135541443-ap-r405-12

AP-R405-12

AUSTROADS RESEARCH REPORT

Design Rules for Bridge Bearings and Expansion Joints


Published June 2012

Austroads Ltd 2012

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.


ISBN 978-1-921991-26-4

Austroads Project No. TS1600

Austroads Publication No. APR405-12

Project Manager Dr Ross Pritchard, DTMR Qld

Prepared by

Dr Hanson Ngo, Ian Steele and Dr Neal Lake ARRB Group

Published by Austroads Ltd Level 9, Robell House 287 Elizabeth Street

Sydney NSW 2000 Australia Phone: +61 2 9264 7088

Fax: +61 2 9264 1657 Email: [email protected]

www.austroads.com.au

Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should

rely on their own skill and judgement to apply information to particular issues.


Sydney 2012

About Austroads Austroads purpose is to:

promote improved Australian and New Zealand transport outcomes

provide expert technical input to national policy development on road and road transport issues

promote improved practice and capability by road agencies.

promote consistency in road and road agency operations.

Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure and Transport, the Australian Local Government Association, and NZ Transport Agency. Austroads is governed by a Board consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:

Roads and Maritime Services New South Wales

Roads Corporation Victoria

Department of Transport and Main Roads Queensland

Main Roads Western Australia

Department of Planning, Transport and Infrastructure South Australia

Department of Infrastructure, Energy and Resources Tasmania

Department of Lands and Planning Northern Territory

Department of Territory and Municipal Services Australian Capital Territory

Commonwealth Department of Infrastructure and Transport

Australian Local Government Association

New Zealand Transport Agency.

The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road transport sector.


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CONTENTS

INTRODUCTION ............................................................................................................................ 1 1.1 Background ........................................................................................................................... 1 1.2 Aims ...................................................................................................................................... 1 1.3 Scope .................................................................................................................................... 1 1.4 Outline ................................................................................................................................... 1

PART I BRIDGE BEARINGS ....................................................................................................... 2 1 INTRODUCTION ................................................................................................................... 3 2 LITERATURE REVIEW ON BRIDGE BEARINGS................................................................. 4 2.1 General .................................................................................................................................. 4

2.1.1 Historical Development ............................................................................................ 4 2.1.2 Functions of Bridge Bearings ................................................................................... 5 2.1.3 General Design Considerations ................................................................................ 5

2.2 Common Types of Bridge Bearings ....................................................................................... 6 2.2.1 Elastomeric Bearings ............................................................................................... 6 2.2.2 Pot Bearings ............................................................................................................ 9 2.2.3 Mechanical Bearings .............................................................................................. 13 2.2.4 Disc and Spherical Bearings .................................................................................. 16 2.2.5 General Causes of Bearing Failure ........................................................................ 18

2.3 Design Codes/Specifications ............................................................................................... 18 2.3.1 AASHTO LRFD Bridge Design Specification .......................................................... 18 2.3.2 European EN 1337 ................................................................................................. 23 2.3.3 Comparison of AS 5100 with AASHTO LRFD and EN 1337 ................................... 27

3 AUSTRALIAN MANUFACTURERS OF BRIDGE BEARINGS ............................................ 28 3.1 Granor Rubber and Engineering Pty. Ltd ............................................................................. 28

3.1.1 Types of Bearings Manufactured ............................................................................ 28 3.1.2 Manufacturer Recommendations ........................................................................... 30

3.2 Ludowici ............................................................................................................................... 32 3.2.1 Types of Bearings Manufactured ............................................................................ 32 3.2.2 Manufacturer Recommendations ........................................................................... 33

3.3 Trelleborg ............................................................................................................................ 34 3.3.1 Types of Bearings Manufactured ............................................................................ 34 3.3.2 Manufacturer Recommendations ........................................................................... 36

4 STATE ROAD AUTHORITY EXPERIENCES WITH BRIDGE BEARINGS ......................... 37 4.1 Popular Types of Bridge Bearings Used Nationwide ............................................................ 37

4.1.1 Elastomeric Bearings ............................................................................................. 37 4.1.2 Pot Bearings .......................................................................................................... 39 4.1.3 Mechanical Bearings .............................................................................................. 40 4.1.4 Mortar Pads ........................................................................................................... 42

4.2 Roads and Maritime Services, New South Wales (RMS) ..................................................... 43 4.2.1 Specifications ......................................................................................................... 43 4.2.2 Design and Construction ........................................................................................ 44 4.2.3 Testing ................................................................................................................... 45 4.2.4 Failures .................................................................................................................. 45 4.2.5 Repair and Maintenance ........................................................................................ 48


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4.3 Roads Corporation, Victoria (VicRoads) ............................................................................... 48 4.3.1 Specifications ......................................................................................................... 49 4.3.2 Design and Construction ........................................................................................ 49 4.3.3 Testing ................................................................................................................... 50 4.3.4 Failures .................................................................................................................. 50 4.3.5 Repair and Maintenance ........................................................................................ 51

4.4 Department of Transport and Main Roads, Queensland (TMR) ........................................... 51 4.4.1 Specifications ......................................................................................................... 51 4.4.2 Design and Construction ........................................................................................ 52 4.4.3 Testing ................................................................................................................... 52 4.4.4 Failures .................................................................................................................. 52 4.4.5 Repair and Maintenance ........................................................................................ 53

4.5 Main Roads Western Australia (MRWA) .............................................................................. 53 4.5.1 Specifications ......................................................................................................... 53 4.5.2 Design and Construction ........................................................................................ 54 4.5.3 Testing ................................................................................................................... 54 4.5.4 Failures .................................................................................................................. 54 4.5.5 Repair and Maintenance ........................................................................................ 55

4.6 Department of Planning, Transport and Infrastructure, South Australia (DPTI) .................... 55 4.6.1 Specifications ......................................................................................................... 55 4.6.2 Design and Construction ........................................................................................ 55 4.6.3 Testing ................................................................................................................... 56 4.6.4 Failures .................................................................................................................. 56 4.6.5 Repair and Maintenance ........................................................................................ 58

4.7 Department of Infrastructure, Energy and Resources, Tasmania (DIER) ............................. 58 4.7.1 Specifications ......................................................................................................... 58 4.7.2 Design and Construction ........................................................................................ 59 4.7.3 Testing ................................................................................................................... 59 4.7.4 Failures .................................................................................................................. 60 4.7.5 Repair and Maintenance ........................................................................................ 60

4.8 Department of Lands and Planning, (formerly Department of Planning and Infrastructure) Northern Territory (DPI) ................................................................................ 60 4.8.1 Specifications ......................................................................................................... 60 4.8.2 Design and Construction ........................................................................................ 60 4.8.3 Testing ................................................................................................................... 60 4.8.4 Failures .................................................................................................................. 60 4.8.5 Repair and Maintenance ........................................................................................ 60

4.9 ACT Department of Territory and Municipal Services (ACT TAMS) ..................................... 61 4.9.1 Specifications ......................................................................................................... 61 4.9.2 Design and Construction ........................................................................................ 61 4.9.3 Testing ................................................................................................................... 61 4.9.4 Failures .................................................................................................................. 61

4.10 New Zealand Transport Agency (NZTA) .............................................................................. 63 4.10.1 Specifications ......................................................................................................... 63 4.10.2 Design and Construction ........................................................................................ 63 4.10.3 Testing ................................................................................................................... 64 4.10.4 Failures .................................................................................................................. 64 4.10.5 Repair and Maintenance ........................................................................................ 64

4.11 Summary on SRAs Practice ................................................................................................ 64 4.11.1 Modes of Failures ................................................................................................... 64 4.11.2 State Specifications and Technical Standards ........................................................ 65 4.11.3 Recommendations ................................................................................................. 65


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5 INVESTIGATION INTO COMMON FAILURES OF BRIDGE BEARINGS ........................... 66 5.1 Proposed Investigation ........................................................................................................ 66 5.2 Elastomeric Bearings ........................................................................................................... 66

5.2.1 Identified Problems ................................................................................................ 66 5.2.2 Root Causes .......................................................................................................... 66 5.2.3 Suggested Maintenance to Rectify the Problems ................................................... 67 5.2.4 Possible Upgrade and Modification ........................................................................ 67 5.2.5 Proposed Changes to AS 5100 for Future Use....................................................... 67

5.3 Pot Bearings ........................................................................................................................ 68 5.3.1 Identified Problems ................................................................................................ 68 5.3.2 Root Causes .......................................................................................................... 68 5.3.3 Suggested Maintenance to Rectify the Problems ................................................... 68 5.3.4 Possible Upgrade and Modification ........................................................................ 69 5.3.5 Proposed Changes to AS 5100 for Future Use....................................................... 69

5.4 Proposed Changes for Other Bearing Types ....................................................................... 71 5.4.1 Plain Pads and Strips ............................................................................................. 71 5.4.2 Mechanical Bearings .............................................................................................. 72

6 PROPOSED BEARING SPECIFICATION DEVELOPMENT ............................................... 73 6.1 Background ......................................................................................................................... 73 6.2 Generic Specification for Spherical and Disc Bearings ......................................................... 73

6.2.1 General Requirements ........................................................................................... 73 6.2.2 Spherical Bearings ................................................................................................. 75 6.2.3 Disc Bearings ......................................................................................................... 75

7 CONCLUSION AND FUTURE DIRECTIONS ...................................................................... 77

PART II EXPANSION JOINTS .................................................................................................. 78 1 INTRODUCTION ................................................................................................................. 79 2 LITERATURE REVIEW ON EXPANSION JOINTS ............................................................. 80 2.1 General ................................................................................................................................ 80

2.1.1 Historical Development .......................................................................................... 80 2.1.2 Functions of Expansion Joints ................................................................................ 81 2.1.3 General Design Considerations .............................................................................. 81

2.2 Common Types of Expansion Joints .................................................................................... 81 2.2.1 Steel Sliding Plate Joints ........................................................................................ 81 2.2.2 Asphaltic Plug Joints .............................................................................................. 82 2.2.3 Cold Applied Sealant Joints.................................................................................... 83 2.2.4 Compression Seal Joints ........................................................................................ 84 2.2.5 Strip Seal Joints ..................................................................................................... 85 2.2.6 Moulded Rubber Joints .......................................................................................... 86 2.2.7 Bonded Metal/Elastomer Joints .............................................................................. 87 2.2.8 Fingerplate Joints ................................................................................................... 88 2.2.9 Modular Joints ........................................................................................................ 89

2.3 Design Codes/Specifications ............................................................................................... 91 2.3.1 Australian Bridge Design Standard AS 5100 .......................................................... 91 2.3.2 AASHTO LRFD Bridge Design Specification .......................................................... 91


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3 AUSTRALIAN MANUFACTURERS OF EXPANSION JOINTS ........................................... 95 3.1 Granor ................................................................................................................................. 95

3.1.1 Types of Expansion Joints Manufactured ............................................................... 95 3.1.2 Manufacturer Recommendations ......................................................................... 101

3.2 Miska ................................................................................................................................. 104 3.2.1 Types of Expansion Joints Manufactured ............................................................. 104 3.2.2 Manufacturer Recommendations ......................................................................... 106

3.3 Trelleborg .......................................................................................................................... 106 3.3.1 Types of Expansion Joints Manufactured ............................................................. 106 3.3.2 Manufacturer Recommendations ......................................................................... 109

3.4 HMS Civils Division ............................................................................................................ 111

4 STATE ROAD AUTHORITY EXPERIENCES WITH BRIDGE EXPANSION JOINTS ........ 112 4.1 Popular Types of Bridge Expansion Joints Used Nationwide ............................................. 112

4.1.1 Fixed Joint ............................................................................................................ 112 4.1.2 Sliding Steel Plate ................................................................................................ 112 4.1.3 Asphaltic Plug Joints ............................................................................................ 113 4.1.4 Pour Sealant Joints .............................................................................................. 113 4.1.5 Compression Seal Joints ...................................................................................... 114 4.1.6 Strip Seal Joints ................................................................................................... 115 4.1.7 Moulded Rubber Joints ........................................................................................ 115 4.1.8 Bonded Metal/Elastomer Expansion Joints .......................................................... 115 4.1.9 Fingerplate Joints ................................................................................................. 116 4.1.10 Modular Joints ...................................................................................................... 116

4.2 Roads and Maritime Services, New South Wales (RMS) ................................................... 117 4.2.1 Specifications ....................................................................................................... 117 4.2.2 Design and Construction ...................................................................................... 119 4.2.3 Testing ................................................................................................................. 120 4.2.4 Failures ................................................................................................................ 121 4.2.5 Repair and Maintenance ...................................................................................... 124

4.3 Roads Corporation, Victoria (VicRoads) ............................................................................. 125 4.3.1 Specifications ....................................................................................................... 125 4.3.2 Design and Construction ...................................................................................... 126 4.3.3 Testing ................................................................................................................. 126 4.3.4 Failures ................................................................................................................ 127 4.3.5 Repair and Maintenance ...................................................................................... 127

4.4 Department of Transport and Main Roads, Queensland (TMR) ......................................... 127 4.4.1 Specifications ....................................................................................................... 127 4.4.2 Design and Construction ...................................................................................... 127 4.4.3 Testing ................................................................................................................. 128 4.4.4 Failures ................................................................................................................ 128 4.4.5 Repair and Maintenance ...................................................................................... 129

4.5 Main Roads Western Australia (MRWA) ............................................................................ 129 4.5.1 Specifications ....................................................................................................... 129 4.5.2 Design and Construction ...................................................................................... 129 4.5.3 Testing ................................................................................................................. 130 4.5.4 Failures ................................................................................................................ 130 4.5.5 Repair and Maintenance ...................................................................................... 133

4.6 Department of Planning, Transport and Infrastructure, South Australia (DPTI) .................. 133 4.6.1 Specifications ....................................................................................................... 133 4.6.2 Design and Construction ...................................................................................... 133


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4.6.3 Testing ................................................................................................................. 134 4.6.4 Failures ................................................................................................................ 134 4.6.5 Repair and Maintenance ...................................................................................... 135

4.7 Department of Infrastructure, Energy and Resources, Tasmania (DIER) ........................... 136 4.7.1 Specifications ....................................................................................................... 136 4.7.2 Design and Construction ...................................................................................... 137 4.7.3 Testing ................................................................................................................. 137 4.7.4 Failures ................................................................................................................ 137 4.7.5 Repair and Maintenance ...................................................................................... 137

4.8 Department of Lands and Planning, (formerly Department of Planning and Infrastructure) Northern Territory (DPI) .............................................................................. 137 4.8.1 Specifications ....................................................................................................... 137 4.8.2 Design and Construction ...................................................................................... 137 4.8.3 Testing ................................................................................................................. 137 4.8.4 Failures ................................................................................................................ 137 4.8.5 Repair and Maintenance ...................................................................................... 138

4.9 ACT Department of Territory and Municipal Services (ACT TAMS) ................................... 138 4.9.1 Specifications ....................................................................................................... 138 4.9.2 Design and Construction ...................................................................................... 138 4.9.3 Testing ................................................................................................................. 138 4.9.4 Failures ................................................................................................................ 138 4.9.5 Repair and Maintenance ...................................................................................... 141

4.10 New Zealand Transport Agency (NZTA) ............................................................................ 141 4.10.1 Specifications ....................................................................................................... 141 4.10.2 Design and Construction ...................................................................................... 142 4.10.3 Testing ................................................................................................................. 142 4.10.4 Failures ................................................................................................................ 142 4.10.5 Repair and Maintenance ...................................................................................... 142

4.11 Summary on Service and Failure Issues ............................................................................ 143 4.11.1 Modes of Failure .................................................................................................. 143 4.11.2 State Specifications and Technical Standards ...................................................... 144 4.11.3 Recommendations ............................................................................................... 144

5 INVESTIGATION INTO COMMON FAILURES OF BRIDGE EXPANSION JOINTS ......... 145 5.1 Proposed Investigation ...................................................................................................... 145 5.2 Compression Seal Joints ................................................................................................... 145

5.2.1 Identified Problems .............................................................................................. 145 5.2.2 Root Causes ........................................................................................................ 145 5.2.3 Suggested Maintenance to Rectify the Problems ................................................. 146 5.2.4 Possible Upgrade and Modification ...................................................................... 146 5.2.5 Proposed Changes to AS 5100 for Future Use..................................................... 146

5.3 Strip Seal Joints ................................................................................................................. 147 5.3.1 Identified Problems .............................................................................................. 147 5.3.2 Root Causes ........................................................................................................ 147 5.3.3 Suggested Maintenance to Rectify the Problems ................................................. 147 5.3.4 Possible Upgrade and Modification ...................................................................... 147 5.3.5 Proposed Changes to AS 5100 for Future Use..................................................... 148

5.4 Bonded Metal/Elastomer Expansion Joints ........................................................................ 148 5.4.1 Identified Problems .............................................................................................. 148 5.4.2 Root Causes ........................................................................................................ 148 5.4.3 Suggested Maintenance to Rectify the Problems ................................................. 148 5.4.4 Possible Upgrade and Modification ...................................................................... 148


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5.4.5 Proposed Changes to AS 5100 for Future Use..................................................... 149 5.5 Fingerplate Joints .............................................................................................................. 149

5.5.1 Identified Problems .............................................................................................. 149 5.5.2 Root Causes ........................................................................................................ 149 5.5.3 Suggested Maintenance to Rectify the Problems ................................................. 149 5.5.4 Possible Upgrade and Modification ...................................................................... 149 5.5.5 Proposed Changes to AS 5100 for Future Use..................................................... 150

5.6 Modular Joints ................................................................................................................... 150 5.6.1 Identified Problems .............................................................................................. 150 5.6.2 Root Causes ........................................................................................................ 151 5.6.3 Suggested Maintenance to Rectify the Problems ................................................. 151 5.6.4 Possible Upgrade and Modification ...................................................................... 151 5.6.5 Proposed Changes to AS 5100 for Future Use..................................................... 152

5.7 Proposed Changes for Other Joint Types .......................................................................... 152 5.8 Selection of Expansion Joints ............................................................................................ 152

6 GENERIC SPECIFICATION FOR BRIDGE EXPANSION JOINTS ................................... 155 6.1 General .............................................................................................................................. 155 6.2 Joint Types ........................................................................................................................ 155 6.3 Requirements .................................................................................................................... 155 6.4 Anchorage of Deck Joints .................................................................................................. 155 6.5 Drainage ............................................................................................................................ 155 6.6 Specific Provisions for Compression Joints ........................................................................ 156 6.7 Specific Provisions for Strip Seal Joints ............................................................................. 156 6.8 Specific Provisions for Modular Deck Joints ....................................................................... 157 6.9 Specific Provisions for Fingerplate Joints ........................................................................... 157 6.10 Joint Sealants .................................................................................................................... 158 6.11 Installation ......................................................................................................................... 158 6.12 Monitoring, Maintenance and Rehabilitation ...................................................................... 158

6.12.1 General ................................................................................................................ 158 6.12.2 Monitoring ............................................................................................................ 158 6.12.3 Maintenance of Joints .......................................................................................... 158 6.12.4 Joint Rehabilitation ............................................................................................... 159

7 CONCLUSIONS ................................................................................................................ 160 REFERENCES ........................................................................................................................... 161


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TABLES

Note: Part I runs from pages 2 to 77. Part II runs from pages 78 to 160.

Table 2.1: Capacity of elastomeric bearings ............................................................................. 7 Table 2.2: LRFD suitability of bearing systems ....................................................................... 19 Table 2.3: AASHTO bearing design limitations ....................................................................... 22 Table 2.4: Pot bearing: comparison of AS 5100 to EN1337 and AASHTO LRFD

(2007) .................................................................................................................... 27 Table 3.1: Main features of Ludowici pot bearings .................................................................. 33 Table 4.1: Specifications for bearings in RMS ......................................................................... 43 Table 4.2: Differences between RMS specification for elastomeric pads and strips

and AS 5100.4 ....................................................................................................... 43 Table 4.3: Failure criteria for different bearing types ............................................................... 45 Table 4.4: Tests required for different pot bearings ................................................................. 45 Table 4.5: Common faults in different types of bearings in NSW ............................................. 45 Table 4.6: Common faults of bearings observed in recent inspections in NSW ....................... 46 Table 4.7: Common failure modes of bearings in Victoria ....................................................... 50 Table 4.8: Hold points in testing of bearings in Queensland .................................................... 52 Table 4.9: Common failures in bearings in Queensland .......................................................... 52 Table 4.10: Friction coefficient of sliding surfaces MRWA ...................................................... 54 Table 4.11: Friction coefficient of sliding surfaces DPTI ......................................................... 56 Table 4.12: Tolerances on compressive stiffness of laminated bearings ................................... 59 Table 4.13: Friction coefficient specified by DIER ..................................................................... 59 Table 4.14: Common failures of bridge bearings ....................................................................... 64 Table 5.1: Differences in requirements for laminated elastomeric bearings ............................. 68 Table 5.2: Comparison of SRAs specifications and AS 5100.4 for pot bearing

materials ................................................................................................................ 69 Table 5.3: Comparison of SRAs specifications and AS 5100.4 for pot bearing

testing requirements ............................................................................................... 70 Table 5.4: Differences in requirements for plain pads and strips ............................................. 71 Table 6.1: Maximum contact stress for PTFE at the ULS ........................................................ 74 Table 6.2: Bearing stress at the ULS ...................................................................................... 75 Table 2.1: Minimum movement range capacity for MBJS........................................................ 93 Table 2.2: Centre beam distribution factor .............................................................................. 94 Table 4.1: Specifications for expansion joints in RMS ........................................................... 117 Table 4.2: Common faults of expansion joints in NSW .......................................................... 121 Table 4.3: Common failure modes of expansion joints in Victoria .......................................... 127 Table 4.4: Common failure modes of expansion joints in Queensland .................................. 128 Table 4.5: Joint selection in MRWA ...................................................................................... 130 Table 4.6: Typical failures of expansion joints in recent inspections ...................................... 139 Table 4.7: Common modes of failure of expansion joints ...................................................... 143 Table 5.1: Applicable movement ranges of popular expansion joints .................................... 153 Table 6.1: Elastomer material requirements .......................................................................... 156


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FIGURES

Note: Part I runs from pages 2 to 77. Part II runs from pages 78 to 160.

Figure 2.1: Typical cross-section of a laminated elastomeric bearing ......................................... 7 Figure 2.2: Design basis of elastomeric bearings ....................................................................... 7 Figure 2.3: Pot bearing ............................................................................................................. 10 Figure 2.4: Extrusion of rubber from pot bearing due to broken seal ........................................ 10 Figure 2.5: Fixed pot bearing ................................................................................................... 11 Figure 2.6: Free floating pot bearing ........................................................................................ 12 Figure 2.7: Pot glided bearing .................................................................................................. 12 Figure 2.8: Mechanical bearings .............................................................................................. 15 Figure 2.9: Watson disc bearings ............................................................................................. 16 Figure 2.10: RJW spherical bearings ......................................................................................... 17 Figure 2.11: Maurer Sohne spherical bearings ........................................................................... 17 Figure 3.1: Granor pot bearing ................................................................................................. 28 Figure 3.2: Granor laminated elastomeric bearing .................................................................... 29 Figure 3.3: Disktron bearings imported by Granor .................................................................... 30 Figure 3.4: Ludowici pot bearings............................................................................................. 33 Figure 3.5: Trelleborg bearing pad/strip.................................................................................... 34 Figure 3.6: A general mobile pot bearing .................................................................................. 35 Figure 3.7: Trelleborg laminated elastomeric bearing ............................................................... 35 Figure 3.8: Maurer Sohne spherical bearing ............................................................................ 36 Figure 4.1: Elastomeric pad bearing ......................................................................................... 37 Figure 4.2: Laminated elastomeric bearing .............................................................................. 38 Figure 4.3: A laminated elastomeric bearing used in Queensland ............................................ 38 Figure 4.4: Laminated elastomeric bearing with lateral restraints ............................................. 39 Figure 4.5: Pot type bearings ................................................................................................... 39 Figure 4.6: A pot bearing used in NSW .................................................................................... 40 Figure 4.7: Fixed plate bearings ............................................................................................... 40 Figure 4.8: Sliding plate bearings ............................................................................................. 41 Figure 4.9: A sliding plate bearing in use in QLD in a bad condition ...................................... 41 Figure 4.10: A fixed rocker bearing ............................................................................................ 41 Figure 4.11: An expansion rocker bearing .................................................................................. 42 Figure 4.12: A rocker bearing used in NSW ............................................................................... 42 Figure 4.13: Mortar pad bearing ................................................................................................. 42 Figure 4.14: Cracking and deterioration of elastomer ................................................................. 46 Figure 4.15: Splitting and bulging of bearing pads ...................................................................... 47 Figure 4.16: Corrosion of a rocker bearing ................................................................................. 47 Figure 4.17: Corrosion and loss of support ................................................................................. 47 Figure 4.18: Corrosion and leakage of rubber ............................................................................ 48 Figure 4.19: Broken seal in pot bearing ...................................................................................... 48 Figure 4.20: Slight bulging of an elastomeric bearing ................................................................. 55 Figure 4.21: Crack in steel roller bearing in Old Mt Barker Overpass bridge .............................. 57 Figure 4.22: Anchor bolt of a plate bearing of Stirling I/C bridge broken down due to

fatigue .................................................................................................................... 57 Figure 4.23: Extrusion of elastomer of a pot bearing in Myall Creek ........................................... 58 Figure 4.24: Extrusion of PTFE of a pot bearing in Two Wells bridge ......................................... 58 Figure 4.25: Failure modes of metal bearings in Bridge 2097 in ACT (1).................................... 61 Figure 4.26: Failure modes of metal bearings in Bridge 2097 in ACT (2).................................... 62


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Figure 4.27: General view of bearing WBEAN1 showing poor mortar pads and vertical curve on bearing base ................................................................................ 62

Figure 4.28: Bearing WBEAN1 showing poor mortar, bearing recessed into abutment top and vertical curve of bearing base ................................................................... 63

Figure 4.29: Bearing WBEAN1 showing minor damage to pads, loss of contact between bearing and top pad and edge of bearing being in the same plane as top and bottom mortar pads ..................................................................... 63

Figure 2.1: Steel sliding plates ................................................................................................. 82 Figure 2.2: Asphaltic plug joint ................................................................................................. 83 Figure 2.3: Pourable sealant joint ............................................................................................. 84 Figure 2.4: Compression seal joint ........................................................................................... 85 Figure 2.5: Strip seal joint ........................................................................................................ 86 Figure 2.6: An example of moulded rubber joint ....................................................................... 87 Figure 2.7: Bonded metal/elastomer joint ................................................................................. 88 Figure 2.8: Finger type expansion joint..................................................................................... 89 Figure 2.9: Example of a modular expansion joint .................................................................... 90 Figure 3.1: Wabo modular joint system .................................................................................... 95 Figure 3.2: ETIC finger joint ..................................................................................................... 96 Figure 3.3: Granor Ausflex strip joint ........................................................................................ 97 Figure 3.4: Components of Granor Ausflex strip joint ............................................................... 97 Figure 3.5: Wabo compression seals ....................................................................................... 98 Figure 3.6: Granor Ausflex compression seal ........................................................................... 99 Figure 3.7: Granor Wizflex expansion joint ............................................................................... 99 Figure 3.8: Two series of Waboflex joints ............................................................................... 100 Figure 3.9: Granor XJS expansion joint .................................................................................. 100 Figure 3.10: Prismo-Thormajoint .............................................................................................. 101 Figure 3.11: Miska bolted-in bridge expansion joint .................................................................. 104 Figure 3.12: Miska cast-in bridge expansion joint ..................................................................... 105 Figure 3.13: Miska compression joint ....................................................................................... 105 Figure 3.14: Miska ZealCrete elastomeric concrete .............................................................. 106 Figure 3.15: Maurer Sohne modular joint ................................................................................. 106 Figure 3.16: Maurer swivel-joist expansion joint ....................................................................... 107 Figure 3.17: Two-way obliquely arranged swivel support bars and joist boxes ......................... 107 Figure 3.18: TESA PHS System .............................................................................................. 108 Figure 3.19: TESA strip seal joints ........................................................................................... 108 Figure 3.20: Transflex expansion joints .................................................................................... 109 Figure 4.1: Some types of fixed joints used in Queensland .................................................... 112 Figure 4.2: Sliding steel plate joint .......................................................................................... 113 Figure 4.3: Asphaltic plug joint ............................................................................................... 113 Figure 4.4: Pourable sealant joint used in Queensland .......................................................... 114 Figure 4.5: Sealant joints used in NSW .................................................................................. 114 Figure 4.6: Compression seal joint using steel angles ............................................................ 114 Figure 4.7: A compression seal joint used in NSW ................................................................. 115 Figure 4.8: A strip seal joint used in NSW .............................................................................. 115 Figure 4.9: A bonded metal/elastomer joint used in Western Australia ................................... 116 Figure 4.10: A fingerplate joint used in NSW ............................................................................ 116 Figure 4.11: Maurer Sohne modular expansion joint ................................................................ 117 Figure 4.12: Loss of seal adhesion........................................................................................... 122 Figure 4.13: Spall and debris blocking the joint ........................................................................ 122 Figure 4.14: Loss of adhesion on sides of the compression seal .............................................. 123 Figure 4.15: Spall and loose nut of a compression joint ........................................................... 123 Figure 4.16: Broken anchor bolts and assembly breaking loose ............................................... 124


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Figure 4.17: Joint blocked with debris and spalls appearing at joint edges ............................... 124 Figure 4.18: Typical deck joint showing minimum transverse reinforcement ............................ 126 Figure 4.19: Wabo joint filled with debris .................................................................................. 130 Figure 4.20: Damaged section of steel angle in a compression seal joint ................................. 131 Figure 4.21: Damaged seal in deck joint .................................................................................. 131 Figure 4.22: Missing seal in deck joint ...................................................................................... 131 Figure 4.23: Failures of a metal fingers-bonded to elastomer joint in Shelley bridge

No. 931 in WA ...................................................................................................... 132 Figure 4.24: Failures of fingerplate type joints in Mt Henry bridge, WA .................................... 133 Figure 4.25: A strip seal on Craig Gilbert bridge, bolts were loosened ..................................... 135 Figure 4.26: A Granor XJS joint with silicone seal failure ......................................................... 136 Figure 4.27: A Felspan joint with debonded rubber .................................................................. 136 Figure 4.28: Damaged asphaltic plug joint in Bridge 1056, cracks along the edges ................. 139 Figure 4.29: A compression seal joint with concrete nosing in Bridge 1147,

deteriorated seal and damaged nosings ............................................................... 139 Figure 4.30: A compression seal joint with steel angles in Bridge 1249, protrusion of

and minor damage/wear to compression seals..................................................... 140 Figure 4.31: An XJS joint in Bridge 2097, cracked nosings ...................................................... 140 Figure 4.32: A Felspan joint in Bridge 2097, damaged joint and missing sections .................... 140 Figure 4.33: A mechanical finger joint in Bridge 2113, a male finger joint segment

broken out ............................................................................................................ 141 Figure 4.34: A deteriorated Transflex joint in Bridge 4087 ........................................................ 141


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SUMMARY

In modern bridge structures, bridge bearings and expansion joints are commonly the source of most maintenance and performance issues. In some situations their performance can lead to the closure/failure of major structures and their maintenance can be extremely difficult and costly to implement. Most of these problems can be overcome if robust design and installation rules are implemented to ensure their design and construction deliver highly durable and reliable components, and that the lessons learnt from practice are fully addressed in the future design and maintenance of these components, thus leading to improved performance.

The report includes the following key topics:

A literature review that summarises the features, usage and performance of bridge bearings and expansion joints throughout the world. The common failures and the proposed solutions to rectify the failures of the most used bearings and expansion joints are discussed.

Information on the State Road Authorities current practice in design, installation and maintenance of bridge bearings and expansion joints. Common failure modes of various types of bearing and expansion joint are derived; the key issues in design, manufacture, construction and maintenance of bridge bearings and expansion joints are presented.

Information on the current practice in manufacture and supply of bridge bearings and expansion joints of Australian providers. The manufacturers recommendations on the solutions to rectifying the identified failures are provided.

An investigation into failures of some common bridge bearing and expansion joint types that have been used in the past and will be continued in new bridge designs. The outcomes of the investigation are incorporated into proposed changes to the current version of the Australian Bridge Design Standard AS 5100 part 4, and recommendations are provided for the design of bearing and expansion joints.

A generic specification for new bridge bearing types spherical and disc bearings to be introduced in Australian practice, and a generic specification for bridge expansion joints in which specific provisions for several popular expansion joint types are also provided.

The fact that each State Road Authority has developed and/or used its own technical guidelines and specifications has brought in a level of inconsistency in the current practice throughout the country. The report also provides recommendations on future directions to overcome this issue.


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INTRODUCTION

1.1 Background In modern bridge structures, bridge bearings and expansion joints are commonly the source of most maintenance and performance issues. In some situations their performance can lead to the closure/failure of major structures, and their maintenance can be extremely difficult and costly to implement. Most of these problems can be overcome if robust design rules are implemented to ensure their design and construction delivers highly durable and reliable bridge components, and that designers make due allowance for their future maintenance so that it can be undertaken in an acceptable and affordable manner.

The project was conceived to develop such guidelines for the design and construction of both bridge bearings and bridge expansion joints to overcome performance issues experienced in Australian bridges. The primary objective is to obtain failure modes of bearings and joints experienced throughout Australia and develop design, construction and/or maintenance solutions to overcome these issues.

1.2 Aims The aims of the overall research project are to:

Review existing Australian and International literature on bridge bearings and expansion joints.

Collect and report State Road Authority (SRA) experiences with the design, construction inspection, maintenance, repair and failures of all types of bridge bearings and expansion joints.

Develop Austroads guidelines addressing critical issues in the maintenance of existing types of bridge bearings and expansion joints and investigate the introduction of new types of bridge bearings.

Identify specific research and development investigations that will deliver the data relevant to understanding the performance of these structures in the Australian environment.

1.3 Scope Bridge bearings and expansion joints specifically designed for seismic requirements or used for special bridge structures are out of scope of this project.

1.4 Outline This report includes two parts: Part I Bridge Bearings and Part II Bridge Expansion Joints. The structure of the two parts is identical.


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PART I BRIDGE BEARINGS


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1 INTRODUCTION The contents of Part I Bridge Bearings are briefly summarised below.

Section 2 presents a literature review that summarises the features, usage and performance of bridge bearings throughout the world. Thus, the common failures of the common type of bearings are identified.

Section 3 describes the current practice in manufacture and supply of bridge bearings of some popular Australian providers.

Section 4 describes SRAs experience in design, installation and maintenance of bridge bearings. Common failure modes of various types of bearing are derived from SRAs information, based on which the key issues in design, manufacture, construction and maintenance of bridge bearings are identified.

Section 5 presents an investigation into failures of some common bridge bearing types that have been used in the past and will be continued in new bridge designs, and proposes changes to the current version of the Australian Bridge Design Standard AS 5100 part 4. Proposed changes for other bearing types are also addressed in this section.

Section 6 proposes a generic specification for new types of bridge bearing to be introduced in Australia practice spherical and disc bearings.

Section 7 represents the key conclusions arising from the project.


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2 LITERATURE REVIEW ON BRIDGE BEARINGS

2.1 General 2.1.1 Historical Development By the end of the 18th century the first bridge structures were built from stone, brick or masonry, which have massive volumes and undergo slight movements due to external impact or thermal changes. The slight movements are accommodated by deformation of the materials or by small displacements of the supports. Thus there were no bearings used in these structures.

The concept of bearings was introduced in the 19th century when cast iron and steel materials were used in bridge construction. Longer span bridges made use of flexible supports to accommodate the contraction and expansion movements at support points due to temperature changes. The first types of bearing were metal plates sliding on one another, or roller bearings. These bearings were popular for a century in steel bridges.

In the 20th century, with the booming construction and development of infrastructure and the development of advanced technologies, more slender bridge structures were built, including large span prestressed concrete bridges. These structures require bearing devices that can accommodate larger rotational and translational movements. As a result, new materials for bearings were introduced, including rubber and a combination of rubber and steel laminates.

In 1932 French railways were the first to make use of rubber pads on a railway bridge at La Plaine St Denis in Paris by placing them underneath steel bearings to absorb vibration.

It was Eugene Freyssinet who first combined rubber and steel into a single product to strengthen the bearing capacity of rubber. In 1952, the first rubber-steel bearings were manufactured by Freyssinet that consisted of a stack of elastomer layers and sheets of tinned metal grillage. The metal grillage was replaced by steel plates in 1956 when the first laminated elastomeric bearings were manufactured (Raina 1994).

Rapid developments in bearing materials in subsequent years resulted in the introduction of neoprene elastomer compounds for bearings in severe climate conditions, the use of stainless steel to resist corrosion in aggressive environments, and the discovery of polytetrafluoroethylene (PTFE) sliding sheets that allow large movement capability for the bearings. Thus, more sophisticated bearing systems were introduced, including pot bearings, and later on, spherical and disc bearings. These bearings can sustain large loads and accommodate multi-directional movements, and can typically meet the requirements of modern bridges with complex actions, such as very wide, large span, multi-span and skew or curve bridges, etc.

In Australia, bridge works are carried out under the authority of independently operated State Road Authorities in the eight states and territories. The previous Austroads Bridge Design Code (Austroads 1992) sets out the requirements for design, manufacture, testing and installation of the most common types of bearings, including mechanical bearings, elastomeric bearings and pot bearings. AS 5100, which was in effect in 2004, added some provisions regarding design criteria for pot bearings and sliding contact surfaces at the ultimate limit states (ULS) as well as testing of elastomer and laminated elastomeric bearings. However, no provisions have been made towards the implementation of spherical and disc bearings.


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2.1.2 Functions of Bridge Bearings In bridge structures, bearings are the connections between the superstructure and the substructure. They not only transfer loads from superstructure to substructure, but also ensure the structure functions as intended so that no part is under excessive stress and/or deformation. In typical girder bridges, the bearings must allow free translations and rotations of the span in the specified directions. In addition, bearings must also restraint the span in predetermined positions or directions.

2.1.3 General Design Considerations Design life

In most of the bridge design codes, bearings are specified to have the same design life as the bridge structure, including long life metallic and non-metallic materials. Experiences from all over the world, however, have shown that bearings of all types have failure modes caused by various factors relating to all stages of the bearing life such as manufacture, installation or maintenance. The failures may occur on any part of the bearing such as the bearing itself, clamping plates or anchor bolts. This leads to the requirements of repair or replacement of the bearing in whole or in part during its service life. Provisions should be made regarding the replaceability of the bearing, including the method of removal, jacking points and loads. Special considerations should be given to jacking space especially for slab type bridges using halving joints, or bridges with possible differential settlements, etc. (Austroads 2009, Lee 1994).

Durability

Corrosion of metal bearing components is one of the most common causes of bearing failures. Thus provisions should be made to avoid corrosion, particularly in aggressive conditions, such as use of stainless steel, use of materials having similar electrode potentials and detailing the bearing in such a way that it does not trap moisture or dirt.

Earthquake design

The capacity of bearings to resist horizontal loads such as earthquake loads should be considered in the design. Restraint can be provided by means of separate dowels or special keys.

Other considerations

A variety of other factors such as spreader plates, uplift, and limit states should be considered in the bearing design, as follows:

Spreader plates: the spreader plates of bearings should be so proportioned so that concentrated loads are sufficiently distributed to ensure that the permissible pressures on the adjacent bridge structure are not exceeded (Raina 1994).

Uplift: if the bearings will be subjected to uplift, they and their fixings must be designed to limit separation of the parts. In particular, rubber should not be allowed to go into tension and sliding surfaces should not be allowed to separate. This would allow dust, grit and other abrasive or corrosive materials to enter and affect the sliding surface (Lee 1994).

Limit states: the bearings should not suffer damage that would affect their proper functioning or incur excessive maintenance during their working life to meet the serviceability limit state (SLS). To meet the ULS, the strength and stability of the bearings should be adequate to withstand the ultimate design loads and movements of the structure (Lee 1994).


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2.2 Common Types of Bridge Bearings 2.2.1 Elastomeric Bearings General features

Being first used in the UK in 1956 and in the USA in 1960 (Fyfe et al. 2006), elastomeric bearings are one of the most popular types of bearings used nowadays for pre-stressed planks, girders and troughs and steel girder spans up to 40 m.

These types of bearings can meet a number of requirements such as capability of translational movement in both longitudinal and transverse directions, rotational movement and vibration isolation for the superstructure. In addition, they have a number of advantages including low cost, ease of installation, long service life, low maintenance, and corrosion protection. Due to high resistance to rotation, they are, however, unsuitable under high rotation or excessive longitudinal movement with low compressive load.

Combined with an effective method of anchoring via friction or dowels through the bearing, elastomeric bearings provide a system that can be used in a wide range of applications. There exist two common bearing forms:

Fixed bearings when the horizontal movement is restrained by the use of steel dowels that extend from the superstructure, through holes in the bearing to the substructure. Alternatively, the superstructure can be fixed to the substructure by separate fixtures to provide restraint.

Expansion bearings the horizontal movement in an elastomeric bearing occurs by the shearing of the rubber such that the top of the bearing moves relative to the bottom of the bearing. Internal or external restraints should be provided to ensure the bearings do not walk out.

Elastomeric bearings include three types, being plain pads, plain strips and laminated elastomeric bearings (LEB) and are used as follows:

Continuous strip bearings of plain elastomer up to 125 x 25 mm in cross-section, which are used to support slabs and pre-stressed concrete planks.

Plain elastomeric bearing pads, having varying thicknesses up to 25 mm, are used to support pre-stressed concrete planks and short span girders where individual bearing supports are required. They are usually rectangular but can also be specified or used as circular. The advantage of circular bearings is that they have uniform characteristics in all directions. This is particularly useful on skew bridges to accommodate the pier movements not along the bridge. These bearings have limited vertical load capacity, shear movement and rotational capacities compared to the thicker laminated bearings described below.

LEB pads are either rectangular up to 600 mm x 600 mm x 293 mm thick or circular up to 880 mm diameter x 309 mm thick (Figure 2.1) and are used to support pre-stressed concrete girders, troughs and steel girders up to 40 m spans. The bearings consist of elastomer reinforced by a number of embedded steel plates. The number of plates and the height of the bearing increase as the bearing capacity and/or movement range increases.


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Source: Austroads (2009).

Figure 2.1: Typical cross-section of a laminated elastomeric bearing

The capacity of the above three types of elastomeric bearings is listed in Table 2.1.

Table 2.1: Capacity of elastomeric bearings

Bearing type Maximum vertical force (kN) Maximum translation

(mm) Maximum rotation

(rad)

Plain pads Strips 1500 15 0.01

Laminated elastomeric bearings 5000 100 0.04 Source: Lee (1994).

Design basis

The design of elastomeric bearings is based on a limit on the combined effects of compressive strain, shear strain and rotational strain (Figure 2.2).


Figure 2.2: Design basis of elastomeric bearings

The proper performance of an expansion elastomeric bearing is contingent on sufficient friction being developed between the bearing and the top and bottom interface material. If the friction is inadequate the bearing will walk out over time as the bearing resets to vertical after each


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successive shearing movement. It is for this reason that the use of neat epoxy resin on the bearing support is not permitted.

Materials

For elastomer manufacture, natural rubber is specified in AASHTO LRFD, BS 5400 and AS 5100.

Synthetic rubbers such as neoprene and chlorobutyl are also used. Although the material is cheaper, it has some disadvantages compared to natural rubber such as poor ratio of elastic modulus to shear modulus, poorer performance and brittleness at low temperatures (neoprene). Chlorobutyl, however, has good abrasion resistance.

Selection of natural or synthetic rubber depends on the site conditions. The selected rubber should have good resistance to the actions of oils, weather, atmospheric ozone, and extreme temperatures. For example, when the lowest temperature is -10 C then natural rubber is selected, when the highest likely temperature is 60 C or when oil or grease may be present, chloroprene is a suitable choice (Lee 1994).

Beside steel reinforced bearings, cotton fabric reinforced (cotton duck) bearings and fibre reinforced bearings have been introduced (Gase & Kaczinski, 2006). The former are stiff against shear and rotation and can accommodate high compressive loads, thus are commonly used with a PTFE sliding surface, while the latter have not proven to offer an economical advantage over the steel reinforced bearings.

Common faults/defects

The following failure modes have been reported in the literature (Fyfe et al. 2006):

cracking of elastomer

excessive bulging or splitting under vertical loads

slippage and irregular contact of the bearing surfaces

migration of bearings from their original as placed seat

changes in rubber properties with aging

ozone cracking.

Recommendations

The following summary of the performance aspects of elastomeric bearings was presented in Austroads (2001):

Although elastomeric bearings have performed well, provisions for jacking and re-positioning should be made due to the long-term creep and shrinkage shortening.

Elastomeric bearings are the first choice of bearings for most small to medium span bridges.

These bearings are not recommended when small loads are coupled with considerable longitudinal movements, for instance, for end spans of continuous bridges.


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Construction issues

In the installation of elastomeric bearings the following issues need to be addressed:

The mortar pad must be planar to ensure uniform bearing and the plan dimensions of the mortar pad must exceed the plan dimension of the bearing to ensure a clearance of at least 25 mm. This is to ensure friction is developed to prevent curling of the edges of the bearing that may lead to tearing of the elastomer.

The safety and stability of tall bearings. The use of tall LEBs to support individual girders presents a stability issue and therefore a safety issue during construction as they may be unstable in terms of rotation and shearing until the in situ deck is constructed. To address this issue, temporary restraints must be provided to prevent the girders sliding off the bearings or rotating and falling. The temporary supports must be fixed before the girder is released from the lifting mechanism.

Effect of crossfall

If girders are placed normal to the deck crossfall the deflection of the girder will have a vertical and a horizontal component. The horizontal displacement may cause the centre of gravity to shift to the point where the girder becomes unstable. The design of the temporary bracing should take this into account, if applicable.

Thermal effects

The stability of bearings and girders can also be affected by thermal effects. Solar heating of one side of a girder may cause the member to bow with the possibility of the centre of gravity shifting horizontally to the point where the girder becomes unstable. To address this issue, temporary restraints must be provided to prevent the girders sliding off the bearings or rotating and falling. The temporary supports must be fixed before the girder is released from the lifting mechanism.

When the member is placed on the bearing a close inspection should be carried out to ensure uniform seating. Rotation of the member under the subsequent dead load of the deck concrete will correct the situation to some degree. However, if a significant gap exists between the bearing and the member, remedial measures will need to be taken. This may involve correcting the levels on the mortar pad.

Laminated bearings under load result in a minor rippling of the vertical surface. Excessive rippling on one side of a bearing compared to the other indicates unequal loading and may lead to splitting of the protruding rubber. Remedial action may be required to address the issue.

Uneven loading of elastomeric bearings results in differential compressive strains and may require remedial action.

2.2.2 Pot Bearings General features

Developed around 50 years ago, pot bearings have been used extensively for large bearings and have load capacities from 1200 to 10 000 kN. They consist of a cylindrical pad of rubber contained in a steel pot attached to, or integral with, a top plate. A close fitting steel piston attached to, or integral with, a bottom plate bears on the rubber pad. The pressure in pot bearings is in the range of 40-50 MPa.


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When rubber is confined in this way under pressure it acts as a fluid and as a result the top plate of the bearing can rock on the pad in any direction. The rubber pad has a number of circumferential bronze sealing rings that are vulcanised or recessed into the top of the rubber (Figure 2.3).


Figure 2.3: Pot bearing

The purpose of the rings is to prevent the extrusion of the rubber from the pot. Instances have occurred where the rubber pad has extruded out of the pot as a result of either excessive clearance between the rings and the internal diameter of the pot or an insufficient number of sealing rings (Figure 2.4).


Figure 2.4: Extrusion of rubber from pot bearing due to broken seal

When load is applied to the bearing, the rubber pad expands and the sealing rings bear against the vertical face of the pot creating the seal. The maximum permitted clearance between the pot and the piston is 1 mm. Pot bearings are much thinner in height compared to bearings used in the past and are ideal as replacement bearings on older bridges.


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Pot bearings are able to support large compressive loads, but the elastomer may leak and the sealing rings may suffer wear or damage (Roeder et al. 1995).

Fixed pot bearings

For fixed pot bearings the bottom plate is fixed to the superstructure by bolting the pot section to a plate cast into the member. Similarly the bottom plate is fixed to the headstock by bolting the piston section to a plate cast into the member. In each case the clearance between the cast-in plates must be sufficient to allow the bolts to be removed to facilitate any future replacement of the bearing (Figure 2.5).


Figure 2.5: Fixed pot bearing

Expansion pot bearings

In expansion pot bearings a thin disc of PTFE is placed into a recess at the top of the pot. A third plate bolted to the superstructure is faced on the bottom with a sheet of polished stainless steel that can slide back and forth on the PTFE disc. As the PTFE has very low friction (coefficient of friction 0.01-0.015) under load, this type of bearing offers very little resistance to the movements of the superstructure. Current standards require that the PTFE discs be dimpled to provide a reservoir for a lubricant applied during manufacture.

The range of sliding movement of a bearing is only limited by the length of the top sliding plate. Similar to fixed bearings, the bottom plate of expansion bearings is fixed to the superstructure by bolting the pot section to a plate cast into the member. The bottom plate is fixed to the headstock by bolting the piston section to a plate cast into the member. In each case the clearance between the cast-in plates must be sufficient to allow the bolts to be removed to facilitate any future replacement of the bearing (Figure 2.6).

Expansion pot bearings are available in two forms the free sliding/free floating bearing (Figure 2.6) or the glided sliding bearing (Figure 2.7).


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Figure 2.6: Free floating pot bearing


Figure 2.7: Pot glided bearing

The free sliding bearing is used where the designer wants the bridge to be able to slide in all directions. The guided sliding bearing is used where lateral restraint of the superstructure is required and also to guide the direction of movement.

The guides are bolted to the sliding plate. In instances where future lateral movement of piers is anticipated e.g. mining subsidence, provision can be made for the replacement of the guides with wider or narrower guide plates to suit the situation.

A combination of a free sliding bearing and a guided sliding bearing is often specified on curved bridges to ensure the bridge is provided with free movement.


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Usage

Pot bearings are used to sustain vertical forces in the range of 1200 kN to 10 000 kN and can accommodate rotations of up to 0.02 rad (Chen & Duan 2003), thus they can be used for a wide range of modern bridges. Low load eccentricity under rotation is one of their strong features. However, they have limited ability to accept rotation at low vertical load.

Common faults/defects

The following faults of pot bearings have been reported (Majumdar & Datta 2006, Fyfe et al. 2006):

leakage of elastomer due to broken seals

abrasion or wear of the elastomeric pad

breakdown of protective system: corrosion of metal components.

Some possible causes of the failures were reported in Majumdar and Datta (2006) and Shiau et al. (2008):

Poor design of components, which leaves little spaces for ease of installation, inspection and maintenance.

Use of inappropriate materials for bearing accessories, such as indicator, scale meter, and sealing rings.

Poor quality corrosive protection layer in fabricated steel bearings.

Excessive rotation, rotation cycles or lateral load.

Inadequate lubrication of the elastomer pad.

Materials

Materials used for pot bearing components include:

Metal components can be made of fabricated steel or stainless steel. For instance, AASHTO LRFD specifies that steel used for pot bearings be structural steel ASTM A 709M, grade 250, 345 or 345W or stainless steel ASTM A 240M.

Sliding surfaces (in glided and free floating pot bearings) usually comprise a PTFE disc and a stainless steel surface. The PTFE is lubricated.

Elastomer pads: according to AASHTO LRFD, elastomeric disc shall be made from a compound based on virgin natural rubber or virgin neoprene with nominal hardness of the range 50-60 on the Shore A scale.

Construction issues

Pot bearings are dispatched with transit bolts to ensure the bearing remains intact in transit. The bolts must be removed once the bearing is in place.

2.2.3 Mechanical Bearings Mechanical bearings are usually made of mild steel or cast iron and include the following bearing types:

Steel plate bearings comprise upper and lower steel plates to slide on the contact surface. The upper plate may be slightly curved. It may become a fixed steel plate bearing if a pin is included. These bearings can be used for spans of less than 15 m (Figure 2.8 a).


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Rocker bearings comprise a curved surface in contact with a flat or another curved surface and constrained to prevent relative horizontal movement. The curve surface may be cylindrical or spherical. The rotation is allowed by rolling of one part on another (Figure 2.8 b, c).

Roller bearings consist of one or more steel cylinders between parallel upper and lower steel plates. Single roller bearings can accommodate rotational movements about the rollers axis and translational movements in the perpendicular direction with the axis. Multiple roller bearings can only provide translational movement, but can also permit rotation by adding another element such as a rocker or knuckle bearing. These bearings can be seen in large span steel truss bridges (Figure 2.8 d, e, f).

Knuckle bearings are another type of rocker bearing. They include two or more members with mating curved surfaces, which may be spherical or cylindrical. Another form is the pin type, where the upper and the lower plates have a concave cylindrical surface mating with a core pin. Knuckle bearings permit rotation by rolling on the contact surface (Figure 2.8 g, h, i).

Leaf bearings consist of a pin passing through a number of interleaved plates fixed alternatively to the upper and lower outer bearing plates. Pin bearings permit only rotational movement, but can be used in combination with roller bearings to

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