may also involve
Special Inspections. With regard t o substructures, foundations, retaining walls and wing walls, BD2l requires that all accessible parts be examined and any defects noted. This should also take into account any evidence of ground movements around foundations and behind abutments and wing walls. Underwater inspection of any submerged parts of structures should also be carried out. Special Inspections are also required for post-tensioned concrete bridges. Unusually these involve more invasive inspection techniques, the main aim of which is t o identify corrosion or the propensity for corrosion of the prestressing tendons. Details for the planning, organisation and methods for carrying out such inspections are given in BASOl93.
3.1.3 Future requirements for inspection
The Highways Agency proposed the introduction of new procedures for bridge inspections, aimed at those aspects of a structure that are most relevant t o its load carrying capacity and durability. The objective of these proposals was to make more effective use of testing, monitoring and the growing range of improved non-destructive testing techniques, and t o improve the quality and consistency of both the inspections themselves and the way in which they were reported. This was t o be done by combining regular visual inspections of the whole structure with a programme of more detailed investigations concentrating on known or suspected areas of deterioration. For each structure, a unique schedule was t o be developed that was specifically designed t o provide the information needed t o assess the structure adequately. However, these proposals have not received universal support and discussions are currently ongoing as t o how t o ensure that this information is acquired.
28
To provide a starting point for assessment, all structures should be subject t o a detailed inspection, the purpose of which should be t o establish precisely the condition of each structure a t the time of its assessment and t o provide a reference against which any future deterioration of the structure can be monitored. Such inspections should record the location of all imperfections and defects on scale drawings complemented with photographs and text. It is anticipated that regular inspections should then be carried out a t programmed intervals, concentrating on the condition of particular parts of the structure where deterioration has been found before or where it is suspected.
3.1.4 The inSpeCtiOtl process
The inspection process starts with the Client’s Brief and should include some or all of the following activities:
0 Review archive and maintenance records for pertinent information (e.g.as-built drawings, soils data, previous inspection reports, Health and Safety File, etc.). 0 A preliminary inspection t o determine access requirements and strategy, particularly in respect of critical elements. 0 Organise traffic management, access equipment, inspection, testing and diving teams, as appropriate. U Carry out element and segmental inspections, recording main dimensions, carriageway and soffit levels (if possible), confirm structural section sizes and condition. 0 Record width, length, location, orientation and pattern of all cracking; size, depth and location of spalled areas; size and location of areas of suspected delamination; size and location of areas of scaling; size, location and description of deposits and staining, especially rust staining; loss of section of any exposed reinforcement; and all other defects and damage. 0 Excavate trial holes t o expose buried members where doubt exists with regard t o detail, dimensions, condition, etc. 0 Carry out in-situ testing and sampling as required (see Section 4.3), implement condition monitoring if appropriate. 0 Record all information relating t o the inspection on sketches, drawings and in photographs, identify areas of specific concern and implement Special and Particular Inspections. 0 Receive, assess and report laboratory and ongoing monitoring results. 0 Establish schedules for implementing and reporting condition monitoring, Particular Inspections and ongoing assessments. Summarise all of the findings from the inspection for assessment in a report which will form the basis for the assessment itself. U Consider a post-assessment inspection, concentrating on those areas where cracking/defects would occur under the serviceability limit state (see Section 6.7).
29
~~~
3 Onopeaion for assessment
3.1.5 Risk assessment and safety issues
The main Health and Safety Issues to be considered in risk assessments for bridge inspections are: U Working a t height 0 Working in confined spaces
0 Working adjacent to traffic 0 Working with and adjacent to 'live' services 0 Working from a boat 0 Working on a railway U Working a t night 0 Working near water (leptospirosis) 0 Use of mechanical access equipment 0 Use of small tools 0 Fire 0 Presence of asbestos3' - (movement joints, waterproofing, pipework, permanent formwork, etc.) U Lead paint. This list is not exhaustive and risk assessments should be carried out for each and every bridge site. For additional advice see HSE best practice In particular where destructive testing is being used the specific and special risks associated with such procedures must be considered.
3.2 Condition assessment
3.2.1 BD21 approach
The actual condition of a concrete structure a t the time of the inspection for assessment could have a major influence on the assessed capacity of the structure and therefore must be taken into account in that assessment.
BD2l states that the assessment resistance shall be determined from the calculated
resistance multiplied by an overall condition factor. It further states that the condition factor, determined on the basis of engineering judgement, shall represent an estimate of any deficiency in the integrity of the structure and may relate to a member, a part of a structure or the structure as a whole. This approach is considered inappropriate for concrete bridges.
3.2.2 CSS bridge condition indicator
30
CSS Bridges Group concluded that to assist the maintenance management of a stock of bridges it is essential to have a 'Condition Indicator' which can be used to determine whether the overall condition is deteriorating over time or not. Their view is that such trends will indicate whether adequate funding is being provided for structures' maintenance work. It was also recommended that all authorities should use a single system in order to ensure consistency and credibility. Therefore, CSS commissioned the development of two guidance documents: A review carried out by the
1. A guidance note on the inspection of highway bridges33. 2. A guidance note on the evaluation of a Bridge Condition Indicator based on element condition data collected during these inspection^^^.
The CSS inspection system is based on a fixed list of 38 elements that occur on highway bridges and a pro forma containing these elements and other data collection requirements has been produced. If an element on the list is present on the bridge then the inspector must record its condition. The condition is recorded as a combination of the Severity and Extent of the defects on the element. The Severity scale ranges from 1 (no defect) t o 5 (failed) and the Extent scale from A (no extent) t o E (extensive). The inspection document provides sound guidance on the classification of defects, with tables and photographs of defects t o assist the inspector. The severity/extent ratings provided by the inspections are used t o evaluate the Bridge Condition Indicator (BCI) score. The BCI score is built up as follows: 1. The element severity/extent alphanumeric ratings are translated to an Element
Condition Score which has a scale of 1 (best condition) to 5 (worst condition). 2. The Element Condition Score is translated to an Element Condition Index, still on the 1 t o 5 scale, which takes into account the importance of the element. (N.B.All 38 elements on the CSS pro forma are classified as being of very high, high, medium or low importance.) 3. The weighted average of all the Element Condition Indices on a bridge gives the Bridge Condition Score, where 5 is the worst possible score and 1 is the best possible score for a bridge. 4. The Bridge Condition Score is translated t o the BCI, the BCI is on a 0 (worst condition) t o 100 (best condition) scale for ease of external reporting. 5. The weighted average of the BCls for a stock gives the Bridge Stock Condition Index,
retaining the 0 t o 100 scale. (N.B. The individual BCls are weighted by deck area.) 6. Guidance is provided on how t o interpret and present the BCI data. The BCI does not represent the true functionality/safety requirements of a bridge, i.e. t o safely carry the required service loads, and as such should not be used in isolation. The BCI is just one component of a full set of Asset Management tools currently being developed by the CSS and Highways Agency in partnership for highway bridges, which includes Reliability and Availability Indicators and Asset Valuation procedures. The BCI will, in the long term, be used in the Asset Management of highway structures for setting targets and the prioritisation of maintenance work. However, in the interim it is being used as a high level management tool t o monitor the trend of bridge condition over time.
3.3 Network Rail's Structures Condition Marking Index (SCMI)
Network Rail's Structures Condition Marking Index (SCM1)34was developed by external consultants for Network Rail as a means of providing a more objective measure of recording the condition of its structures. The scoring takes place during each detailed examination, which occurs every six years. The scoring system and associated algorithm are currently in use for bridges but have also been developed for other structural assets, such as tunnels and retaining walls.
31
The system requires on the first visit the key components of the structure to be identified and the defects associated with that component to be scored based upon their extent and severity. At the following detailed examinations only the scoring of defects is required. The extent and severity scores are material specific. Each structure (or each span in multi-span structures) is scored out of 100, with 100 being perfect condition. The average score for the whole population scored is published by the company in its annual Business Plan and is a requirement of the Office of the Rail Regulator. Extensive trials were undertaken with bridge engineers and examiners to validate the output of the system to ensure that the score accurately reflects the condition of the structure. It does not give a direct indication of serviceability as no account is taken of the assessed loading capacity. The system was not developed to manage directly either safety or work prioritisation, although clearly condition influences decisions in both these areas. The scores are to provide Network Rail with information on the condition of its bridge stock or other asset type primarily nationally, regionally, by route and material type. This information can then used to assist in determining national policies on the management of structures and provide one of a number of measures input into tools used to manage work prioritisation, future funding and route upgrade policies.
3.4 Other work
The BRIME project35identified that in Europe generally current methods of condition assessment use two different approaches for both the assessment of individual elements and of the structure as a whole. The first uses a cumulative condition rating which is derived from the condition of individual elements, while the second uses the condition rating of the bridge element in the worst condition as the condition rating of the structure itself. The condition assessment is based on bridge inspection and similar procedures, although with different intervals between inspections, being used in different countries. An alternative procedure is to use damage classification systems36.More recently, this approach has been developed as part of the CONTECVET project37and the findings have been implemented by the British Cement Association as part of a structures management package for the Hong Kong Buildings Department. The BRIME project also investigated the use of damage categorisation models as a basis for an expert system to assess damage and deterioration, although they concluded that: research is needed to improve the methods developedfor categorising damagedareas, , , this requires the development of a larger database and the addition of new parameters. ‘I.
. . further
’I
32
3.5 Preferred approach
Where an inspection for assessment of a concrete bridge highlights areas of deterioration such as spalling, loss of reinforcing bar cross section, etc. then that deterioration must be measured and recorded to enable it to be taken into account in the assessment of individual elements. Procedures for doing this have been investigated by WS at kin^^^ in work for the Highways Agency and by Webster3’ as part of the CONTECVET project3’. Recommendations as to the most appropriate methods for bridge assessment are given in Chapter 7. Similar advice to that given above appears in a technical audit of BA79 undertaken by Parsons Brinckerhoff310for the Highways Agency, and the latter are currently considering these and other recommendations as draft amendments to BD44. It is intended that these amendments will permit various forms of deterioration to be taken into account in the condition assessment of individual elements. Chapter 9 gives information on specific material factors that may need to be taken into account.
3.6 References
3.1
HIGHWAYS AGENCY, Draft for Comment, Interim Advice Note on Asbestos Management Application to the Strategic Road Network, (2004)
3.2 HEALTH & SAFETY EXECUTIVE, Bestpracticeguides 3.3 CSS, Bridge Condition Indicators Volume 1 CommissionReport, Volume 2 Guidance Note on Bridge Inspection Reporting (Addendum No 1, May 2004), Volume 3 Guidance Note on EvaluationofBridge ConditionIndicators (Addendum No 1, May 2004) 3.4
NETWORK RAIL Structures Condition Marking Index, Company code of Practice RTICEICI041
3.5 WOODWARD, RJ eta/, Bridge Management in Europe (BRIME), Deliverable D14 Final Report, Contract N o ’ RO-97SC.2220, BRIME. March 2001.
3.6 SODERQUVIST,The Finnish practice and experience regarding bridge inspection and management, The managementof Highwaystructures,Highways Agency, 22-23 June 1998.
3.7 BRITISH CEMENT ASSOCIATION et a / , A vabdatedusers manualforassessing therexidualservicelife ofconcretestructures, EC Project Ref. IN 309012, CONTECVET, BCA, Camberley, 2001
3.8 WS ATKINS, Deterioratedstructures- The assessmentofdeteriorated reinforced concrete bridge structures, Highways Agency Final Report, November 2000.
3.9 WEBSTER. M. The assessment of corrosion-damagedconcretestructures, PhD Thesis. Birmingham University.July 2000. 3.10 PARSONS BRINCKERHOFF, TechnicalAudit ofthe applicationofBA79 and A review ofbridge assessmentfai/ures on the Motorway and Trunk Roadnetwork,Final Reports HBR80616 - Highways Agency Contract 2/41 9, Parsons Brinckerhoff, December 2003
33
a 4. In-situ and laboratory testing 4.1 Background and introduction
Testing is closely associated with the Inspection process, in terms of establishing/confirming assessment parameters (Section 3,1), and in particular for condition assessment, where some form of deterioration is involved (Section 3.2 and Chapter 9). It is important t o have clear objectives in selecting test methods and in developing a test programme t o ensure that these are met, with an acceptable level of confidence.
Precision of tests and sampling regimes are crucial in this regard, particularly for the key stage of data interpretation.
1
Testing may be required for any of the following reasons: 1. To establish/confirm representative mechanical properties for the concrete, reinforcement or prestressing tendons, for use in subsequent structural analysis. 2. To locate the position of reinforcement or prestressing tendons, and t o establish the
extent of any section loss or concrete spalling. 3. To identify possible deterioration mechanisms, and t o establish which one is dominant.
4.To quantify relevant material properties and characteristics, relevant to current and possible future deterioration, under the measured/assessed local micro-climate and general environmental conditions. In all of this, the purpose is t o augment and enhance the overall Inspection for Assessment process, and t o produce data on the general condition of the structure and/or t o serve as input for subsequent structural analysis. The above objectives require the availability of a wide range of test methods, which may either be non-destructive or intrusive, carried out on site or in the laboratory. This is an evolving field, but details of current relevant test methods are given at the end of this Chapter, together with some guidance on choice, and application in practice. This Chapter can only give a brief review, and this is done in the context of practice and recommendations in relevant Highways Agency documents, most notably in BD21, BD and BA44, BD and BA63, and the Bridge Inspection Guide.
4.2 Current testing practice
34
The emphasis here is on practice in determining assessment parameters for subsequent structural analysis in accordance with Highways Agency guidance documents, i.e. on objectives 1 and 2 in Section 4.1
L
4.3 In-SitU sampling and testing
The extent of in-situ sampling and testing on any particular structure varies from structure to structure, depending on the existence or otherwise of as-built and maintenance records, the form of the structure and its general condition. Sampling and intrusive investigations should be carried out in areas which will not impair the capacity or the durability of the structure, and such sites should be made good using appropriate materials. As a minimum, in-situ sampling and testing should include a cover survey, verified by
local breakout if possible. This is often carried out in conjunction with half cell potential measurement, sampling for chloride ingress profiles, and depth of carbonation measurement, all of which help to give an indication of the condition of the structure and the potential for future problems. Occasionally, resistivity testing is also undertaken to provide information regarding the likelihood of ongoing corrosion. Further in-situ operations can include: exposure of an area of reinforcement to determine size, type, spacing and condition; the recovery of cores; and the recovery of steel reinforcement samples. In areas of concrete where delamination is suspected, e.g. deck or beam soffits, a tapping or sounding survey with a light hammer is useful in detecting potential spalling and loss of cover. Information regarding in-situ stress measurement of concrete and steel is given later in this Chapter.
’
Durability of concrete structures - Investigation, repair, protection4 contains information on planning, test and inspection techniques and on the interpretation of condition surveys. Further information is contained in CBDG Technical Guide 2 - Guide to testing and monitoring the durability of concrete
4.4 Laboratory testing
Laboratory tests may include the determination of chloride and cement content plus the determination of estimated in-situ cube strength. Steel tensile testing may also be carried out. Testing to identify possible deterioration mechanisms and related material properties (objectives 1 and 4 in Section 4.1) involves significant laboratory testing.
4.5 ConCrete parameters for assessment
The worst credible strength (WCS) for concrete in an element can be determined from a minimum of three estimated in-situ cube strength results using a formula given in BA44, where estimated in-situ cube strengths are determined from concrete core tests. It is important to recognise that the BA44 formula was derived on the basis of the variation in the estimate of concrete strengths from core test results at a particular
‘location’, within which the variation of concrete strength is small, and not across an entire structure. The use of this formula to estimate concrete strengths for an entire structure or even element must be approached therefore with caution since test results at one location may well be unrepresentative of the material properties a t another.
35
4 In-situ and laboratory testing
Non-destructivetechniques such as Schmidt or Rebound hammer may be used to demonstrate that core locations are representative of the properties of wider regions of a structure or to identify the need to take additional cores. Although Schmidt or Rebound hammer tests can be calibrated against a core test they are no real substitute for core tests in their own right when feasible. It is advisable that the value obtained from the BA44 formula is compared with values obtained from other methods and an appropriate value selected by engineering judgement. Table 4.1 illustrates four different approaches that might be considered, with example values based upon an actual set of 5 core test results with estimated in-situ cube strengths of 84.5, 86.0,97.0, 83.5 and 83.0N/mm2.The mean value of this set of test results is 86.8N/mm2and the standard deviation 5.82N/mm2.This is greater than the standard deviation assumed in deriving the BA44 formula and illustrates that in this case it may be prudent to use a worst credible strength somewhat lower than the value determined using the BA44 formula. In fact, it actually shows that the cores do not all come from a ‘location’ within which the variation of concrete strength is small. When larger numbers of cores are taken, it is common for some to be below the WCS. Strictly, the BA44 formula gives an indication of the worst likely mean strength, and is only valid if either the variation in concrete strength is small or the behaviour is sensitive to mean concrete strength, rather than the strength in critical local areas. It is emphasised that the second approach below, based on using the BA44 formula in conjunction with the lowest recorded core strength, is included only as an aid to establishing a sensible concrete strength for elements when the number of cores taken is limited. It might be appropriate when it was considered that only a particular core was likely to be representative of the concrete in a critical area. In practice, it is often necessary to use cores such as these, which do not come from one ‘location’, to obtain a single WCS for the whole structure or element. Strictly, the most correct statistical approach is to use the T distribution which allows for uncertainty in determining the standard deviation as well as the mean. However, when the number of cores is small, it becomes necessary to use more judgemental approaches, considering such aspects as the lowest core result or, when results are similar, the possibility that this may be coincidence. Table 4.1 Examplesof four different approaches.
I
Method
I
Description
I
Estimated WCS (N/mm*
4.6 Reinforcement parameters for assessment
The reinforcement parameters required for assessment are type, size, condition, spacing, location and yield strength. AS previously noted, type, size and condition are often best determined by breaking open a small area of concrete for examination, whilst spacing can be determined using a cover meter. When it comes to yield strength, BA44 states that a minimum of three samples should be taken for tensile testing. These samples should be of similar type and diameter and should be taken from low stress areas. Steel samples taken from concrete cores are not recommended for tensile testing. Determining the worst credible strength from a set of three samples requires engineering judgement. BA44 states that the formula for concrete strengths may also be used for reinforcing steel. However, the characteristic strength of the same three samples can be lower than the value calculated by the BA44 formula, depending on the spread of results, and the formula therefore does not provide the worst credible strength. Statistical analysis of a large number of samples tested by the TRL indicates that the worst credible strength tends towards the mean strength with a sufficiently large population. Indeed, testing reinforcing bars can show a sizeable variation and may also show significantly greater yield strengths than the characteristic values specified during design. It is noteworthy, however, that variations in reinforcement yield strengths within a structure will essentially be averaged because the number of bars that collectively contribute to the flexural strength of a section or yield-line. In such cases therefore, the use of the WCS, which is actually an indication of the mean strength, is more correct than in cases where behaviour is sensitive to the local strength of individual bars. In dealing with variations in reinforcement strength, some comfort can also be drawn from the fact that, typically, assessments are based on reinforcement yield strengths, whereas before flexural col[apse occurs some strain hardening of the reinforcement will generally occur, particularly for significantly under reinforced sections and mild steel bars.
~
4.7 In-SitU Stress measurement - concrete
There are two basic techniques that have been widely used: slot-stress and hole drilling. Both rely on measuring the strain relief around a hole which is cut into the concrete. The semi-circular slot, typically about 150 to 300 mm diameter, is cut with a diamond saw and the hole, generally about 75 to 100 mm diameter, with a diamond tipped corer. Both techniques are semi-destructive and care has to be taken to ensure that the
In the case of slot-stress, the strain can be restored by jacking the slot apart. This jack load can be related to the initial stress state. It is also possible to determine the initial stress state by using results of finite element analysis backed up by laboratory testing from the strain releases across and on either side of the For hole drilling, then the relaxation in strain as the hole is progressively drilled can be
related to the initial strain and, hence, stress state by semi-empirical methods. These are established and correlated by both finite element analysis and laboratory testing. As part
37
4
of this work, a circular jack was also developed with can be used to determine the inplane elastic modulus of c o n ~ r e t e ~ ~ , ~ ~ . Care should be taken to allow for the variation in concrete physical properties as well as the internal stress state. The strains and stresses in a cross-section can be affected by locked in stresses due to differential shrinkage and creep resulting from construction and subsequent load history as well as transient temperature fluctuations. The technique has been successfully applied to a number of prestressed concrete bridges. The field accuracy of the technique is plus or minus 2N/mm2.
4.8 In-situ stress measurement - steel reinforcement 4.8.1 Blind hole drilling
A similar principle 3 the slc stress technique can be used for measuring steel resses and is sometimes referred to as blind hole drilling. This entails drilling a small hole in a reinforcing bar or prestressingtendon and measuring the strain relief around the hole. This can be calibrated to give the steel stress and hence force.
One of the problems with this technique is that reinforcing bars have built in residual stress profiles which are caused by the rolling process. Prestressing wires also have drawing residual stresses. This stress profile has to be known, hente the need to calibrate against an identical or a t least similar bar -which would need to be taken from the structure. Additional complications can arise from the stress concentrations caused by the deformations on deformed bars although these can be ground off before strain gauging and drilling. The accuracy is unlikely to be less than plus or minus 20N/mmZ which is often of little use for reinforced structures. Tests carried out in the mid-1980s by WS Atkins to explore this technique were not pursued for this reason.
4.8.2 Cut bar method
The application is to expose a bar in the structure, strain gauge it,&$ then to cut it (with minimum heat). The strain relaxation then gives the in-situ stress in the bar. The bar is then reinstated with a full penetration butt weld which would re-tension the bar due to weld shrinkage. This technique was referred to in a paper presented in 199446in respect of a box culvert. The significance for culverts is that the actual soil pressures are somewhat indeterminate and the readings allow the capacity available for carrying live load to be estimated. The drawbacks are that dead load stress levels can be low; the stress measured after exposing the bar over sufficient length is the average between points where it is properly anchored; actual stresses are dependent on the amount of tension contained by the concrete. However, if the reinforcement stresses are predicted to be very high but there are no signs of cracking to confirm this then using the technique could be justified.
38
I
4.9 Geophysical techniques
4.10 References
The most commonly used geophysical technique in the investigation of concrete bridges is surface impulse radar. This technique can be used t o determine hidden detail in abutments or other elements and can also be used t o detect the presence of reinforcement adjacent to remote faces. However, specialist interpretation is required, quite often coupled with trial holes or cores for calibration.
4.1 MAYS,cc (ed.),Durabi/ityofConcreteStructures-Investigation, Repair, Protection. E&FN Spon, London, 1992 4.2
CONCRETE BRIDGE DEVELOPMENT CROUP, Guide to testing andmonitoring the durabiOtyofconcretestructures. Technical Guide 2, The Concrete Society, Camberley, 2002
4.3
FORDER, S. Ca/ibrationofsaw cutting techniquefor insit ustress determination MEng Project Dissertation, University of Surrey, 1992
4.4
MEHRKAR-ASL, 5, Direct measurement ofstresses in concretestructures,PhD Thesis, University of Surrey, 1988.
4.5
MEHRKAR-ASL, S. Concrete stress-relief coring theory and application, ProceedingsofFP Symposiumon Post-tensioned Concrete Structures, London, 25-27 September 1996, pp 569-576.
4.6
CHALKLEY, C, Proof load testing - a recent proof load test, and its results, Proceedingsofa Surveyorconferenceon Hidden Strength-Load Testingfor Bridge Assessment, London, February, 1994.
39
D Loading
5. Loading 5.1 General PrinCipkS
Loading rules are given in BD2l for short and medium span bridges and in BD50 for long span bridges. These documents are fundamental to the assessment loads for all types of bridge and reference should be made to them for the rules and explanatory notes. The main purpose of this Chapter is to explain the principles and point to references that provide further background details. There have been several changes to loading rules in BD21 since it was first introduced, one being to account for the increase in gross vehicle and axle loads. The most recent changes allow the use of bridge specific live loading for short and medium span bridges, where good surface condition and low traffic flow lead to less onerous requirements (See Section 5.2). Assessment loading is limited generally to the application of dead, superimposed dead and Type HA live loads. (Dead loads are the weight of the structural (i.e. load bearing) elements whereas superimposed dead loads are the weight of the non-structural elements, such as surfacing, parapets, services and miscellaneous street furniture.) Type HA live loading consists of a uniformly distributed load (UDL) together with a knife-edge load (KEL), which can be determined as described in BD21. For assessment purposes, the values specified by this Standard have to be factored to give the Assessment Live Loading (ALL). Although BD2l only covers permanent and Type HA live loads, most bridges that can carry the 40t ALL have also been checked for Type HB loading and, therefore, should now also be checked for STGO loading (See Section 5.4).
5.1.1 Scope O f h e loading
The Type HA (design) loading given in BD2l allows for the effects of 40 tonne vehicles and includes a contingency margin for unforeseeable changes in traffic patterns. For assessment, reduction factors are applied to this Type HA loading to give the various ALL levels with no contingency provision. The 40 tonne ALL covers the effects of normal vehicles (Authorised Weight Regulation) of up to 40 tonne gross vehicle weight [including 41 tonne 6 axle lorries, 44 tonne 6 axle bimodal articulated lorries and draw bar trailer combinations and 44 tonne 6 axle general haulage lorries. (Although the gross weights of these vehicles exceed 40 tonnes, the load is distributed over six axles, rather than the five axles of the 40 tonne vehicle, as well as a longer wheelbase. Thus, the load effects generated by these vehicles are lower than those caused by the 40 tonne vehicle.)] and 11.5 tonne axle weight. For cases where structures are found to be incapable of carrying the full 40 tonne ALL,
loading criteria are given which correspond to specified limits on gross vehicle weights Special loading criteria are also given for fire engines. The Type HA UDL and KEL given in BD2l are generally only suitable for modelling longitudinal load effects. Alternative loads are given for the effects of vehicles on trough
40
,
decks, short span masonry arches, decks with main members that span transversely including skew slabs with significant transverse action, and buried concrete box structures with cover greater than 0.6 m. A recent change requires longitudinal elements to be checked using specific vehicles where there is a low capacity for transverse distribution.
5.1.2 Background to assessment live loading
I
The Type HA ALL that is given in B D 2 l for short spans (2-50 m length) is derived from an ultimate or extreme loading as opposed to a working load. This ultimate load was derived from first principles and is based on the assumption that the worst credible load that can reasonably be expected to occur in the lifetime of the bridge will be equivalent to some multiple of Type HA loading. It has been shown that this extreme load has a return period of 200,000 years or a 0.06% chance of occurring in 120 years. The values of nominal HA loading that are given in BD2l were determined therefore by dividing the extreme loading by 1.5. Four elements have been used to generate the extreme loads. They are: 1. Loading from Authorised Weight (AW) vehicles 2. Impact 3. Overloading 4. Lateral bunching.
Details of these are given in Annex C of BD21 The loading has been derived for a single lane only. It has been assumed that if two adjacent lanes are loaded there is a reasonable chance that they will be equally loaded The various factors that have been used in determining the loading are span dependent. (The exception is impact for the single vehicle case.) The derived loading has taken account of the possibility of convoys of eight or more HCVs, but, on more lightly trafficked routes, the probability of having a bridge completely filled with heavy vehicles is small. The loading is conservative therefore for lightly trafficked medium span structures. The impact factor that has been used was derived from measurements taken on motorway overbridges, which were of modern construction and where the road surface and bridge joints should have been in good condition. The overload factors were derived from a sample survey of approximately 3500 vehicles and thus can be assumed to be typical of what may occur at any time, or in any place in the country. The loading that is derived using these procedures is considered therefore to be fairly universal in its application and to reflect situations that can occur a t any bridge site.
41
D Loading
5.2 Bridge Specific loading
For short and medium spans, bridge specific live loading is derived by the application of a load reduction factor (K) t o account for different traffic flows and road surfaces. Bridge Specific Assessment Live Loading (BSALL) for long span bridges is based on traffic surveys and is described in BD50. Specific guidelines for Trunk Roads are given in BD2l and appropriate K factors can be determined from graphs [the K diagrams) for loaded lengths between 2 and 50 m and six combinations of traffic flow and surface condition. If a bridge is found t o be inadequate at the 40 tonne load level, the permissible weight restriction level is the highest capacity for which the K value in the appropriate K diagram
is less than the live load capacity factor C, derived using the available live load capacity. For bridge specific reliability-based assessments a different loading model is required. For the Highways Agency this has been developed in a project described by Coopers’, the result of which is a probabilistic loading model which accounts for traffic flow, road surface characteristics and the dynamic response of the bridge under consideration.
5.2.1 Bridge Specific probabilistic loading mode[
For a site-specific reliability-based assessment, the load effect [bending moment or shear force) 5,, a t any section can be taken t o be a random variable expressed as: S,, = BSLL x R,, x DAF
Where BSLL = the Basic Static Live Load effect [defined below) = a random multiplier t o model the uncertainty in the load effect due t o the static weight of vehicles DAF = a Dynamic Amplification Factor t o account for the dynamic amplification of the static load effect due t o the impact of axles and the dynamic response of the bridge.
R,,
The Basic Static Live Load effect at the critical section is determined by loading the relevant influence surface with the following: 0 A uniformly distributed load (UDL) of 27 kN/m run across a width equal t o the
smaller of the notional lane width or 3 m,placed centrally within the lane. 0 Two axle loads, each 300 kN, with a 1.2 m spacing between axles, placed anywhere
within the lane t o maximise the load effect at the section considered. This follows the geometrical configuration of the load model in Eurocode 1, Part 3 because it was felt that the tandem axle configuration provides geometrically realistic force distributions on the structure. Each axle consists of two wheels a t a 2 m track width placed centrally within the lane. The same loading is used for both Lane 1 and Lane 2 but a reduced UDL of 7 kN/m run and two axle loads of 100 kN each are used for the remaining lanes. The carriageway is divided into notional lanes according t o BD21.
42
The random multiplier R,, is assumed to have a Cumbel distribution, the parameters of which are given separately for bending and shear in Cooper” and they depend on the span and traffic flow rate. The maximum value of this extreme type distribution also depends on the number of repetitions of a load event within the given reference time. Bridge natural frequencies are considered to be closely correlated to the span length. The Dynamic Amplification Factor (DAF) is assumed therefore to have a Normal distribution, the parameters for which are based on span, the number of loaded lanes and the pavement condition.
5.3 Highway Surfacing effects
Significant research has taken place to enable Bridge Specific Loadings to be determined (see Section 5.2). Application of this work first appeared in the 1997 version of BD21. Not only were the notional lane widths modified from previous calculation methods but the number of HCV movements and the road surface condition were also taken into account. Three traffic flow categories (High, Medium and Low) and two road surface categories (Good and Poor) were defined. This gave six possible combinations of effects, e.g. Hp (High-Poor) and Mg (Medium-Good), and six separate graphs of Reduction Factor, K, against Loaded Length, I ,were produced. The category Hp gave the worst loading condition whereas the category Lg gave the least onerous loading condition. The assessment engineer was left with the task of determining whether the road surfacing was ‘Good’ or ‘Poor’ quality. The three traffic flow categories are defined as follows in BD21: 1. High (H) 2. Medium (M) 3. Low (L)
Flow > 70 70 > Flow > 7 7 > Flow
Flow is equal to the total annual two-way HCV traffic over the bridge divided by 8760 where HCV is defined as goods vehicles that are over 3.5 tonnes maximum permissible gross vehicle weight. The 1997 version of BD2l required road surface categories to be determined using equipment described in TRL Report L R I 125”such as the High Speed Road Monitor (HRM). The acceptance criteria quoted approximated to the mid threshold value for roads with 50 mph speed limit or greater as given in Highway Standard HD2gS3.This technique was reasonable for application to high speed roads which were regularly checked using equipment such as HRM, but proved to be problematical for local roads where the use of such specialised vehicles was rare and the criteria quoted were inappropriate for roads with lower speeds and greater rates of change in gradient. The 2001 version of BD2l simplified the process by allowing the subjective determination of road surface quality. For instance, it is stated that “Motoways and trunk roads may generally be consideredas ‘good’surface category if they are maintainedand repairedbefore they deteriorate to ‘poor’surface”. The ‘poor’ classification is applied if, when driving a vehicle over the bridge in free flow traffic condition, any of the following applies:
43
5 Loading
0 Subsidence - vehicle bounces 0 Sub-base deterioration - vehicle pitches locally U Surface deterioration -visual deterioration of surfacing or steps in expansion joints
Additionally, where practicable, the assessment should be confirmed by observation of HGVs crossing the structure (a full description of the criteria is contained in BD21). If a quantitative assessment of the road surface condition is carried out then it is now permissible t o use the criteria in HD29 appropriate t o the speed of the road carried by the bridge. ‘Good’ roads are those in categories 0 and 1 as given in HD29 whereas ‘Poor’ roads are those in categories 2 and 3. Measurements are required t o be taken between points 20 m beyond the bridge. This increase over the previous figure of 5 m is because road profile unevenness at bridge abutments is a major cause of significant dynamic loading on bridge structuress4. The results of these changes mean that for minor roads the maintenance of a ‘Good’ category may require maintenance at more frequent intervals than would be demanded by consideration of the pavement alone. It may also be necessary t o enter into agreements between the bridge owner and the highway maintenance authority t o ensure that adequate standards of maintenance are achieved. Specialised inspection requirements should be included on bridge files.
5.4 Road traffic and abnormal loads
There are two main classifications of road traffic in the UK. These are: 1. Normal vehicles: The large majority of vehicles using the highway network are regarded as ‘normal’ traffic, which covers cars, light goods vehicles, rigid and articulated vehicles and heavy goods vehicles up to a gross weight of 44 tonnes. These vehicles comply with the Road Vehicles Construction and Use (C&U) Regulations 1998 and Authorised Weight (AW) Regulations 1998. 2. Abnormal vehicles: These are vehicles, either empty or laden, which do not comply with C&U Regulations. This non compliance with the Regulations could be because: 0 The load carried exceeded dimension limits
0 The vehicle and load exceeded weight limits 0 The vehicle had been designed for carrying outsized loads and/or for particular purposes. These vehicles include mobile cranes, construction plant and low loaders carrying exceptional industrial loads (e.g. electrical transformers, machine presses, etc.). These vehicles and payloads are commonly referred t o as an Abnormal Indivisible Load (AIL) There are two types of abnormal vehicles:
1 , Special Types General Order (STGO) vehicles: This group includes vehicles that do not comply with the AW Regulations but comply with The Motor Vehicles (Authorisation of Special Types) General Order (STGO Regulations). Under Article 18 of the STGO Regulations, the maximum gross vehicle weight and maximum axle weight are 150 tonnes and 16.5 tonnes respectively.
44
Loading 5
2. Special Order (SO) vehicles: This group includes vehicles that do not comply with the AW Regulations or STCO Regulations and is covered by Section 44 of the 1988 Road Traffic Act. SO vehicles have maximum axle weights of greater than 16.5 tonnes or gross vehicle weights in excess of 150 tonnes. Both types of Order work in conjunction with the Road Vehicles (Construction and Use) Regulations. The movement of an AIL is covered by the provisions of Section 44 of the Road Traffic Act 1988 and is required to be notified to the appropriate authorities (police, Highway Authorities and bridge owners). Indivisible load movements that take place under the STCO do so under a number of conditions. Some of the C&U Regulations apply and, depending on the size and weight of the load, some special conditions. The STCO divides vehicles into three categories depending on their weight. The chief criteria are summarised in Table 5.1. If the Category 1 or 2 axle limit is exceeded, then the load must be moved under the higher category 2 or 3. All vehicles must carry a 'Category Sign'. In addition to the above, the STCO also makes special provision for 'Engineering Plant', which may be self-propelled or towed. For example: a mobile crane or bulldozer, which cannot meet some of the C&U Regulations because of the task it is designed to do. Any vehicle moving in excess of 150 tonne gross vehicle weight, 16.5 tonne axle weight, 5.0 m wide, or 27.4 m long (excluding the length of the tractor unit) requires the permission of the Highways Agency. Two types of AIL require signed authorisation, as shown in Table 5.2.
Table 5.1
'
Categoriesof Special Types General Order vehicles.
'9
Category
Maximum gross weight of combined vehicle
Maximum axle weight
Minimum number of axles
Speed limits (mph)
Other conditions
I
45
- -
5.4.1 Purposes O f highway structure assessment
5.4.2 Design and assessment
Dimensions and weights
5 m < width < 6.1 m and < 150,000 kg and 150,000 kg or >6.1 m wide or >27.4 m long
Required authorisation
STGO VSE - Form VR1
Special order under Section 44 of the RTA 1988 from the AIL Section of the VSE
The prime purpose of highway structure assessment is to verify that the structure is capable of carrying the intended road traffic safely. The secondary purpose is to determine the carrying capacity or the critical pinch point of the route from assessment of the structures on it, if all structures on that route were assessed.
Since 1973 (BE5) it has been a requirement to design ‘other public roads’, ‘Principal roads’ and ‘Trunk Roads and motorways’ for 30 (120 tonne), 37.5 (150 tonne) and 45 (180 tonne) units of HB loading respectively. These design load requirements are now embodied in the current UK highway loading Standard BD37.This Standard defines Type HA and Type HB load models, which are intended to allow for the loading effects of ‘normal’ and ‘abnormal’ vehicles respectively. UK highway bridge assessment loadings for normal traffic are given in BD2l. Until 2001 there was no assessment load model for abnormal vehicles, so the Type HB load model in BD37 was also used for assessment. The HB model was derived in the late 1940s. An assessment load model for AILS was developed because the HB model was found to be unduly conservative when used in assessment of short span structures. Furthermore, the HB model could not be readily correlated to the current fleet of abnormal vehicles. Consequently, assessments in HB units do not fully meet the prime purpose of assessment.
5.4.3 BD86 - Assessment of highway structures for the effects Of Types General Order (STCO) and Special Order (SO) Vehicles
For short span structures with loaded lengths of less than 50 m, load assessment models for abnormal vehicles have been developed based on the load effects from actual STCO vehicle weights and configurations and traffic data. These models can be used to assess the load effects from STCO vehicles more accurately than the HB load model in BD37. In addition to meeting the purposes of AIL assessment, the BD8655load models enable the following augmentations: Attainment of higher load capacity ratings, particularly for structures with loaded lengths of less than 10 m.
-’
I
I 0 Flexibility t o modify various factors such as dynamic amplification factor, overload factor, etc. t o suit a specific structure. Consistent levels of safety for structures of different spans and for different STCO and SO vehicle movements.
5.4.4 BD86 Load Models sv Vehicles
Five load models (SV vehicles) were derived t o represent the load effects from the axle arrangements permitted under the STCO Regulations, which produce the most severe load effects. The five SV vehicles are: 1. SV80 covering Category 2 STCO vehicles 3.0 m wide up t o 80 tonnes. 2. SVIOO covering Category 2 STCO vehicles 3.0 m wide up t o 100 tonnes. 3. SV150 covering Category 3 STCO vehicles 3.0 m wide up t o 150 tonnes. 4. SV-Train consisting of the SV150 STCO vehicle drawn by a 3.0 m wide tractor unit (locomotive) . 5. SV TT covering military Tank Transporters 3.7 m wide up t o 105 tonnes.
The SV80, SVIOO and SV150 vehicles have axle spacings of 1.2 m, with a variable length at the centre between two bogies of 1.2 m, 5.0 m or 9.0 m. The 1.2 m spacing reflects the findings of the database studies and the STCO Regulations. The 5.0 m and 9.0 m spacings are only used above specified loaded lengths for load effects with two or more peaks in the influence line (e.g. for continuous span structures). Inevitably this may require a large number of load cases t o be considered and therefore general guidance has been given in BD86 t o assist in reducing the number. The most significant difference between the SV load models and HB model is that the latter tends t o be over conservative for spans less than about 10 m where the two adjacent heavy axles tend t o dominate. This is because STCO vehicles have a greater number of lighter axles, which tend t o spread the load more than HB loading.
5.4.5 BD86 - HB Conversion Charts
A number of simple conversion charts based on influence lines have been derived t o facilitate the conversion of SV rating t o HB units. These enable the methods contained
in BD86 for the assessment and management of STCO movements t o be used for most bridges that have previously been assessed for Type HB loading. Examples of the use of the conversion charts are given in the Standard, together with some limitations on applicability.
5.4.6 Assessment O f Abnormal Load Movements using BD86
.
Guidance is given in BD86 to allow abnormal load movements t o be managed where a structure has already been assessed for SV vehicles, or where HB conversion charts may be used. The first stage of the assessment for the particular notified STCO vehicle is a simple screening assessment. The screening compares the vehicle type, gross weight,
axle weight and axle spacing of the notified STCO vehicle with limits appropriate t o the
I
SV vehicles. The STCO vehicle may pass if its gross weight is less than the gross weight of the appropriate SV vehicle multiplied by the corresponding Reserve Factors for the load effects being considered. The original SV assessment would have given the Vehicle Ratings and Reserve Factors for the structure. Prior t o a rigorous SV assessment, the HB t o SV conversion charts may be used t o obtain Vehicle Ratings and Reserve Factors. Note that the Vehicle Rating for a structure is defined as the most onerous SV vehicle that can safely pass over the structure (i.e. the vehicle with the smallest Reserve Factor greater than 1.O). Reserve Factor is defined as the factor on the assessment SV load required t o reach the first failure. When a STCO vehicle does not satisfy the axle weight and spacing characteristics of the SV vehicles, a detailed assessment is carried out. The load effects caused by the STCO vehicle are compared with those of the SV vehicle, multiplied by the Reserve Factors, using one or more influence lines most appropriate for the bridge. If load effects due t o the STCO vehicle still exceed those of the factored SV vehicle, further refinements such as reductions in dynamic amplification factor and/or overload factor and associated loading, can be made where justifiable. These reductions depend on the extent to which the movement can be regulated with regard t o keeping the structure clear of associated normal traffic, restricting the speed of the STCO vehicle over structures and certification by the haulier of the load carried.
5.5 References
5.1
COOPER,DI, Development of short span bridge-specific assessment live loadlng In /nternationa/symposiumonsafetyof bridges (Das, PC, ed.) Institution of Civil Engineers and the Highways Agency, London, 4-5 July 1996, Thomas Telford, 1997.
5.2
TRANSPORT RESEARCH LABORATORY, Measurementandassessment ofunevenness on malorroads. Report LR1125, TRL Ltd, Crowthorne, 1984.
5.3 HIGHWAYS AGENCY, HD29 Structural Assessment Methods. Design ManuallorRoadsandBridges, Vo1.7, Section 3, The Highways Agency, London. 2001
48
5.4
JORDAN, P, Unpublished TRL Report PR/CE/18/98, Comment on Departmental Standard BD21/97. RoadSurface Categories, 1998
5.5
HIGHWAYS AGENCY, BD86. The Assessment of Highway Bridges for the Effects of Special Types General Order (STGO) and Special Order (SO) Vehicles, Design ManualforRoads andBridges, The Highways Agency, London, 2003.
6. Analysis for assessment 6.1 Assessment principles 6.1.I General PrinCipkS
Reserves of strength can exist but remain unrecognised if full advantage is not taken of the range of analytical methods available for the assessment of concrete bridges. As a result, in some instances, assessments can be excessively conservative and adequate structures may be condemned as unsafe. Analytical methods appropriate for bridge assessment vary greatly in their complexity and in their cost of application. Therefore, it is appropriate generally to begin an assessment with a simple method and then to extend the analysis if a shortfall in capacity is identified. Details of specific analytical approaches are set out in this Chapter. Usually, it is the strength and safety of a structure that is the principal concern when an assessment is undertaken. The guidance provided is focussed therefore on assessment a t the ultimate limit state.
6.1.2 Analytical procedure
Typically, the assessment of concrete bridges is undertaken using the following simple procedure: U Establish load effects, using an analytical method chosen by the assessing engineer. 0 Compare load effects with capacities, determined in accordance with BD44 and
’,
BA446 ‘.
Improvements in assessment ratings may be achieved therefore through judicious selection of analytical methods or by recognising unnecessary conservatism in the capacities determined. However, this simple approach to assessment does not recognise one of the principal reasons why concrete bridges are able to carry far greater loads than might be expected; their capacity for redistribution. For a concrete bridge to collapse it is necessary for a failure mechanism to form. Therefore, the maximum load a structure can carry safely is not necessarily governed by the load that causes the first ‘region’ within a structure to reach its capacity (i.e. to yield), as often assumed in design. If a structure is sufficiently ductile and provided alternative load paths exist, additional loading can be carried safely by the structure through redistribution until sufficient ‘regions’ are yielding for collapse to occur. Generally, concrete bridges have both adequate ductility and redundancy to enable significant redistribution so that there is a sizeable difference between the load a t ‘first yield’ and the ultimate collapse load. If the benefits of redistribution are to be realised in assessment, the simple approach of separating load effect calculations and capacity calculations cannot be retained generally. Instead, a form of analysis that accounts for both load effects and capacities in combination must be adopted, such as plastic methods or non-linear numerical methods, usually non-linear finite element analysis (NLFE). Methods that can account for
49
6 Analysis for assessment
redistribution require careful application, in particular, in understandingthe implications of the ductility of concrete structures. However, if distress occurs at the serviceability limit state, a structure may be considered to be ‘unsafe’ and usage may be restricted as a result.
6.1.3 Lower bound and Upper bound methods
The methods used for the analysis of concrete structures a t the ultimate limit state are underpinned generally by Plastic Theory (see, e.g., Clark6.3).When an elastic analysis or a strip method is used, the resulting distribution of stresses is in equilibrium with the applied loads and therefore, provided the stress nowhere exceeds the capacity of the structure, a lower bound assessment of the strength of the structure is obtained. Numerical methods, such as grillage and finite element analysis, can be used to find distributions of stresses that satisfy equilibrium approximately. The results of applying such methods can be considered generally as lower bound assessments, although strictly this is not the case since equilibrium is not satisfied exactly. Methods, such as yield-line analysis, identify a failure mechanism but do not check stresses everywhere. They are upper bound methods therefore. They can be particularly useful in bridge assessment since they account directly for a structure’s capacity for redistribution. The degree of complexity of different methods of analysis and uses are shown in Table 6.1.
6.1.4 Ductility
‘2
Plastic methods implicitly assume that structures are ductile. Generally, concrete structures are sufficiently ductile for this assumption to be valid and some guidance on limits is given in standards. However, establishing precise bounds on the-ductility requirements for plastic analysis to be valid is not ~traightforward~.~,~.~. Non-linear numerical methods, such as non-linear finite element analysis, can be used to account for the effects of limited ductility. However, such methods can be highly complex (and therefore costly) to apply, and particular care and expertise is required to ensure that the results are reliable.
.
6.1.5 Differences from design
Table6.1
Superficially, assessment is similar to undertaking analysis and checking sections in design. There are, however, important differences which should alter the approach fundamentally. If this is not appreciated, it can result in a significant waste of resources
- 1 Increasing complexity I Analysis approach
I Example
on unnecessarily sophisticated analysis. More seriously, it can result in structures being strengthened or even condemned when, in fact, they are satisfactory. Even quite minor repairs, strengthening or modifications to existing bridges are very expensive. This contrasts sharply with the situation in design. A structure that does not yet exist, is very easy to change. Therefore, the designer chooses a convenient analytical approach and then adjusts the structure to comply with the results of the analysis. In assessment, in contrast, the approach should be to alter the analysis to suit the structure. There are a wide variety of analyses normally which will give different but safe assessments. The further the structure is from being statically determinate, the greater the range of possible solutions. The analytical approaches used in design are often conservative. Alternative approaches are available which give more realistic results. These approaches are often, although not always, more expensive. The high cost of strengthening existing structures makes it more likely to be worth using them in assessment. However, whilst the designer aims for a consistent margin of safety, and hence uses consistent analytical approaches, the margin of safety in existing bridges may vary wildly. Because of this, there are also cases where a very simple conservative analytical approach is quite adequate to show that a structure is satisfactory. Another difference between design and assessment relates to the relative importance of the different limit states. In design, serviceability is often critical. In assessment, the first and most important responsibility is always to ensure safety, to ensure that the ultimate strength is adequate. BD2l says that pre-1965 bridges will not normally be checked for serviceability.The serviceability checks in BS 5400 Part 4, particularly the stress check, impose a severe limitation on the benefit that can be gained from using non-elastic analyses. Without this limitation, the scope for using plastic and other inelastic methods is much greater. The effect of these differences is that a wider range of analytical approaches are used in assessment than in design. Faced with a beam and slab bridge to analyse for design, most engineers would use a linear elastic analysis, probably using a grillage model and section properties based on gross-concrete sections. They would tend to use the same form of analysis for all such bridges. In assessment, it may be appropriate to use a simple static analysis, a grillage based on a variety of section properties, a threedimensional elastic finite element model, a plastic analysis, a non-linear finite element analysis, a load test or a combination of these approaches.
6.1.6 Structural condition
One of the most important differences between design and assessment lies in the fact that assessments are undertaken on actual structures that typically have been in service for some years. As a result their condition may not be as it was intended when designed either as a result of construction errors or subsequent deterioration. Assessments should be undertaken on the basis of the actual properties of the structure, as built and as matured. Further guidance on such issues is included in Chapter 9.
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6.1.7 Global and local analysis
In assessment, as in design, it is usual (although not universal) to consider global and local analysis separately. In a beam and slab type deck, the global analysis gives the moments and forces in the beams and diaphragms. If there are no intermediate diaphragms, it will also give significant transverse moments in the slab. These moments, which are known as global transverse moments, have to be considered in the slab design. The distinction between global and local analysis does not arise in slab type bridges. The overall or global analysis of these can be treated in a similar fashion to global analysis in beam and slab bridges. However, particularly for smaller bridges, it will often be easier to use analytical approaches more often used for deck slab analysis.
6.2 Simple methods 6.2.1 Strip method
The simplest way to determine forces and moments in the beams of a beam and slab structure is to use a static load distribution ignoring the capacity of the deck to distribute loads. The approach can also be used to determine the moments in a section of a slab deck, although when axle loads (rather than uniform lane loads) are considered, it will be necessary to assume some degree of spread of the individual wheel loads. Because it is so simple, this method is useful as an independent check when more sophisticated methods are used. A static load distribution provides a set of forces which are in equilibrium. It is therefore
a lower bound (safe) method according to plastic theory. Hence, if a structure has adequate strength according to such an analysis, it is safe. If only a strength assessment is required, there is no reason to go to a more sophisticated analysis. The analysis also gives conservative figures for serviceability assessment of longitudinal members. However, it cannot provide any information about transverse moments since it ignores them. Hence, if serviceability assessment of transverse members is required, a more sophisticated analysis is needed.
6.2.2 BA16 method
6.2.3 Upper bound limit and check
52
BA16 provides a simple assessment method using empirical charts. This approach is more restricted in application than a static distribution and cannot be used for HB loading, but it can be less conservative. There is also a similar approach for transverse members but this is still more restricted in application.
The opposite extreme from a static load distribution is to assume the distribution is perfect and compare the total longitudinal moment and vertical shear across the width of a bridge with the total available capacity. This is an unsafe method (it is related to yieldline analysis) but it does give useful information. If the bridge fails this test, no amount of improved load distribution will make it work. It provides an upper bound to strength and also another useful check on other analyses. The distribution analysis should always give results which fall between those obtained using a static and perfect load distribution.
If both the section and the loading are fairly uniform across the width of the bridge, there may be little difference between a static and ideal load distribution, in which case there is little advantage in doing more advanced distribution analysis.
6.3Elastic methods 6.3.1 Elastic grillage
If the simple methods do not show the bridge to be adequate, the usual next step is to use the type of elastic methods used in design. Before embarking on such an analysis, it is worth reviewing its limitations. The increase in strength comes only from improving the distribution of load between the beams. This distribution comes as a result of the transverse moments. The distribution analysis cannot increase the total moment capacity of the bridge. This means that if the bridge has no transverse moment capacity, or if all the beams are fully loaded in the simple analysis, a distribution analysis cannot increase the capacity.
Section properties When elastic analysis is applied to reinforced concrete structures, BD44 (like 855400: Part 4) allows considerable freedom in the choice of section properties. It allows them to be calculated from either cracked or uncracked sections. Either approach leads to satisfactory, but sometimes significantly different, results. In design, uncracked properties are used almost invariably. The main reason for this is that cracked section properties can only be calculated after the reinforcement has been designed, whilst the primary object of the analysis is to determine the required amount of reinforcement. Reinforcement can be designed directly using uncracked concrete properties whilst, if cracked properties are used, the process becomes iterative. In assessment, this disadvantage of using cracked properties does not arise.
Cracked v uncracked properties In a slab with isotropic reinforcement, it makes no difference to the predicted moments whether cracked or uncracked properties are used as the longitudinal and transverse stiffnesses are equally affected. However, most bridges are far from being isotropic. In the longitudinal direction they may be heavily reinforced or they may have prestressed concrete or steel beams. In any of these cases, the choice of approach will have little or no effect on stiffness. Transversely, however, they may be very lightly reinforced and the uncracked stiffness can be as much as ten times greater than the cracked stiffness. This is often the case in older structures which were designed using a static load distribution and so have very little transverse steel. When such structures are assessed using a conventional grillage based on gross-concrete properties, the transverse steel often appears overstressed. However, a more realistic assessment using uncracked transverse stiffnesses will show the transverse steel to be less highly stressed a t the expense of giving a less favourable distribution between beams.
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Varying section properties Reducing the transverse stiffness can result in an analysis that suggests that the longitudinal members are inadequate.Sometimes analysis using cracked properties gives overstressed longitudinal members, whilst analysis using uncracked properties suggests that the transverse steel is inadequate. In such cases, an analysis with intermediate properties could show that both are satisfactory. Using intermediate properties is also a useful technique when the top and bottom steel is different, and it is not obvious whether the section will be in sagging or hogging. It will be safe to use section properties calculated from the greater steel area. The strength is then checked using the appropriate area.
Torsionless gri llages Another property that frequently has to be varied in analysis for assessment is the torsional stiffness. The beams in many older structures were not designed for torsion and will appear to have inadequate torsional strength. However, a grillage analysis using reduced or zero torsional stiffness will still produce a safe assessment. This is a useful technique for assessing skew slabs, particularly where the transverse reinforcement is either skewed or very light. A torsionless skew grillage can be used with the elements running in the reinforcement directions. The moments from this can be used directly to check the reinforcement without the need for post-processing, The approach is safe but it is not valid for serviceability assessment. In particular, it will not predict the top cracking which can arise in the obtuse corner of simply supported slabs.
Slab reinforcement Whilst beams can be checked directly from grillage results, more interpretation is required in slabs, particularly skew slabs. The reinforcement in these is normally designed, and often assessed, using Wood-Armer equations, usually via post processors on the grillage or finite element program. The solution is derived by determining the minimum total steel to resist the moments a t the point. This approach has limitations in assessment but is almost always conservative as is explained in more detail in Section 7.5.
6.3.2 Elastic finite e[efTlentS
Elastic finite element analysis is used in various ways in assessment and the main ones will now be considered.
Plate models of slabs Plate finite element analysis is sometimes used for geometrically complicated slab decks
because, with modern programs, it is easier to assemble the models. Finite elements give a better representation of the behaviour of elastic plates than grillage models. However, as a reinforced concrete slab cracks, the moments tend to approach the distribution given
by a grillage using cracked section properties. If isotropic plate elements from simpler programs are used this loses the ability to take advantage of cracked properties or of reduced stiffnesses to allow redistribution. In extreme cases of highly skewed slabs with light transverse reinforcement placed parallel with the abutments, rather than perpendicular to the main steel, the cracked elastic stiffness in the stiffest and least stiff direction can differ by a factor of 100. It is common for a conventional assessment based
54
on isotropic elastic finite element analysis to give such a structure a significantly lower assessed capacity than a static load distribution. This problem can be avoided by using more sophisticated programs which allow orthotropic properties.
When plate analysis is used with either point loads or point supports, some care is needed in interpreting the results. The peak moment intensity under a point force in an elastic thin plate finite element analysis is very ‘mesh sensitive’; it tends towards infinity as the element mesh is made finer. Because the real slab has a finite thickness, and the load has a finite size, this peak does not arise in practice. The problem can be relieved by modellingthe size of the load or support. However, for the ultimate limit state it is considered appropriate to spread the peaks over a reasonable width; perhaps three times the slab depth. Another consequence of mesh sensitivity is that if the elements are too big in relation to slab thickness, it is possible to get unsafe results with the analysis failing to pick up moment peaks which can be important.
Beam and slab decks In a finite element model of a beam and slab deck, the beams and the parapet upstands can be modelled using offset beam elements with the slab modelled with plates. This is more realistic than a grillage analysis in the sense that it gives a more realistic representation of the interaction between string course, slab and beams. It also enables the real behaviour of intermediate diaphragms (or, more commonly, steel cross girders) which are separate from the slab to be modelled. However, a finer than normal element mesh is required to achieve this. If this type of analysis is done with a fine enough element mesh, the results can be used directly to check the slab reinforcement, avoiding the requirement for a separate local analysis. This approach used to be very expensive but, as computer power becomes cheaper, it becomes more economic to use it in a wider range of circumstances. It is preferable to use a program which enables the plate elements to be made orthotropic, even if the initial analysis is undertaken using isotropic properties. If, as often happens, initial analysis gives excessive stresses in the transverse steel, the analysis can then be adjusted by changing the element properties. This is much easier than transferring to a completely new analysis.
Three-dimensional models Full three-dimensional finite element analysis of box girder and cellular structures is a very powerful and, with modern hardware and software, reasonably economic way of obtaining realistic serviceability assessments. However, its use in strength assessment is more restricted. It does have application on major structures where, for example, it can be useful in determining the implications and causes of observed stresses and behaviour.The difficulty in strength assessment is that code approaches for determining bending and shear strength work on whole sections. Thus, having undertaken a sophisticated analysis which gives elastic stresses throughout the section, moments and shears for comparison with code predicted capacities can only be obtained by integrating the stresses over the section. The results are very similar usually to those obtained from a simple beam analysis.
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8 Analysis for assessment
6.3.3 Westergaard and Charts
Global analysis As noted above, it is possible and now sometimes economic to analyse a whole bridge
with a finite element model with a fine enough mesh to use the results directly to check the slab reinforcement. However, it is more usual to use less detailed analyses and these call for a separate analysis to determine the local moments in the deck slab due to wheel loads. These local moments have to be combined with the global transverse moments from the global computer model: the moments induced in the deck slab by its action in distributing load between beams. Only coexistent values (values occurring in the same piece of slab under the same load case) need to be combined.
Nodal loading In order to avoid the combination of global and local moments being over-conservative, it is necessary to ensure that the results in the global model do not include any component due to local moments. This is ensured if there are no nodes between the beams and the loads are applied to the nodes rather than using the option available in many programs to apply them to the members.
Slabs The methods used to determine local moments can also be used to analyse slab structures and are usually quicker to use than a grillage for short span structures where only a few wheels have to be considered. Conversely, on major bridges with wide spans between beams or webs, it may be easier to analyse the deck slab using finite elements or a grillage.
Westergaard The simplest approach is that due to Westergaard66.This considers a one-way spanning simply supported slab allowing for its finite thickness. It gives the maximum moment per unit width under a single wheel as: M, = 0.2107 [0.4825
+ Io~(s/c,)]
My= M, - 0.0676P
Where P = value of point load s =slabspan C, = 2[,/(0.4c2+ h2)] c = diameter of loaded area h =slab thickness Strictly, 5 should include the spread through the surfacing allowed by 8037 but not the spread through the slab as this is allowed for in the analysis. In reality, most slabs are built into beams and are continuous. An approximate
correction for this is to reduce M,and Mygiven by the above equations by 0.07P and 0.1065P respectively.
When the slab span is longer than a critical value, normally 1.7 times load spacing, the effect of more than one wheel has to be considered. Westergaard derived solutions for this and they are also given by R o ~ and e ~elsewhere. ~
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Pigeaud Charts Older bridges often had intermediate diaphragms so that the slab was two-way spanning. A simple elastic way of analysing these for concentrated loads using charts has e ~by ~Reynolds been derived by Pigeaud and the necessary charts are given by R o ~ and and Steedmad8.They can be used for one-way spanning slabs as well but other methods are usually easier.
Influence surfaces The other common way to undertake elastic analyses of slabs is using published influence surfaces. These are the two-dimensional equivalent of influence lines. They plot the bending moment intensity (for example) at a particular point in a slab due to point loads applied a t all positions in the slab. To use them the wheel load area, including the spread allowed by the code, is plotted on a copy of the chart and a numerical integration of the influence factors over this area undertaken. For simply supported, one-way spanning slabs, the approach gives very similar answers to Westergaard. However, for continuous slabs it has the advantage of giving a more realistic account of the effect of continuity, giving reduced bottom steel requirements, and of giving a value to check the top steel against. The best known influence surfaces are due to Pucher6’. There are others, however, including some which enable slabs with haunches to be considered, as well as skew slabs.
6.4 P h S t i C equilibrium methods
The lower bound theorem of plastic limit analysis states that if a distribution of stresses can be found in equilibrium with an applied loading, and which nowhere exceeds the capacity of the structure, then that load can be safely carried by the structure. If a structure is statically indeterminate (i.e. it has some redundancy), then different equilibrium distributions of stresses can be found and any one of these would be acceptable as the basis of a plastic lower-bound analysis. This theorem underpins many of the approaches used in the design of structures, particularly elastic methods, see, e.g., Clark63.It is important to recognise, however, that this theorem is strictly only applicable to ductile structures and cases where displacements are small. Furthermore, it is essential that equilibrium is satisfied everywhere throughout the structure. The ductility of reinforced concrete sections in flexure can be assessed from their rotation capacity. Considerable research has been focused on establishing the rotation capacity of reinforced concrete sections, a good review of which is given in CfB/NP Bulletin d’lnformation No. 2436’0.The rotation capacity of concrete sections is governed either by concrete crushing or by reinforcement fracture. Criteria are included in BD44 for both of these cases, although the expressions given do not appear to be consistent with the latest research in this field, as described in CfB/NP Bulletin d’lnformation No. 243. Particular care should be taken when considering structures which are either heavily reinforced or which are lightly reinforced with reinforcement that itself has limited ductility. Structures that are moderately reinforced typically have a high degree of ductility, making them suitable for plastic analysis.
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If lower-bound plastic methods of analysis are to be used, it is also necessary to consider carefully the shear capacity requirement for the structure. Typically, this may be done by ensuring that the shear capacity of the structure is sufficient for both an elastic analysis and for the lower-bound equilibrium stress distribution.
6.4.1 Plastic redistribution
If an elastic analysis is undertaken for a structure with a particular applied loading, the resulting distribution of stresses will be in equilibrium with the applied loading, and therefore provide a plastic lower-bound stress distribution. further lower-bound stress distributions may then be found be adding any self-equilibrating stress distribution to the elastic result. (A self-equilibrating stress distribution is a non-zero distribution of stresses throughout a structure, in equilibrium with itself and with zero external applied loads, except for support reactions. Parasitic moments that develop in continuous prestressed structures are an example of a self-equilibrating stress distribution.) This approach is commonly used in the design of continuous beams in buildings, where moments are redistributed from mid-span to supports, or visa-versa, by adding a moment distribution consisting of straight lines between supports (i.e. a self-equilibrating moment distribution). Such moment distributions can be derived from considering the effect of a small rotation occurring through the formation of a plastic hinge. A similar approach can be used in the analysis of reinforced concrete bridges. This is
most simply undertaken by considering the effect of a small rotation in a yield line across the supports if there is a hogging deficiency a t this location or a t mid-span if there is a shortfall in sagging capacity. The degree of redistribution permitted by BD44 can be checked by comparing the rotation required in the yield line with the specified limits. More sophisticated self-equilibrating stress distributions can be developed.
6.4.2 Hillerborg Strip method
The Hillerborg strip method6”,612is an alternative lower-bound plastic method in which a distribution of stresses in a structure, in equilibrium with an applied loading, is found by dividing the structure into longitudinal and transverse strips, and assigning a proportion of the loading applied to the region where a longitudinal and transverse strip intersect, to each of the two strips. The sum of the loads applied to the longitudinal and transverse strips in the intersecting region must be equal to the applied loading. The method is very convenient for designing two-way spanning slabs in buildings. The approach is less well suited however to the assessment of one-way spanning structures (such as most bridge decks) with complex loading patterns. Yield lines are usually a more appropriate approach as is outlined in the following section
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6.5 Yield-line analysis
6.5.1 P r i n C i p k S
Yield-line analysis is a long established method of using the plasticity of reinforced concrete slabs in order to obtain greater capacity than that obtained by elastic analyses. It was pioneered by K W Johansenin his doctorate thesis in 19436’3. Geometrically compatible plates of a bridge deck are deflected under load to simulate a failure mechanism. Each plate is bounded by straight lines and the boundaries form plastic hinges with the reinforcement yielding such that a mechanism forms. The work done in deflecting the load is equated to the work done in yielding the reinforcement along the plate boundaries. See Figure 6.1.
6.5.2 Upper bound SO[UtiOnS
This method provides an upper bound solution so it is necessary to examine all conceivable combinations and permutations of plate configurations to determine the failure mode
Figure 6.1 General principles of yield-line analysis for a simply supported slab considering one simple mechanism.
7 I (I Plate A
Plate B
L
I Sagging yield line
Plan Slab simply supported on t w o sides subject t o uniformly distributed load
Unit displacement
Section A-A External work done = Load on Plate X displacement of centroid
Section A-A Internal work done =Yield moment of reinforcement crossing the yield line
X (OA
+ OB)
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Awalysis for assessment
which gives the lowest ratio of applied work to internal work. Therefore, higher partial safety factors are used usually for yield-line analysis in order to allow for this uncertainty. An upper bound to the load capacity is derived from equating external and internal work done.
6.5.3 Slab decks
Yield-line analysis is a simple and effective analytical technique for slabs. Care is needed in the analysis as any given mechanism gives an upper bound for strength. Published and affinity theorems can be helpful in finding the worst mechanism and the corresponding strength, particularly for short spans where fan mechanisms around individual wheels have to be considered in order to identify the worst case. In the simple example shown in Figure 6.2, the circular fan is a significantly worse mechanism than the easier to analyse square. Work equations for deflection of 6 (uniform slab sagging capacity M/unit width, hogging capacity m/unit width):
Circle P ~ = I -d4+m(?) ~Mr6
r
:.
P = 2~ (M
+ m)
Square
:. P=8 ( M + m ) ~ T ( M m)
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