Publication No. FHWA-IF-11-016

January 2011

Framework for Improving 

Resilience of Bridge Design

U.S.Department of Transportation

Federal Highway Administration
Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation in 

the interest of information exchange. The U.S. Government assumes no liability for use of the 

information contained in this document. This report does not constitute a standard, specification, 

or regulation.

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The Federal Highway Administration provides high-quality information to serve Government, 

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continuous quality improvement.
Framework for Improving Resilience of 

Bridge Design

Report No. FHWA-IF-11-016

January 2011

i 

Technical Report Documentation Page 

1. Report No.

FHWA-IF-11-016

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Framework for Improving Resilience of Bridge Design

5. Report Date

January 2011

6. Performing Organization Code

7. Author(s)

Brandon W. Chavel and John M. Yadlosky

8. Performing Organization Report No. 

9. Performing Organization Name and Address

HDR Engineering, Inc.

11 Stanwix Street, Suite 800

Pittsburgh, Pennsylvania 15222

10. Work Unit No.

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

Office of Bridge Technology

Federal Highway Administration

1200 New Jersey Avenue, SE

Washington, D.C. 20590

13. Type of Report and Period Covered

Technical Report

September 2007 – January 2011

14. Sponsoring Agency Code

15. Supplementary Notes

Contracting Officer’s Technical Representative (COTR): Raj Ailaney, HIBT-10

16. Abstract

 Bridges are an integral and important part of the highway infrastructure system and need to be designed to provide the 

necessary safety for the traveling public. Bridge failures can result in the disruption of commerce and services, significant repair 

costs, and most importantly the loss of human life. Performing a failure analysis during design, coupled with the review of past 

bridge failures, can help to avoid the need to initiate investigations and perform forensic engineering after a failure. This is the 

motivation for the development of this Framework.

 The development of this Framework considers bridge failures that resulted in collapse, service closures, major repairs, or 

other significant issues that occurred while the bridge was in service or during construction. A fault tree methodology is adopted 

in the Framework, where lessons from past bridge failures are used extensively to identify potential events that could lead to a 

bridge failure. A bridge designer, conscientiously or unconscientiously, goes through a fault-tree analysis mentally while 

ensuring that the design is devoid of weaknesses that could lead to bridge malfunction or failure. 

 This Framework can provide a starting point for the less than senior engineer to jump start a conscience evaluation 

process, and is expected to be of interest to students and instructors of bridge engineering, bridge owners, bridge designers, 

inspectors, fabricators, contractors, and maintenance personnel.

17. Key Words

Bridge Failure, Lessons Learned, Fault Tree, Bridge 

Design, Bridge Construction

18. Distribution Statement

No restrictions. This document is available to the public through 

the National Technical Information Service, Springfield, VA 

22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No of Pages 22. Price

 Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized 
ii
iii

Framework for Improving Resilience of Bridge Design

Table of Contents

FOREWORD.................................................................................................................................. 1

ACKNOWLEDGMENTS .............................................................................................................. 2

1.0 INTRODUCTION ................................................................................................................. 3

1.1 Definition of Failure ........................................................................................................ 4

1.2 Redundancy...................................................................................................................... 4

1.3 Bridge Inspection and Maintenance................................................................................. 5

1.4 Design Errors and Omissions........................................................................................... 6

1.5 Quality Control and Quality Assurance Programs........................................................... 6

1.6 Fault Tree Diagram.......................................................................................................... 7

2.0 Steel Girder Bridge Failure Framework ................................................................................ 9

2.1 Design/Operation Category ........................................................................................... 10

2.1.1 Superstructure – Girders....................................................................................... 11

2.1.2 Superstructure – Cross Frames / Diaphragm ........................................................ 19

2.1.3 Superstructure – Bearings..................................................................................... 19

2.1.4 Superstructure – Concrete Deck/Railings............................................................. 21

2.2 Inspection Category ....................................................................................................... 22

2.3 Construction Category ................................................................................................... 23

2.3.1 Steel Erection........................................................................................................ 24

2.3.2 General Construction ............................................................................................ 26

2.4 Fabrication Process Category ........................................................................................ 28

3.0 Precast Concrete Girder Bridge Failure Framework ........................................................... 31

3.1 Design/Operation Category ........................................................................................... 32

3.1.1 Superstructure – Girders....................................................................................... 32

3.1.2 Superstructure - Bearings...................................................................................... 41

3.1.3 Superstructure - Concrete Deck/Railings.............................................................. 41

3.2 Inspection Category ....................................................................................................... 42

3.3 Construction Category ................................................................................................... 43

3.3.1 Girder Erection...................................................................................................... 44
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3.3.2 General Construction ............................................................................................ 44

3.4 Fabrication Process Category ........................................................................................ 44

4.0 Substructure Failure Framework.......................................................................................... 46

4.1 General........................................................................................................................... 47

4.2 Concrete Substructure.................................................................................................... 51

4.3 Steel Substructure .......................................................................................................... 51

5.0 Complex Bridge Failures – Lessons Learned ...................................................................... 53

5.1 Steel Truss Bridges........................................................................................................ 53

5.1.1 Quebec Bridge, 1907 ............................................................................................ 53

5.1.2 Yadkin River Bridge, 1975................................................................................... 54

5.1.3 Minneapolis I-35W Bridge, 2007 ......................................................................... 54

5.2 Suspension Bridges........................................................................................................ 56

5.2.1 Tacoma Narrows Bridge Collapse, 1940.............................................................. 56

5.2.2 Silver Bridge, 1967 ............................................................................................... 56

5.3 Cable-Stayed Bridges..................................................................................................... 57

6.0 Conclusions.......................................................................................................................... 58

7.0 References............................................................................................................................ 59
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List of Figures 

Figure 2-1 Steel girder bridge fault tree, showing top categories only which include Design 

and/or Operation, Inspection, Construction, and Fabrication. .................................... 9

Figure 2-2 Portion of the steel girder bridge fault tree showing the Design and/or Operation 

category for superstructures only, with several Events and Basic Events provided. 11

Figure 2-3 Photos showing the before and after of the elimination of trough-type drainage 

details by making simple steel spans continuous during a rehabilitation project. .... 12

Figure 2-4 Photo of fractured girder of the US 422 Bridge showing a crack in the web emanating 

for intersecting welds (taken from [17]) .................................................................... 15

Figure 2-5 Dunn Memorial Bridge Interchange Rocker Bearing Failure: (a) Sketch of the bridge 

elevation from Pier 9 to Pier 13; (b) Photo of the failed rocker bearings at Pier 11 

showing them tipped over (taken from [29]). ............................................................ 20

Figure 2-6 Photo of failed rocker bearing at Pier 10S of Birmingham Bridge Approach Span 

showing the rocker tipped over with girder sitting on top of failed bearing (taken 

from [30]). ................................................................................................................. 21

Figure 2-7 Portion of the steel girder bridge fault tree showing the Inspection Category only, 

with three Basic Events provided. ............................................................................. 22

Figure 2-8 Portion of the steel girder bridge fault tree showing the Construction Category only, 

with several Events and Basic Events provided. ....................................................... 23

Figure 2-9 Curved girders displace vertically and rotate out-of-plane causing the top and bottom 

flanges to displace out-of-plane unless the rotation is prevented by supports or 

bracing. ...................................................................................................................... 25

Figure 2-10 Photo showing the use of temporary supports during erection of a curved steel Igirder bridge. The use of temporary supports help to control the geometry of the 

structure during steel erection. .................................................................................. 26

Figure 2-11 Photo showing the cantilever forming brackets which will support formwork that 

will be used to construct the concrete deck overhang on the simple span steel girder 

superstructure. ........................................................................................................... 27

Figure 2-12 Portion of the steel girder bridge fault tree showing the Fabrication Category only, 

with four Basic Events provided. .............................................................................. 29

Figure 3-1 Precast Concrete Girder Bridge fault tree showing the top categories only which 

include Design and/or Operation, Inspection, Construction, and Fabrication. ......... 31

Figure 3-2 Portion of the precast concrete girder bridge fault tree showing the Design and/or 

Operation category for superstructures only, with several Events and Basic Events 

provided. .................................................................................................................... 33

Figure 3-3 Photo showing results of vehicle impact and subsequent corrosion damage to 

prestressing strands from a prestressed concrete box beam in Lake View Bridge over 

I-70 (taken from [43]) ................................................................................................ 34
vi

Figure 3-4 Sketch showing the elevation of de la Concorde overpass; the simple central span of 

the bridge consisted of adjacent box girders, which rested on a beam seat that was 

built into the thickened slab that cantilevered from the abutments at each end (taken 

from [52]). ................................................................................................................. 38

Figure 3-5 Photo of the bottom face of a tested beam subjected to fire while in service showing 

that the concrete has deteriorated such that sufficient pathways through the concrete 

to the bottom layer of prestressing strands have developed (taken from [56]). ........ 40

Figure 3-6 Photo showing the spans of the US-90 bridge completely unseated and submerged in 

the water due to storm surge induced loading (taken from [57]). ............................. 41

Figure 3-7 Portion of the precast concrete girder bridge fault tree showing the Inspection 

Category only, with three Basic Events provided. .................................................... 42

Figure 3-8 Portion of the precast concrete girder bridge fault tree showing the Construction 

Category only, with several Events and Basic Events provided. .............................. 43

Figure 3-9 Portion of the precast concrete girder bridge fault tree showing the Fabrication 

Category only, with several Basic Events provided.................................................. 45

Figure 4-1 General substructure fault tree with main categories of Concrete Substructures, Steel 

Substructures and General Substructures, and several basic events shown for each. 46

Figure 4-2 Sketch showing potential scour locations for a general bridge crossing over a river in 

which the concrete supports are located within the limits of the waterway.............. 47

List of Tables

Table 1-1 Symbols used in typical fault tree diagrams, describing Events, Basic Events, Or Gates 

and And Gates. ............................................................................................................ 7
1

FOREWORD 

The motivation for developing the “Framework for Improving Resilience of Bridge Design” is to 

perform failure analysis during design to avoid the need to initiate failure investigation and 

perform forensic engineering after an in-service failure. Lessons from past bridge failures are 

used extensively in the fault-tree analysis to identify potential events that could lead to a bridge 

failure. A bridge designer goes through a fault-tree analysis mentally in making sure that the 

design is devoid of weaknesses or hot-spots that could lead to bridge malfunction or failure. 

This is generally adequate for the smaller, simpler and more common types of bridges. For the 

more complex bridges, it is desirable to perform a fault-tree analysis to systematically determine 

all contributing factors or events that could lead to a bridge failure. The contributing factors or 

events can then be considered and carefully addressed in the design by the bridge designer. The 

fault-tree methodology is also conducive to using the collective knowledge, experience, and 

skills of engineering professionals in a group environment to develop or review a fault-tree 

analysis of a more complex design and take steps to enhance the resilience of bridge design for 

safety, quality, and economy. 

Illustrative examples of fault-tree analysis are given in the “Framework for Improving Resilience 

of Bridge Design” for several types of bridges, including superstructures and substructures. The 

illustrative examples provide the tools for identifying potential failure modes and give 

suggestions for giving due considerations in the design to prevent such potential failures and to 

improve the resilience of bridge design. 

The bridge designer has the most influence in the quality and performance of his/her design. 

However, the bridge designer needs the cooperative and collaborative efforts of the fabricators, 

contractors, inspectors, and maintenance personnel to fully meet the intent of quality and 

resilience of the design. 

The Framework is expected to be of interest and use to students and instructors of bridge 

engineering, bridge owners, bridge designers, inspectors, fabricators, contractors, and 

maintenance personnel. 

The constructive review comments on the final draft from many engineering professionals are 

very much appreciated. The readers are encouraged to submit comments for enhancements of 

future edition of the Framework to Myint Lwin at the following address: Federal Highway 

Administration, 1200 New Jersey Avenue, S.E., Washington, DC 20590. 

M. Myint Lwin, Director 

Office of Bridge Technology
2

ACKNOWLEDGMENTS 

The authors would like to acknowledge the encouragement and guidance provided by Myint 

Lwin, the Director of the Office of Bridge Technology, and Raj Ailaney, the Contract Manager 

of the Office of Bridge Technology during the development of the Framework. The authors also 

would like to thank the engineering professionals that provided very useful review comments on 

the final draft of the Framework. 
3

1.0 INTRODUCTION 

Bridge failures can result in the disruption of commerce and services, significant repair costs, 

and most importantly the loss of human life. Bridges rarely experience complete failure during 

non-extreme events, however when such failures do occur, the results can be catastrophic: 

The collapse of Schoharie Creek Bridge in 1987 resulted in the loss of 10 lives.

The collapse of a curved I-girder ramp under construction in 2005 resulted in the death of 

a construction worker. 

The August 2007 collapse of the I-35W Bridge in Minneapolis, Minnesota killed 13 

people and injured 145 others. 

Bridges are an integral and important part of the highway infrastructure of the Nation and need to 

be designed to provide the necessary safety for the traveling public. The review of past bridge 

failures allows bridge designers to apply lessons learned to new design projects and to the 

preservation of existing structures which will help prevent future failures. 

The intent of this document is to provide a framework that can be employed by bridge designers

during the design process that can help to minimize bridge failures while in service and/or during 

construction. Use of this methodology is not a panacea for the prevention of bridge failures, but 

is intended to provide bridge designers with a tool for identifying potential failure mechanisms 

and design accordingly to prevent failures. Addressing potential failures during design is much 

less costly and painful than failure investigation and forensic engineering carried out after an 

actual failure event. This is accomplished by highlighting design considerations that could 

reduce failures that might not be readily apparent in current design specifications. The 

framework developed within this document will incorporate a review of past bridge failures 

either during construction or in service to identify and assess the causes and determine what 

bridge designers can do to reduce the likelihood of future failures. The main purpose of this 

framework is to advise bridge designers to “think outside the box” and potentially go beyond 

governing design specifications during the design process. Doing this addresses potential 

failures that a particular bridge design may be susceptible to, and make provisions during design 

to prevent such failures. It should be noted that not all of the potential failure scenarios 

described in this document can be designed for/investigated during the design phase for every 

bridge, but designers should be aware of these failure scenarios for general knowledge purposes. 

These failure scenarios deserve further study to make sure they are adequately addressed in 

design. 

The development of this framework considers bridge failures that resulted in collapse, service 

closures, major repairs, or other significant issues that occurred while the bridge was in service 

or during construction, which could have been mitigated in the design process; similar to a 

lessons-learned approach. For each bridge type considered in this document, a fault tree is 

developed to describe the design issue/characteristic that may lead to a failure. Although fault 

trees can be used to numerically investigate the probability of failure, the use of fault trees in this 

framework is intended solely as a visual representation of potential bridge failure scenarios. 

Through these fault trees, the framework addresses design provisions and/or methods that can be 

employed to mitigate specific issues/characteristics that can lead to failures. Suggested 

guidelines and methods are developed from the current AASHTO LRFD Bridge Design 

Specifications, as well as from past specifications, the archival literature, and “rules of thumb.” 
4

The primary intended audience for this document is relatively less experienced bridge designers 

faced with the design of any number of bridge types, and are not familiar with past bridge 

failures that could have been prevented. However, the broader audience is truly all bridge 

professionals; the industry as a whole can benefit from the sharing of past bridge failure causes,

which can aid in the prevention of future bridge failures. Furthermore, this document does not 

fully cover each type of failure with an in-depth discussion. The intent of this document is to 

provide the designer with a “first line of defense,” and if the reader needs further information, 

references are cited. Potential failure scenarios are highlighted throughout this document, 

however it is up to the designer to determine whether or not the failure scenario needs to be 

investigated further as it relates to the particular bridge being designed. 

1.1 Definition of Failure 

Failure is defined herein as the inability of a bridge or one of its primary load-carrying 

components to no longer perform its intended function. For bridges under construction or in 

service, this framework considers the term failure in two different contexts: 1) collapse and 2) 

critical defect. Herein, a bridge collapse is the failure of all or a substantial part of the bridge 

where full or partial replacement may be required. The term critical defect refers to the 

condition in which the structure has undergone some deformation, section loss, or similar 

undesirable condition, but has not collapsed and can be repaired or retrofitted. Additionally, 

delays during construction and/or fabrication can be considered as a critical defect in the overall 

bridge construction process.

Failures can be caused by one, or a combination of the following (not inclusive): errors in 

design, detailing, or construction; unanticipated affects of stress concentrations; lack of proper 

maintenance; the use of improper materials or foundation type; or the insufficient consideration 

of an extreme event. Design and detailing errors, omissions, or flaws could lead to failure of a 

structure. Also, there could be a loss of the original design intent during the detailing process. A 

construction failure can occur due to the incorrect installation of structural members or 

temporary supports. A deviation from the approved construction procedures may also result in a 

failure. The lack of proper maintenance can result in corrosion leading to deficient members. 

The use of incorrect materials, concrete deck admixtures for example can also result in a type of 

failure. However, it has been shown through various studies [1]

1

that a bridge failure is most 

likely to be caused by an extreme event, with the most prevalent type being flooding and scour. 

1.2 Redundancy 

Redundancy is typically defined as the ability of the bridge system to sustain damage without 

collapse. In a non-redundant bridge system, the failure of any one critical member may result in 

the collapse of all or a portion of the bridge system. In a redundant system, two or more 

components must fail before the bridge system collapses. There are three types of redundancy 

that can exist for bridges: 

Internal Redundancy - Internal redundancy relates to the fact that the failure of one 

element of a member will not result in the failure of other elements of the members. For 

 

1

 Numbers in brackets refer to References at the end of this document. 
5

example, riveted plate girders and multiple eyebar truss members have internal 

redundancy. In a riveted plate girder, if a crack begins in one of the elements, it will not 

propagate directly into adjacent elements. Welded plate girders and rolled sections do 

not have internal redundancy. 

Structural Redundancy - Structural redundancy refers to the redundancy that exists as a 

result of the continuity within the framing element. A statically indeterminate structure, 

such as a continuous beam, could be classified as being structurally redundant. 

Load Path Redundancy - Load path redundancy is related to the ability of the structure 

carrying load following the loss of a single member. A bridge such as a two-girder 

superstructure is classified as non-redundant because it does not have any alternative load 

paths. The failure of a single girder in a two-girder bridge could result in failure of the 

entire bridge system. Another example is a single column pier.

Redundancy has a significant role in the prevention of bridge failures. A non-redundant bridge is 

more susceptible to a failure since it is more likely to have a reduced number of members or no 

alternate load path. At the present time, the AASHTO LRFD Bridge Design Specifications [2] 

only recognize load path redundancy as described above. A redundant structural system is 

defined in the AASHTO LRFD Bridge Design Specifications as a system of element and 

components whose failure is not expected to cause collapse of the bridge. A redundant system 

can receive some benefit in the AASHTO LRFD Bridge Design Specifications [2] with the use of 

a load modification factor ( ) to account for redundancy, if the designer and/or owner feel this is 

warranted. Good and cost-effective design practice incorporates as much redundancy as can be 

justified economically, sometimes going beyond the minimum design requirements. 

Additional information regarding redundancy in highway bridge superstructures can be found in 

NCHRP Report 406 [3]; and likewise for highway bridge substructures in NCHRP Report 458 

[4]. 

1.3 Bridge Inspection and Maintenance 

In accordance with the National Bridge Inspection Standards (NBIS), every bridge, in general, is 

to be inspected at regular intervals not to exceed twenty-four months [5]. Regularly scheduled 

inspections enable bridge owners to routinely recognize the general conditions of the bridge and 

help to detect any potential problems that could lead to a failure. Bridge inspectors must meet 

defined qualifications and recognize the importance of observations made during an inspection. 

These qualifications are specified through the NBIS program, but may vary if more rigorous 

requirements have been implemented at the State level. Additionally, the bridge and the details 

themselves must be readily inspectable. The bridge designer must consider whether or not the 

details that have been developed can be inspected with relative ease. Obviously, some design 

details can not be inspected by typical means and methods. For example, prestressing tendons in 

a prestressed concrete girder, or internal pistons of a pot bearing, are virtually impossible to fully 

inspect. Post-earthquake inspection of some foundations would require earth moving equipment 

for inspection. 

Bridges must also be maintained. A good maintenance program will help to reduce the potential 

for deterioration that leads to a bridge failure. Steel bridges often require cleaning and repairs 
6

associated with corrosion. Likewise, concrete bridges often require concrete repairs related to 

concrete degradation and reinforcement corrosion. If bridge maintenance is not routinely 

performed, deteriorated areas in need of repair will increase, resulting in the increased potential 

for a bridge failure. 

1.4 Design Errors and Omissions 

Errors and omissions during the design of a bridge can have serious consequences. The designer

must provide calculations that are performed in accordance with the bridge owner’s governing 

design specifications and standards and meet the appropriate standard of care. The structural 

details, such as shear connectors, reinforcing bars, or gusset plates, must be suitable and meet all 

design requirements. The bridge design plans must be developed in accordance with the design 

calculations performed by the bridge designer. In addition, the correct materials must be 

specified in the contract plans, and in accordance with the bridge owner’s specifications. 

Failure in any of these aforementioned tasks will result in, at the very least, an unsuccessful 

bridge process, or at worst, a failure of the bridge itself. The framework developed herein does 

not necessarily consider the effects of an incorrect design for each specific bridge type, since 

design errors and omissions can occur in many places during the design process. A well 

established QC/QA program can help to reduce errors and omissions. 

1.5 Quality Control and Quality Assurance Programs 

The Bridge Owner plays the most important role in the quality and success of a project. The 

Bridge Owner must clearly establish the requirements and expectations of a project. These 

requirements and expectations must be communicated and understood by the bridge designer and 

the contractor. The owner, the designer and contractor are then expected to work together to 

meet the requirements and expectations. 

Quality Control/Quality Assurance (QC/QA) programs are formal office or organizational 

procedures or practices for ensuring that the owner’s requirements and expectations are fully 

met. A QC/QA program provides checks and balances within an organization to assure quality 

in the final products. QC/QA programs are implemented at different levels or phases of work 

activities. For example, in the design phase, the bridge designer is responsible for making sure 

his/her calculations and drawings are accurate and meeting the requirements of the design. The 

bridge designer is performing QC of his/her own work by establishing a procedure for selfchecking the work for accuracy and correctness. On the other hand, the reviewer, practicing QA, 

is responsible for independently checking the work of the bridge designer to assure accuracy and 

correctness in meeting the design requirements and expectations of the bridge designer. In 

construction, QC is the responsibility of contractor to ensure that the quality of the work is 

carried out in compliance with the contract provisions. On the other hand, the owner is 

responsible for practicing QA to assure that the contractor is carrying out the work in accordance 

with the contract. 

A good QC/QA program is a deliberate and systematic approach to reduce the risk of introducing 

errors and omission into a design. The likelihood of a failure in any design process is increased 
7

if standardized procedures are not established and followed. In some cases, the root cause of a 

bridge failure can be traced back to a failure to create or follow a good QC/QA program. The 

implementation and adherence to a good QC/QA program will likely reduce the possibility of 

failure of the overall bridge design process. 

For major bridge projects involving unusual, complex, and/or innovative features, a peer review 

may be desirable to raise the level of confidence in the quality of design and construction. A 

peer review is generally a high-level review by a special panel of professionals specifically 

appointed by the Bridge Owner to meet the needs of the project. Peer review is an effective way 

to improve quality and to reduce the risk of errors and omissions. 

1.6 Fault Tree Diagram 

A fault tree diagram is a graphic model that shows parallel and sequential failure paths that can 

lead to an undesirable outcome: in this case a bridge failure. The fault tree diagram is helpful in 

determining potential failure modes and their interactions in a complex system such as a bridge. 

A fault tree diagram is developed in a top-down direction. In this application the top event is the 

failure of the bridge. The events immediately beneath the top event lead to the execution of the 

top event. Successor events and conditions that most directly lead to the predecessor events are 

then determined. This process is repeated at each successive level of the fault tree until the 

diagram is complete. Table 1-1 provides an explanation of the symbols used in the fault tree 

diagrams contained in this document. Specific information regarding the use of fault trees in 

engineering type applications can be found in Haasl et al. [6]. 

Table 1-1 Symbols used in typical fault tree diagrams, describing Events, Basic Events, Or 

Gates and And Gates. 

Utilizing a fault tree diagram, a bridge is modeled to demonstrate/determine the critical failure 

paths. The fault tree diagram illustrates the structural component interactions, redundancy, 

actions/causes such as corrosion or fatigue, and environmental impacts such as flooding or scour. 
8

The failure paths depicted in the fault tree diagrams are intended to provide bridge designers 

with a means to improve bridge designs and prevent future failures. If bridge designers 

understand critical failure modes related to the particular bridge being designed, they may be 

able to employ additional analyses or design calculations to investigate potential failures and 

determine strategies to assess and rectify issues not typically addressed during the design phase. 

The general fault trees presented in this document are qualitative; however these fault trees can 

be used in a quantitative sense as well. The vulnerability of a bridge can be determined by a 

numerical evaluation that employs the probability values of each basic event. Once the 

probabilities of the basic events are determined, Boolean algebra can be performed. This will 

result in the probability of failure of the top event (in this case, a bridge failure) and the relative 

importance ranking of each path of the fault tree. LeBeau and Wadia-Fascetti [7] discuss the use 

of Boolean algebra to determine the probability of failure of different failure path mechanisms 

for the collapse of the Schoharie Creek Bridge. Similarly, Daniels et al. [8] perform a 

quantitative vulnerability assessment of several steel bridges, employing a method analogous to a 

fault tree analysis. Both of these studies show that fault trees have both qualitative and 

quantitative advantages that could be employed by a bridge designer during the design process. 

The reader can refer to these studies for information regarding a numerical evaluation associated 

with fault trees in bridge design. 
9

2.0 STEEL GIRDER BRIDGE FAILURE FRAMEWORK 

A general fault tree for the case of a steel girder bridge failure is developed and presented in this 

section. As discussed earlier, failure in this framework refers to a total collapse of the bridge 

system or an event that renders the bridge unfit for service. In general, a fault tree tends to be 

project specific, such as the case of the failure tree developed by LeBeau and Wadia-Fascetti [7] 

for the collapse of the Schoharie Creek Bridge. However, for this framework, in an attempt to 

include all steel girder bridges, both I- and box-girders, the fault tree presented in Figures 2-1, 

2-2, 2-7, 2-8, and 2-12 are developed for a general case. The general fault tree developed below 

points out events that could cause a failure in the bridge, which could be addressed during the 

design of the structure. 

The fault tree developed for steel girder bridges assumes that the bridge is designed and 

constructed according to the governing specifications for normal design loads as well as required 

extreme events. It also assumes that regular inspections and maintenance are performed over the 

service life of the bridge. 

The fault tree is established with the top event, the Steel Girder Bridge Failure, as shown in 

Figure 2-1. The failure can develop from four different categories; Design/Operation, 

Inspection, Construction, or Fabrication. These four categories are joined by an Or Gate, which 

means any one of the four conditions can result in a failure. A bridge specific fault tree will be 

more refined than the general case provided in this framework. Also, not all of the 

aforementioned conditions will necessarily apply to the specific bridge in question, but the 

designer should be aware of all of the possible events on the fault tree. For example, there will 

be some construction and steel erection aspects that do not necessarily fall under the bridge 

designer’s control, but more so for an engineer working for a contractor, fabricator, or steel 

erector. These aspects are presented here so bridge designers understand and take into account 

the entire process of the design, fabrication, construction, and inspection of the bridge. 

Figure 2-1 Steel girder bridge fault tree, showing top categories only which include Design 

and/or Operation, Inspection, Construction, and Fabrication.

The Design/Operation category alludes to the fact that a failure, either a collapse or a critical 

defect, can occur while the bridge is in service. Inspection refers to the fact there may be a 
10

problem with the regular inspection resulting from a design that does not facilitate inspection of 

the bridge components. The Inspection category does not intend to encompass problems with 

the actual bridge inspection, but highlight issues that can be addressed during design that will 

facilitate bridge inspection. A failure of the bridge process can occur during the Construction of 

the steel girder bridge, whether it is a collapse or a problem that results in delays. The steel 

Fabrication process is also subject to errors and problems, which could result in a failure of the 

bridge process. These are all conditions that the bridge designer must be aware of, and give due 

consideration to, when designing a steel girder structure. Each of the four categories is 

developed into a more detailed fault tree. 

2.1 Design/Operation Category 

The fault tree that follows the Design/Operation category is shown in Figure 2-2. While in 

service, a bridge failure can result from either a failure of the superstructure or substructure. A 

detailed representation of the superstructure fault tree is shown in Figure 2-2, while the 

substructure is shown later in Figure 4-1. 

A failure of the steel girder superstructure can be caused by a failure in any one of the 

superstructure components; the most severe are those occurring in the girders, cross frames or 

diaphragms (in curved girder bridges), bearings, or concrete deck. Again, an Or Gate is used to 

join these fault scenarios, meaning that a failure of anyone of these components will cause a 

failure of the superstructure. A failure of the superstructure will then trigger a design/operational 

failure of the bridge. 
11

Figure 2-2 Portion of the steel girder bridge fault tree showing the Design and/or Operation 

category for superstructures only, with several Events and Basic Events provided.

2.1.1 Superstructure – Girders 

2.1.1.1 Corrosion / Adequate Drainage Details 

Girder failure can be caused by one or more events, as shown in the fault tree, Figure 2-2. For 

example, inappropriately detailed drainage components can often become clogged and not 
12

provide for the removal of water and deicing salts from the structure. Thus, deterioration 

(corrosion) caused by the combination of water and deicing salts can cause the failure of the 

main bridge girders. This is depicted in Figure 2-2 in the lower left corner of the figure. 

The importance of adequate drainage was evident in the collapse of the I-95 Mianus River 

Bridge in Greenwich, Connecticut, in 1983. The bridge, completed in 1958, used floor beams 

that framed into two main girders, and employed pin-and-hanger assemblies in the two main 

girders. Maintenance personnel testified that drains in the bridge were difficult to keep open 

because the scuppers and downspouts were too small and downspout slopes were too shallow 

and changed direction too abruptly [9]. Additionally, much of the down-spouting was 

inaccessible or difficult to repair. In a 1973 resurfacing project, the curb drains were covered 

with steel plates and asphalt, forcing the water, deicing salts, and debris to drain through the 

expansion joints. The troughs beneath the expansion joints, and just above the pin-and-hanger 

connections and bearings systems, became clogged with debris. 

The failure investigators determined that one of the pin-and-hanger assemblies was subjected to 

a significant amount of corrosion resulting from the clogged troughs and drainage modifications. 

It was concluded that corrosion packout of the pin-and-hanger assembly was primarily 

responsible for the lateral displacement of the hanger on the pin, and subsequent collapse of the 

particular suspended span of the bridge [9, 10].

It is important to locate drainage scuppers and connecting elements in regions of the 

superstructure where, if they become clogged with debris, they will cause the least damage. It is 

also important that the down-spouting be accessible for inspection, maintenance, and repair, and 

has the necessary slopes and connections to prevent clogging. In addition, it may be 

advantageous to make the steel superstructure continuous and eliminate expansion joints, thus 

eliminating drainage details that cause problems. An example of where simple spans are made 

continuous, to eliminate poor drainage details is shown in Figure 2-3. Adequate drainage and the 

layout of the drainage elements will help to prevent girder corrosion. 

a) Before Rehabilitation b) After Rehabilitation

Figure 2-3 Photos showing the before and after of the elimination of trough-type 

drainage details by making simple steel spans continuous during a rehabilitation 

project.
13

In addition, girders that are part of an overpass structure are prone to deicing salt-spray. Once 

deicing salts are applied on the roadway below, traffic on that roadway will cause the salt and 

water combination to become airborne, with some of the deicing salt/water solution splashing 

onto the steel bridge girders overhead. If the girders in these splash zones are not adequately 

protected against corrosion, such as protection through proper painting methods, the girders will 

deteriorate, leaving the bridge system vulnerable to critical defects. 

2.1.1.2 Fatigue and Fracture 

There have been several reported steel girder bridge failures caused by either fatigue or fracture 

of the girder steel. A significant amount of research has been performed regarding fatigue over 

the past 50 years. This research has led to the categorization of various bridge details in the 

current design specification that can be used by the bridge designer to investigate fatigue in the 

design process. The AASHTO LRFD Bridge Design Specifications [2] has separated fatigue into 

two categories: load induced fatigue and distortion induced fatigue. In general, when provisions 

concerning fatigue sensitive details are followed, the potential failure of a bridge due to fatigue is 

significantly reduced. However, it should be noted that fatigue cracking due to out-of-plane 

distortions of the girder web, at the diaphragm connection plates of multiple girder bridges, has 

resulted in the largest number of cases of fatigue cracking in steel bridges. 

Distortion-induced fatigue is addressed in the current specifications, but not in as much detail as 

load-induced fatigue. Distortion-induced fatigue crack growth generally results from small 

deformations, usually out-of-plane, in localized areas, and may not be readily apparent during the 

design process. Fisher et al. [11] provide several examples of details that can be susceptible to 

distortion-induced fatigue: 

Transverse stiffeners cut short of the tension flange.

Rotation of a floor beam attached to the main girder web. 

Web gaps in multi-girder bridges at the diaphragm connections. 

Web gaps in box girder bridges at the internal diaphragms.

Lateral bracing connections. 

Distortion-induced fatigue details must be given consideration by bridge designers. For 

additional information regarding fatigue, other than the current specification, the reader is 

referred to Demers and Fisher [10], Fisher et al. [11], Fisher [12], and Fisher et al. [13]. 

A brittle fracture can result in the failure of the steel girder, causing a collapse of the entire 

system or a critical defect that must be repaired. Steel girder fractures can occur with no 

warning. Sometimes the details that lead to a fracture are not easy to inspect. Compared to 

fatigue cracking, the number of brittle fractures in highway bridges has been relatively small 

over the past 40 years [14]. 

Several brittle fracture failures have been noted in the archival literature: 

In January of 1977, a fracture was discovered through the welded bottom flange and web 

splice of an I-girder on the I-79 Ramp Bridge crossing the back channel of the Ohio River 

near Pittsburgh, Pennsylvania. The fracture was brittle with little or no apparent plastic 

deformation of the steel [15]. The crack in the 30 in. wide by 3.5 in. thick bottom flange 
14

opened approximately 1.75 in., and extended upward through the full depth of the 132 in. 

by 0.5 in. web plate. The crack terminated at the underside of the top flange. The 

fractured girder was a main girder of a three-span, two-girder-floorbeam type bridge 

system. The crack initiated in a butt weld detail that was prepared using an early version 

of the electroslag welding process. At the time, the electroslag welding process had been 

used in building design but not in bridge design. The I-79 Bridge was one of the first 

bridges to employ the electroslag weld process. Soon after this failure, the FHWA 

prohibited the use of electroslag welding in bridge members subjected to tension. 

However, in March 2000, the FHWA issued a memorandum allowing the use of NarrowGap Improved Electroslag Welding (NGI-ESW) that can be used as an alternative 

welding procedure in bridge structures [16].

In 1975 the Lafayette Street Bridge in St. Paul Minnesota had a fracture in one of the 

main girders near the center span inflection point. The Lafayette Street Bridge was also a 

three-span, two-girder-floorbeam type bridge system. This structure, similar to the I-79 

structure, was capable of carrying traffic after the fractures and without collapse. This 

was attributed to the bridge system behaving like a torsionally stiff box [11]. 

Redundancy was provided by the continuity of the three span structure, as well as the 

lateral bracing, floorbeams/stringers, and concrete deck and shear connectors, even 

though the bridge was a two-girder system. 

In December 2000, the Hoan Bridge, in Milwaukee, Wisconsin, experienced a brittle 

fracture in all three girders in a cross section of the three-span continuous plate girder 

south approach unit. Two of the girders had full depth fractures. The lateral gusset plates 

were slotted to fit around the transverse connection plates. It was determined that all 

three girder web fractures initiated from the geometric restraint and resulting triaxial 

stress at the intersection of the lateral bracing gusset plate and transverse connection plate 

welded connection with both intersecting and overlapping welds [17]. 

In 2003, a fracture was found during a routine inspection on one of the main girders of 

the US 422 Bridge over the Schuylkill River in Pottstown, Pennsylvania. The bridge is a 

six-span two-girder steel structure, and employed a lateral bracing system near the 

bottom flange. The fracture, shown in Figure 2-4, was located in a positive moment 

region, and initiated at the intersecting welds connecting the web, lateral bracing gusset 

plate, and transverse connection plate approximately 3 in. above the bottom flange [17]. 

The fracture propagated upward in the web, above the gusset plate, and also propagated 

downward and completely severed the bottom flange. 
15

Figure 2-4 Photo of fractured girder of the US 422 Bridge showing a crack in the web 

emanating for intersecting welds (taken from [17])

The US 422 Bridge and Hoan Bridge fractures resulted from constraint-induced fracture, CIF 

[17]. Conner et al. [17] states that three conditions must be present for a detail to be vulnerable 

to CIF. 

1. There must be an area of high stress concentration that locally magnifies the stress level 

occurring in the web plate. 

2. The local stress concentration must occur in an area where the plate is under high 

constraint, preventing local yielding. 

3. There must be sufficiently high tensile stresses present at the detail, including dead load, 

live load, and residual stresses. 

Furthermore, Connor et al. [17] provide prevention strategies that can be employed during design 

to mitigate constraint-induced fracture. 

During the design process, and more so for a non-redundant system, designers may need to 

investigate the potential failure of a steel girder due to a brittle fracture in a flange and/or web. 

Finite element programs and computer simulations can be used to investigate the loss of 

continuity in a bottom flange for example. Alternatively, when possible, connections should be 

detailed and material specified appropriately to avoid fracture critical details and/or members. 

Fracture toughness requirements are specified in the AASHTO LRFD Bridge Design 

Specifications, Article 6.6.2 [2] for given temperature zones (specifically Table 6.6.2-2). All 

main components and connections subjected to tensile forces, including those subject to a 

reversal of stress, are subject to mandatory Charpy V-notch fracture toughness testing and should 

be specified as such on the design plans. Another primary reference for designers regarding 

fracture in steel structures is by Rolfe and Barsom [18]. 

Also, per the AASHTO LRFD Bridge Design Specifications, Article 6.6.2 [2], the bridge 

designer is responsible for determining if any steel bridge component is a fracture critical 

member. The designer is also responsible for clearly specifying those fracture critical members 

on the design plans. Fracture critical members are defined as a pure tension member or tension 

components of a bending member in which the failure of that member would result in a collapse 

of the bridge, i.e., no alternate load path. 
16

2.1.1.3 Overload 

An overload, such as an illegal non-permitted load or an overload during construction, could 

cause the steel girder to fail, leading to a failure of the bridge. All bridges are designed for a 

given design load and possibly permit load, in accordance with current AASHTO LRFD Bridge 

Design Specifications and as modified by State bridge design specifications. Pedestrian bridges 

need to be designed for pedestrian loads, and some vehicle loads, depending upon the owner’s 

requirements. In most cases, there is an inability to monitor whether or not a vehicular bridge 

has been overloaded by a non-permitted load. 

Bridge designers need to be aware that overloads do indeed occur on most bridges. During 

design it may be necessary to investigate owner specified loads, as well as a potential overload 

condition for vehicular routes that are more likely to experience non-permitted vehicles, based on 

an owner’s recommendation. 

2.1.1.4 Vehicle and Vessel Collisions 

A vehicle or vessel collision with a steel girder superstructure could cause a failure of the bridge. 

Overpass bridges, which cross over an interstate for example, can be subjected to an over-height 

vehicle collision. Instances of these do occur, such as special loads, or oversized loads, which 

may exceed the clearance provided. Similarly, a bridge over a navigable waterway could be hit. 

During the bridge design process, the bridge designer must verify that the design team has 

adequately addressed the clearances required by the owner. If a risk of impact from over height 

vehicles or vessels reasonably exists, steps could be taken to either mitigate the potential 

collision (use of a sacrificial beam to protect load-carrying girders) or evaluate the impact on the 

structural system. 

2.1.1.5 Fire / Extreme Heat

During an extreme heat event there can be a significant loss of stiffness in the girder and its 

connections. A vehicle or vessel fire below a steel girder bridge could cause the girders, as well 

as other bridge components, to fail resulting in a failure of the bridge. 

In April of 2007, an extreme fire event did occur on a highway in Oakland, California which 

caused the collapse of the over passing multi-I-girder steel structure. A gasoline tanker truck 

caught on fire, and came to rest under the overhead structure, which funnels traffic onto the San 

Francisco – Oakland Bay Bridge. A thermal threshold was exceeded, causing the steel girders to 

buckle, leading to the collapse of the span. 

Bridge designers need to be aware of the fact that a fire can occur below a steel girder 

superstructure. Depending on the importance of the structure, it may be necessary to investigate 

the bridge behavior due to an extreme heat event to ensure that a collapse does not happen. 

Alternatively, a design could be developed that would allow some delay before a collapse would 

occur, which would minimize the potential loss of life.
17

2.1.1.6 Blast / Terrorist Attack 

Blast loads resulting from explosions, triggered during a terrorist attack for instance, that occur 

below the bridge deck will impart large uplift forces on the superstructure. The forces may cause 

the girders to slip off of the bearings, deformations of the steel girders, or reduce the composite 

behavior of the deck and steel girders. Similarly, an explosion on the bridge deck may cause 

large areas of the deck to fail, the girders to deform, or reduce the composite behavior of the 

deck and steel girders. In both cases, the explosion may result in a sustained fire, which could 

lead to failure of the superstructure. 

Depending on the importance and vulnerability of the structure being designed, the bridge 

designer may need to consider the effects of blast loads on the superstructure during the design 

process. The designer, owner, and/or security personnel should perform a risk assessment to 

determine the threats that a particular bridge may be subjected to. If the risk is low, a simple 

overpass for example, then the consideration of blast loads may not be warranted. However, if 

the bridge is an important structure, and/or the risk of a terrorist attack is high, the designer may 

need to consider the effects of blast loads on the superstructure. The effects of blast loading can 

typically be investigated through computer simulations and the use of finite element analysis 

procedures. It may be difficult to develop a design that would resist all types of terror attacks, 

however a design could be developed that would delay collapse and allow the preservation of 

human life.

For additional information regarding the consideration of blast loads and terrorist attacks in the 

design process, the reader is referred to the following references: 

The FHWA assembled a Blue Ribbon Panel on Bridge and Tunnel Security to address 

issues related to the vulnerability of bridges and tunnels to terrorist attacks [19]. The 

panel investigated funding requirements associated with future research and 

implementing countermeasures against terrorist attacks. 

Through risk management techniques, Williamson and Winget [20] outline methods to 

mitigate the risk of terrorist attacks against critical bridges providing cost effective 

security measures, discussion of blast effects on bridges, and structural design guidelines. 

Ray [21] describes a risk based methodology that can prioritize the needs for terrorist 

attack mitigation for specific bridge components. 

Winget et al. [22] discuss how incorporating physical security and site layouts in the 

design process can help to mitigate terrorist attacks. The authors also discuss the 

potential effects of blast loads on bridges, and provide structural design solutions that 

may be employed by the bridge designer to reduce these effects. 

2.1.1.7 Poor Details 

For some steel girder bridges, certain details may prove unusually costly, unusually time 

consuming, or difficult to fabricate and/or erect. If the detail is too difficult to fabricate or erect, 

then delays may result in the fabrication process or construction. For more complicated details, 

for example, cross frames at a severely skewed pier, it is worthwhile for the bridge designer to 

contact fabricators and obtain their opinion on the ability to fabricate the detail in question. 

Similarly, a designer should contact a steel erector concerning a complicated erection scheme or 
18

connection assembly. It will help to prevent delays, extra costs, and potential failures during the 

bridge project if the designer investigates complicated details during the design process. 

Additional information regarding general steel girder bridge details can be found in the 

AASHTO/NSBA Steel Bridge Collaboration document Guidelines for Design Details [23], and 

in Practical Steel Tub Girder Design [24] specifically for steel box girders. 

2.1.1.8 Flood / Storm Surge 

A bridge with a small vertical clearance over a waterway could be vulnerable to damage 

resulting from a debris flow in a flood situation. If the vertical clearance is small, it is possible 

that the girders of the structure will cause flood debris to be stopped at the bridge. This debris 

stoppage and water flow could lead to additional lateral loads on the steel girders that were 

unanticipated during design. Wardhana and Hadipriono [1] found that 16 cases of bridge failure 

(any type of bridge) derived from debris flows in the same year, 1995, resulting from flash 

flooding in Madison County, Virginia. For additional discussion on storm surge, see section 

3.1.1.10. 

2.1.1.9 Seismic

Seismic loads can be one of the most critical extreme events that must be considered by bridge 

designers. Depending on the geographic location of the structure, seismic loads may govern the 

design. Several bridge failures, both steel and concrete, have been reported on in textbooks and 

the archival literature, most notably are bridge failures associated with the 1989 Loma Prieta 

earthquake in California. It is not the intent of this document to list all of the noted bridge 

failures due to seismic activity, potential design procedures, and load mitigation techniques, but 

to highlight the need to consider earthquake loads when necessary. However, it should be noted 

by bridge designers that in many cases bridge failures due to seismic loads are caused by 

inadequate bridge seat lengths and/or the use of non-ductile details in both the super and 

substructure elements. 

The reader can refer section 3.10 of the current AASHTO LRFD Bridge Design Specifications

[2] for earthquake design loading. However, the bridge designer should be aware that local 

and/or individual State specification may take precedence, or be more restrictive than the 

AASHTO LRFD Bridge Design Specifications. It is the responsibility of the bridge designer to 

use the correct seismic design specifications. 

Additional information regarding seismic loading and design in bridges can be found in the 

following references (not an all-inclusive list): 

“NCHRP Report 472: Comprehensive Specification for the Seismic Design of Bridges,” 

[25] 

“Recommended LRFD Guidelines for the Seismic Design of Highway Bridges, Part I: 

Specifications and Part II: Commentary and Appendices,” [26]. These guidelines are 

based on NCHRP Report 472.

Seismic Design and Retrofit of Bridges [27].
19

2.1.2 Superstructure – Cross Frames / Diaphragm 

Cross frames and diaphragms are generally considered secondary members in straight girder 

bridges. However, in horizontally curved steel girder bridges cross frames and diaphragms must 

be considered primary members, and designed as such. The cross frames in a horizontally 

curved girder bridge transmit forces necessary to provide equilibrium, therefore these forces 

must be considered in design. 

A failure of a cross frame in a steel girder superstructure does not necessarily mean that the 

structure will collapse, but it does result in a critical defect that must be assessed. Similar to the 

girders in a steel girder superstructure, the cross frames can fail due to fatigue and overload 

considerations. 

2.1.2.1 Use of Oversized Bolt Holes 

Oversized bolt holes are sometimes used in the connections between the cross frames and the 

girders in steel girder bridges. It is believed by some in the bridge industry that oversized bolt 

holes facilitate the steel erection, providing additional flexibility to make the connections. While 

this practice may be suitable for straight steel girder bridges, it is, in most cases, not suitable for 

horizontally curved and/or skewed steel girder bridges. The control of geometry is extremely 

important during the steel erection of a horizontally curved and/or skewed steel girder bridge. 

The use of oversized holes will likely compromise the geometric control necessary to 

successfully assemble the bridge components. As the girder lines are erected across a curved 

I-girder bridge, the girders will displace and rotate more due to the extra space of the oversized 

holes. This additional displacement and rotation will often cause the girders to be at a location 

that is not in accordance with the final top of steel elevations typically shown in design plans. 

The bridge designer must understand the potential pitfalls associated with the specification of 

oversized bolt holes. 

2.1.3 Superstructure – Bearings 

Bearing failure is a critical defect that could lead to collapse or partial collapse of a bridge. One 

primary cause of a bearing failure is a longitudinal and/or lateral force that exceeds the capacity 

of the bearing to resist that force and subsequently causing the girders to slip off the substructure 

seat. Such a force could result from an extreme wind event, water (associated with flooding), an 

earthquake loading, or vessel/vehicle collision. Rocker bearings are particularly susceptible to 

longitudinal forces, and are therefore not typically employed in new bridge designs and are often 

replaced as part of bridge preservation programs. 

Elastomeric bearing pads can exhibit failure modes such as crushing, delamination, and slippage. 

Slippage tends to be a significant serviceability issue for neoprene bearing pads, as reported by 

McDonald et al. [28]. Refer to the paper by McDonald et al. [28] for recommendations on

preventing slippage of elastomeric bearing pads. 

Unless significantly exceeding allowable displacements, thermal movements alone don’t 

typically cause bearing failure, but can be a major contributor. Longitudinal and lateral restraint 
20

or provision for displacement must match the assumptions used in the analysis. This is 

especially true when modeling curved and/or skewed girder bridges, since an inconsistency in 

the analysis may exceed the bearing’s load capacity or permitted motion, or cause distress in 

other structural elements. 

Rocker bearings can be susceptible to failure resulting from what is known as “ratcheting.” 

Debris and/or corrosion material can build-up on the rocker seat area and can prevent the bearing 

from moving freely as intended by design, thus imparting longitudinal forces into the support 

structure. The build-up of material, coupled with the movements caused by thermal cycles can 

result in the “ratcheting” effect of a rocker bearing. Two recent rocker bearing failures highlight 

this issue: 

On July 27, 2005, in Albany, New York, two steel I-girder spans of the I-787 Ramp AC 

structure (Dunn Memorial Bridge Interchange) fell of their support bearings at Pier 11

supporting the expansion ends of two different two-span units (see Figure 2-5). The 

authors of the forensic investigation indicated that the failure can be most likely 

attributed to the fact that the Span 12 bearings became overextended, due to their 

inability to return toward vertical during time of expansion of span 12, thus pushing Pier 

11 southward to accommodate expansion [29]. During periods of Span 12 contraction, 

the bearings could tip further north becoming more overextended, and repeated thermal 

cycles would cause the bearings to “ratchet” further northward, until they became 

unstable [29]. 

A similar rocker bearing failure occurred in a steel plate girder approach span of the 

Birmingham Bridge in Pittsburgh, Pennsylvania, in February 2008. The rocker bearings 

at Pier 10S tipped over, dropping the steel girders and roadway approximately 8 inches. 

Based on the forensic investigation, the rocker bearings most likely tipped over at the pier 

due to a combination of events, including improper installation and a leaking expansion 

joint above the bearings that contributed to and accelerated the corrosion of the bearings. 

Collecting debris also allowed the ratcheting effect [30]. A photo of the tipped bearing at 

Pier 10S is shown in Figure 2-6. 

(a) (b) 

Figure 2-5 Dunn Memorial Bridge Interchange Rocker Bearing Failure: (a) Sketch of the 

bridge elevation from Pier 9 to Pier 13; (b) Photo of the failed rocker bearings at Pier 11 

showing them tipped over (taken from [29]).

NORTH
21

Figure 2-6 Photo of failed rocker bearing at Pier 10S of Birmingham Bridge Approach 

Span showing the rocker tipped over with girder sitting on top of failed bearing (taken 

from [30]).

2.1.4 Superstructure – Concrete Deck/Railings 

The failure of a concrete deck in a steel girder superstructure is not likely to cause a collapse of 

the bridge, but could result in a critical defect on the bridge. Cracking of the concrete, combined 

with corrosion of the steel reinforcement could lead to a serviceability issue and poor ride quality 

for the users of the bridge. To prevent a deck failure, the designer must take steps to reduce 

cracking in the deck and corrosion of the reinforcement. For example, concrete admixtures to 

reduce shrinkage cracking, corrosion inhibiting devices, use of epoxy-coated reinforcement, 

and/or providing additional concrete cover to the steel reinforcement are methods that can be 

employed by the designer, in consultation with the owner’s specifications, to reduce the 

likelihood of a concrete deck failure. The reader can refer to NCHRP Synthesis 333: Concrete 

Bridge Deck Performance for additional information regarding design and construction practices 

intended to improve the performance of bridge decks [31]. Topics in NCHRP Synthesis 333 

include factors that contribute to the durability of concrete, performance of various deck 

protection systems, and lessons learned in design, construction, and maintenance of concrete 

bridge decks. 

Additionally, bridge traffic barrier systems, otherwise known as railings, must meet a certain 

crash test level requirement in accordance with the AASHTO LRFD Bridge Design 

Specifications [2]. There are six bridge railing test levels specified, and named TL-1 though Tl6. The various test levels are intended to evaluate performance factors of the railing, including 

structural adequacy, occupant risk, and post-impact behavior of the test vehicle. TL-4 bridge 

railings are typically satisfactory for interstate bridge designs. In several States, bridge owners 

have standard bridge railings that are to be used in by the bridge designer. However, if there are 

no specific State standards, the owner must specify which of the test levels is most appropriate 

for the bridge site [2].

The reader is also referred to section 3.1.1.2 of this document for discussion of creep and 

shrinkage in concrete, and section 3.1.1.3 regarding concrete properties. 
22

2.2 Inspection Category 

The fault tree that follows the Inspection category is shown in Figure 2-7. Inadequate inspection 

can contribute to a failure of the bridge by not finding, assessing, reporting, and initiating repair 

actions for a problem. Therefore details should be incorporated into the design by the designer

to facilitate inspection of the bridge. 

Figure 2-7 Portion of the steel girder bridge fault tree showing the Inspection Category only, 

with three Basic Events provided.

In addition to providing access, the details themselves must be inspectable. Elements that are 

difficult to inspect are typically problematic to maintain, which could lead to a condition in 

which fatigue cracks or other flaws may go undetected. During the design process, the designer

should verify that all areas of the structure can be reasonably accessed, inspected, and 

maintained.

Adequately sized manholes must be provided in steel box girder bridges, which allow bridge 

inspectors access into the box itself. In a continuous box girder bridge, manholes must also be 

provided in the internal full depth diaphragms at the supports, which allow bridge inspectors to 

pass through from span to span. Information regarding manhole details in steel box/tub-girder 

bridges can be found in Practical Steel Tub Girder Design [24]. 

Furthermore, for bridges where access to the girders and bridge components may be difficult 

from underneath or by a snooper truck, it may be necessary to have walkways installed, or at 

least hand rails installed on the girders. Forethought by the designer during the design process,

concerning the inspection of the structure, can help to prevent a potential failure of the bridge 

system. 

In some cases, for unique steel girder bridges, it may be advantageous to have a bridge 

maintenance manual developed specifically for the bridge in question. This document would

include items that the designer knows may be potential causes for structural problems in the 

future. Additionally, the manual should show what structural elements are fatigue sensitive 

and/or fracture critical so that the bridge inspectors know what elements should be given 

additional attention during the bridge inspection. 
23

2.3 Construction Category 

The fault tree that follows the Construction category is shown in Figure 2-8. This part of the 

overall fault tree may not necessarily apply to the bridge designer, but may be more applicable to 

a specialty engineer working for a contractor or steel erector. However, it is necessary that the 

bridge designer is aware of the potential construction issues when developing a new bridge 

design in order to facilitate (and/or simplify) bridge construction. 

Figure 2-8 Portion of the steel girder bridge fault tree showing the Construction Category only, 

with several Events and Basic Events provided.

The Construction category is divided into two failure paths: Steel Erection and General 

Construction, as shown in Figure 2-8. This approach is used because the general construction 

path is typically applicable to all other bridge types, and will be referenced in latter portions of 

this framework. In the case of a steel girder bridge, a failure during the steel erection or during 

the general construction of the bridge can result in an overall failure of the bridge system. A 
24

failure may be a total or partial collapse during steel erection, or may be classified as a critical 

defect, such as a connection misalignment or limited crane access. 

2.3.1 Steel Erection

A failure during the Steel Erection of a bridge will trigger a failure on the construction path of 

the framework, as depicted in Figure 2-8 and 2-1. As shown in Figure 2-8, there are several 

events that can cause the steel erection to be considered as a failure. These events acting alone 

may cause a failure, or the combination of any set of these events can cause a failure.

Wind loads, for example, can cause lateral forces that can overstress the girder flanges, or 

overload tie-down assemblies. If a girder is not properly braced during erection, wind loads may 

in fact cause the girder to roll over and possibly off of the supports. The girders need to be 

adequately braced for all lateral loads.

Bracing (temporary or permanent) is typically needed to resist wind loads and to provide stability 

for the girders. During steel erection, I-girders are often placed onto the supports with large

unbraced lengths. These large unbraced lengths cause steel plate I-girders to be susceptible to 

lateral torsional buckling during steel erection. If the erected girder has not been designed for 

this temporary condition, has an inadequate number of brace points, or the bracing does not 

provide the necessary stiffness, the girder will buckle.

Several instances of inadequate bracing during steel erection have been reported. One such 

incident occurred on May 15, 2004, in Golden, Colorado, at the I-70/C-470 interchange, when a 

steel I-girder collapsed during a highway bridge-widening project. The girder, which was 

erected two nights prior, fell onto the roadway below, killing three occupants of a vehicle. The 

NTSB determined that the probable cause of the girder collapse was the failure of the girder’s 

temporary bracing system and deficiencies in the installation of the girder and the bracing [32].

On May 16, 1995, a partially erected steel girder bridge on Tennessee Highway 69 collapsed. 

Wojnowski et al. studied the collapse, providing insight into the state of stress and buckling 

stability of bridge superstructures during steel erection [33]. The paper illustrates the effect that 

the lack of bracing has on the stability of a partially erected structure. 

Bolted splices and connections require special attention during the steel erection. In accordance 

with AASHTO LRFD Bridge Construction Specifications [34], Article 11.6.5, splices and field 

connections shall have one-half of the holes filled with bolts and cylindrical erection pins (half 

bolts and half pins) before installing and tightening the balance of high-strength bolts [34]: this is 

a minimum condition. These bolting requirements, as well as the tightening requirements, must 

be specified by the bridge designer, in accordance with the owner’s specifications, in the steel 

erection procedure. If these requirements are not stated in the steel erection procedure, field 

bolts may not be properly installed. The incorrect installation of field bolts could lead to 

unpredicted girder displacements or a girder collapse. 

Displacements during the steel erection should be evaluated during development of the steel 

erection procedure by the specialty engineer working for a contractor. This is especially critical 
25

for horizontally curved and/or skewed girder bridges. Curved girders will rotate out-of-plane, as 

shown in Figure 2-9, causing the top and bottom flanges to displace out-of-plane, unless the 

rotation is prevented by supports or bracing. Large rotations and displacements may make 

connecting components difficult. Additionally, for straight, curved, and skewed bridges, 

deflections and rotations at field splice locations should be investigated to ensure that the 

connections can be made. 

Figure 2-9 Curved girders displace vertically and rotate out-of-plane causing the top and 

bottom flanges to displace out-of-plane unless the rotation is prevented by supports or bracing.

Thermal movements during steel erection need to be considered by the specialty engineer 

engaged by the contractor. Bridges are typically designed for an ambient temperature of 68°F. 

If the bridge is erected when the temperature is higher or lower, there is potential for connection 

and component misalignments. 

In steel I-girder bridges, lateral bracing (temporary or permanent) may be required to control 

lateral deflection due to wind loads. Longer spans (greater than 300 feet) generally have issues 

related to deflections due to wind. In their Bridge Design Standards, PennDOT [35] has 

requirements for lateral bracing applicable to erected steel superstructures, before the placement 

of the concrete deck. PennDOT Bridge Design Standard, BD-620M, states the permissible 

lateral deflection of a span due to wind loads is the span length divided by 150 (L/150). (Other 

States may have similar standards that may need to be followed; the PennDOT example is 

provided for reference only.) The bridge designer should investigate the need for lateral bracing 

during the design process to verify that the bridge, as shown on the design plans, is 

constructable. The engineer that is developing the steel erection sequence must consider wind 

loads, and their effect on the erected steel superstructure as well as during stages of the erection 

sequence. 
26

During steel erection, especially for horizontally curved and/or skewed girder bridges, it is 

important that the erector control the geometry of the structure throughout the erection sequence. 

The use of temporary support towers (as shown in Figure 2-10), bracing, guy wires, or holding 

cranes are methods that can be used to control geometry. If the erected geometry of the 

superstructure deviates from the anticipated geometry, misalignments of the bearings, cross 

frames, or expansion joints can result. In more complex cases, the specialty engineer should 

provide the steel erector the anticipated superstructure geometry at each erection stage. 

Figure 2-10 Photo showing the use of temporary supports during erection of a curved steel Igirder bridge. The use of temporary supports help to control the geometry of the structure 

during steel erection.

2.3.2 General Construction 

As shown in Figure 2-8, there are several events related to the General Construction of a steel 

bridge that may cause a failure of the bridge system. Some of these events are required to be 

investigated during the design process by the bridge designer, such as the girder constructability 

checks. 

In accordance with the AASHTO LRFD Bridge Design Specifications [2], Article 6.10.3, 

constructability checks of the steel girders are required during the design process. These checks 

indicate whether the girder will have adequate strength during critical stages of construction. 

Insufficient consideration of constructability during design could result in a failure during 

construction. Some items that need to be considered as part of the constructability checks 

include: 

Deck Placement Sequence - The concrete deck is often placed in multiple pours, resulting 

in girders being composite in some locations, and noncomposite in others at various 

times. A particular deck placement sequence can create temporary moments in the 

girders that are larger than the final noncomposite moments after the entire placement is 

complete. 

Deck Overhang Loads - Cantilever forming brackets (shown in Figure 2-11) are typically 

used along the exterior girders to construct the concrete deck overhangs. The eccentricity 

of the deck weight and other construction loads (screed, formwork, construction live 

load) acting on the bracket applies a torsional moment on the exterior girder. The 

overhang bracket will transmit loads to the top flange and the girder web, depending on 

the bracket and web height. The overhang bracket should terminate at the web-bottom 
27

flange junction. However, if it does not (a deep girder for example) then the potential for 

web buckling needs to be investigated. 

Wind loads - Wind loads also need to be considered during the deck placement. The 

combination of loads associated with the deck placement and wind can result in an 

overstress condition. Typically, the specialty engineer may need to specify a certain 

wind speed at which the deck can be placed, in order to reduce the lateral loads acting on 

the girder. 

Figure 2-11 Photo showing the cantilever forming brackets which will support formwork that 

will be used to construct the concrete deck overhang on a simple span steel girder 

superstructure.

A study related to a failure of a steel plate I-girder bridge during deck placement has been 

reported on by Waddle and Wang [36]. A three-span continuous composite plate girder bridge 

having end spans of 215 ft and a center span of 290 ft was studied. It was determined that local 

buckling in the web and that lateral torsional buckling of the plate girder occurred during or 

immediately following a particular stage of deck placement, before sufficient composite action 

was reached. The authors noted that a critical process to be accounted for in design is during the 

construction when all or parts of the structures act in a noncomposite fashion. 

In 2002, workers were placing a concrete deck for a new pedestrian bridge in Marcy, New York, 

when the bridge suddenly collapsed, killing one construction worker and injuring nine others. 

The steel superstructure was a single trapezoidal box girder which spanned 170 ft, with no top 

flange lateral bracing. The girder buckled due to global elastic-torsional buckling when 

approximately 60 percent of the wet concrete had been placed. Recommendations resulting from 

a forensic investigation of the collapse [37] are: 1) require top flange lateral bracing in 

trapezoidal box girders; 2) require stability checks because long span girders can have stability 

limitations, with or without top flange lateral bracing. 

Yura and Widianto [38] performed additional studies regarding the Marcy Pedestrian Bridge 

collapse. The authors further investigated the causes related to the lateral torsional buckling of 

the cross section, the need for lateral bracing in trapezoidal box girders, and the similar buckling 
28

case of a twin I-girder system. A twin I-girder system could be susceptible to global lateral 

torsional buckling at various stages of a steel erection sequence. A relatively simple equation 

was developed that could be used to approximate the buckling strength of a twin I-girder system. 

During steel construction, temporary support towers, also known as falsework, may be used to 

reduce loads in the girders and help control geometry. Often, a temporary support may be 

selected from a manufacturer’s catalogue or stock available at local contractor equipment rentals, 

based on the vertical load the supports must resist. The specialty engineer must check that the 

assumptions made in the capacity of the temporary support tower by the manufacturer are 

consistent with the conditions in the field. Temporary support towers from a catalogue often do

not provide any lateral support so additional provisions are often needed if lateral restraint is 

required. Furthermore, the specialty engineer may need to specify how the ground is to be

prepared for the foundations of temporary support towers.

During the design process, the bridge designer should consider limitations on the size and weight 

of field pieces. They should not be too heavy to be lifted by typical erection equipment which 

can access the site or too large (height, length) to transport and deliver by normal means. If the 

bridge can not be constructed by typical methods, this should be noted in the design plans.

Traffic management during the construction must also be considered. If road or lane closures are 

required to erect the bridge components, the bridge designer, in coordination with traffic 

engineers, should verify if closures will be allowed by the governing agency. Coordination early 

in the design process will help to eliminate potential conflicts when the structure is being erected. 

Qualified field inspectors are required during erection to verify that components are placed 

within the specified tolerances at each stage of erection. If the components are not placed in the 

correct locations at the beginning of erection, it can compound problems as additional field 

pieces and/or components are erected. The inspectors need to be familiar with the given type of 

construction, and the known pitfalls that can occur. 

2.4 Fabrication Process Category

The fault tree that follows the Fabrication category is shown in Figure 2-12. This part of the 

overall fault tree will apply to the bridge designer as well as the specialty engineer working for a 

contractor or steel erector. As shown in Figure 2-12, there are four basic events shown that can 

trigger a failure in a bridge project related to the fabrication of the steel superstructure. 
29

Figure 2-12 Portion of the steel girder bridge fault tree showing the Fabrication Category only, 

with four Basic Events provided.

In accordance with the AASHTO LRFD Bridge Design Specifications (2008), Article 6.7.2, for 

horizontally curved and/or skewed I-girder bridges, the contract documents are to clearly state 

the intended erection position of the girders and under which load condition that position is to be 

achieved. The intended erected positions are: girder webs theoretically plumb, or girder webs 

out-of-plumb. Common conditions at which the erected positions can be theoretically obtained 

are: no-load condition, steel dead load condition, or full dead load condition. When designing a 

curved and/or skewed I-girder bridge, the bridge designer needs to consider the detail condition 

that will be specified and how the fabricator can achieve that detail condition. If the intended 

erected position and load condition are not clearly identified in the contract plans by the 

designer, a failure of the erection process or long term problems may result. Article 6.7.2 and 

the commentary of the AASHTO LRFD Bridge Design Specifications [2], discusses this topic in 

great detail, and it should be reviewed by the designer for additional information. Further 

discussion on this topic is available in the AASHTO/NSBA Steel Bridge Collaboration 

document Guidelines for Design for Constructibility [39]. 

Additionally, if the bridge designer does not fully consider the consequences of detail conditions, 

a failure or critical defect may occur. This has been shown in studies conducted by Chavel and 

Earls [40, 41] regarding the detailing and erection of the horizontally curved span of the steel Igirder Ford City Veterans Bridge. The intended detail condition for the subject structure was for 

the girder webs to be plumb under steel dead load. This was to be achieved by detailing the 

cross frames for a web plumb position at the steel dead load condition, while the girders were 

detailed for the web-plumb position at the no-load condition, creating a detailing inconsistency, 

resulting in formidable fit-up problems and an increase in erection costs [41]. The choice of 

detail condition and methods of achieving the detail condition (for example, webs plumb at steel 

dead load) could have a significant effect on the success or failure of a curved and/or skewed Igirder bridge project. 

Shipping and handling of girder field pieces must be considered by the bridge designer during 

the design process. Permits may be required and could put constraints on the design. If the 

fabrication is to be done in one State, and delivered to another, the fabricator must verify permit 

requirements for each State. The designer will need to assess length, weight, curved girder 

sweep, and allowable times of transit when necessary. The steel I-girder Lake Creek Bridges in 
30

northern Idaho required that the girders be shipped between different States. The permits in one 

State allowed shipment during the day, while the adjacent state only allowed shipment of the 

same piece at night [42]. This transporting issue resulted in construction delays. Longer girder 

field pieces may be too heavy to lift with typical erection cranes and transported by typical 

methods. If longer girder field pieces are used in the design, the bridge designer should contact a 

fabricator and/or transporter to verify that the necessary equipment and permit capabilities are 

available. 

For complex structures, such as horizontally curved and skewed I-girder bridges, the designer

should determine if full shop or partial assembly is appropriate prior to shipment of the bridge 

components. Verifying bracing connections in a full shop assembly will add confidence that the 

same pieces will fit under field conditions. However, this added confidence will come with an 

additional cost. AASHTO LRFD Bridge Construction Specifications [34], Articles 11.5.3 and 

11.8.3.7, provide the designer with requirements concerning shop assembly. 

Proper inspection of the bridge components in the fabrication shop will help to prevent failures 

in the fabrication process and subsequent failures during steel erection. If an error is undetected 

during fabrication, additional failures may result throughout the bridge project. The inspectors 

need to be familiar with the given type of bridge being fabricated, and be aware of the difficulties 

that can occur in the fabrication process.
31

3.0 PRECAST CONCRETE GIRDER BRIDGE FAILURE FRAMEWORK 

A general fault tree for the case of a precast concrete girder (prestressed and/or post-tensioned) 

bridge failure is developed and presented in this section. As discussed earlier, failure in this 

framework refers to a total collapse of the bridge system or an event that results in a critical 

defect. In their study of recent bridge failures, Wardhana and Hadipriono [1] noted that concrete 

beam/girder bridge failures have the third highest number of occurrences, following steel girder 

and steel truss bridges. Twenty-nine of the 503 failures recorded and studied were failures of 

concrete beam/girder bridges. Similar to the steel girder bridge framework, this concrete bridge 

framework is developed for a general case, as shown in Figures 3-1, 3-2, 3-7, 3-8, and 3-9. 

The fault tree developed for precast concrete girder bridges assumes that the bridge is designed 

and constructed according to the governing specifications for normal design loads as well as 

required extreme events. It also assumes that regular inspections and maintenance are performed 

over the service life of the bridge. 

The fault tree is established with the top event, the Precast Concrete Girder Bridge Failure, as 

shown in Figure 3-1. The failure can develop from four different categories; Design/Operation, 

Inspection, Construction, or Fabrication. These categories are joined by an Or Gate, which 

means any one of the four conditions can result in a failure. Similar to the Steel Girder Bridge 

Failure Framework, not all of the aforementioned conditions will necessarily apply to the bridge 

designer, but the designer should be aware of all of the events on the fault tree. These aspects 

are presented here so bridge designers understand and take into account the whole process of the 

design, fabrication, and construction of the bridge. 

Figure 3-1 Precast Concrete Girder Bridge fault tree showing the top categories only which 

include Design and/or Operation, Inspection, Construction, and Fabrication.

The Design/Operations category alludes to that fact that a failure, either a collapse or critical 

defect, can occur while the bridge is in service. Inspection refers to the fact that there may be a 

problem with the routine inspection such that the design does not permit inspection of some of 

the bridge components. A failure can also occur during the construction of the concrete girder 

bridge, whether it is a collapse or a problem that results in delays (a failure of the overall 
32

process). The fabrication process is also subject to errors and problems, which could result in a 

failure of the overall bridge process. These are all categories that the bridge designer should be 

aware of, and give due consideration to, when designing a concrete girder superstructure. Each 

of the four categories is developed into a more detailed fault tree in this section. 

Some of the events shown on the fault trees associated with a concrete girder bridge failure are 

the same as those shown previously for a steel girder bridge failure. The reader will be referred 

to previous discussions concerning failure events which are similar between the bridge types.

3.1 Design/Operation Category

The fault tree that follows the Design/Operation category is shown in Figure 3-2. While in 

service, a bridge failure can result from either a failure of the superstructure or substructure. A 

detailed representation of the superstructure fault tree is shown in Figure 3-2, while the 

substructure is shown in Figure 4-1. 

A failure of the concrete girder superstructure can be caused by a failure in any one of the 

superstructure components, but mainly the girders, bearings, or concrete deck. Again, an Or

Gate is used to join these fault scenarios, meaning that a failure of any one of these components 

may cause a failure of the superstructure. A failure of the superstructure will then trigger an 

operational failure of the bridge. For example, poor reinforcement and/or duct detailing, too 

much water in the concrete mixture, or corrosive constituents can cause an operational failure of 

a concrete girder bridge.

3.1.1 Superstructure – Girders

3.1.1.1 Corrosion / Inadequate Drainage Details

The failure of the girders can be caused by one or more events as shown in the fault tree, Figure

3-2. In addition to locating drainage scuppers and connecting elements based on design criteria, 

it is important to locate drainage elements in regions of the superstructure where they will cause 

the least damage if they become filled with debris. It is also important that the down-spouting be 

accessible for inspection and repair, and have the necessary slopes and connections to prevent 

clogging with debris. In addition, it may be advantageous to eliminate drainage details at 

support and/or expansion locations by making the concrete superstructure continuous and 

eliminating trough type drainage details when possible. Adequate drainage and the layout of the 

drainage elements will help to prevent corrosion of the reinforcing elements. Deterioration 

(corrosion) of the mild reinforcement or tensioned strands caused by the combination of water 

and deicing salts, can cause the failure of the main bridge girders. The corrosion of the mild 

reinforcement or tensioned strands will cause the girder concrete to crack and spall, furthering 

the possibility of a main girder failure. This is depicted in Figure 3-2 in the lower left corner of 

the Figure. 

Additionally, girders that are part of an overpass structure are prone to deicing salt-spray. Once 

deicing salts are applied on the roadway below, traffic passing through will cause the salt and 

water combination to become airborne, with some of the deicing salt/water solution splashing 
33

onto the bridge girders overhead. Salt-spray, and its effect on rate of corrosion, was one of the 

potential causes of the collapse of the exterior girder of the Lake View Bridge over I-70 in 2005 

in Washington, Pennsylvania. Based on the pre-test inspections conducted, it appeared that the 

most exterior prestressing strand damage resulted from vehicle impacts damaging the concrete 

cover, followed by corrosion of the exposed strands [43]. A photo of the another beam in the 

structure, that did not collapse, showing the damaged girder and corroded strands is provided in 

Figure 3-3.

Figure 3-2 Portion of the precast concrete girder bridge fault tree showing the Design and/or 

Operation category for superstructures only, with several Events and Basic Events provided.
34

Woodward [44] reports on a segmental post-tensioned concrete bridge that collapsed in 1985 due 

to corrosion of the post-tensioning tendons. The investigation showed that corrosion of the 

tendons occurred where they passed through the segmental joints. Corrosion more than likely 

occurred because the tendons were inadequately protected at the concrete segment joints, 

allowing chlorides from deicing salts to penetrate the ducts. 

Additional information regarding concrete bridges and corrosion can be found in Concrete 

Bridges in Aggressive Environments [45], and similar documents available through American 

Segmental Bridge Institute (ASBI) and the Portland Cement Association (PCA). 

Figure 3-3 Photo showing results of vehicle impact and subsequent corrosion damage to 

prestressing strands from a prestressed concrete box beam in Lake View Bridge over I-70 (taken 

from [43]).

3.1.1.2 Creep and Shrinkage 

Bridge designers must take into account the effects of creep and shrinkage during the design of 

concrete girder superstructures (and decks of steel girder superstructures). Creep and shrinkage 

can lead to cracks in the concrete, allowing water, salt, and other materials to penetrate the 

concrete. Once these materials penetrate the concrete, a corrosive environment can develop, and 

result in a failure of the concrete girder. Also, the effects of creep and shrinkage can increase 

deflections in beams and/or cause loss of prestressing forces in prestressed girders. 

Creep is the increase in strain with time due to a sustained load. When concrete is initially 

loaded an instantaneous elastic strain develops. If this load remains on the member, creep strains 

will develop over time. Creep is affected by the ratio of the sustained stress to the strength of the 

concrete, the humidity of the environment, the dimensions of the concrete element, the 

composition of the concrete, and temperature. At high temperatures, such as in a fire, large creep 

strains can occur. Additionally, creep is most significant when the concrete has a high cement 
35

paste content. Concrete containing large aggregate fractions creep less because only the cement 

paste creeps and is restrained by the aggregate.

Shrinkage is the shortening of concrete during curing. The primary type of shrinkage, called 

drying shrinkage is the decrease in volume of concrete when it loses moisture, occurring after the 

concrete has attained its final set, and most of the chemical hydration process in the cement paste

has occurred. 

There are several factors that affect drying shrinkage: 

Aggregate content - the aggregates act to restrain the shrinkage of the cement paste, 

therefore concrete with high aggregate content tend to be less vulnerable to shrinkage.

Water to cement ratio - the higher the water to cement ratio the more vulnerable the 

concrete element is to shrinkage.

Humidity - shrinkage strains are largest for relative humidity of 40 percent or less.

Temperature - shrinkage is less likely to occur at low temperatures.

Admixtures – admixtures used to accelerate the hardening of the concrete tend to increase 

shrinkage effects. Air-entraining agents have little effect on shrinkage. The effect that an 

admixture has on shrinkage varies depending on the particular admixture.

Surface Area / Volume – as the ratio of surface area to volume increases, the effects of 

shrinkage will increase.

More detailed discussions regarding creep and shrinkage and their effects can be found in 

concrete design textbooks, such as MacGregor [46] and Nawy [47].

3.1.1.3 Concrete Properties

The quality of the concrete, and its components, are particularly important to the behavior and 

endurance of a concrete superstructure. The bridge designer and/or owner must ensure that 

appropriate materials are specified and the contractor must provide components, including 

aggregate, water, admixtures, or grout, which meet specifications and do not adversely affect the 

concrete quality. There are several factors that affect the strength and behavior of concrete that 

bridge designers should be cognizant of, with the more important ones being:

Water to cement ratio – the strength of concrete is significantly dependent upon the water 

to cement ratio. Typically, a water to cement ratio of 0.45 corresponds to a compressive 

strength of 5000 psi, while a ratio of 0.65 corresponds to a compressive strength of 3500 

psi.

Type of cement – different cements have a different rate of at which they gain strength.

Cementitious materials – a portion of the cement is sometimes replaced with a 

supplementary material, such as fly ash or silica fume. These materials, referred to as 

pozzolans, possess little or no cement-like properties, but will react with calcium 

hydroxide to form cement-like compounds. 

Aggregate – the concrete strength is affected by the strength, texture, grading, and size of 

the aggregate. 

Other factors include moisture and temperature conditions during curing, age of the 

concrete, and the rate of loading.
36

3.1.1.4 Grout Properties and Grouting

In post-tensioned superstructures, the grout used will have a significant effect on the behavior 

and endurance of the bridge. The post-tensioning ducts and anchorages must be completely 

filled to provide a permanent protection for the post-tensioning steel and to develop a bond 

between the steel and the surrounding concrete. If voids remain due to accumulated bleed water,

improper venting, or carbonated grout, corrosion of the post-tensioning strands can result. The 

quality of the grouting and the use of appropriate details for ducts and anchorages are critical for 

the performance of the structure.

The ASBI Grouting Committee [48] has reported on several structures in Florida that have 

experienced corroded post-tensioning tendons directly resulting from problems related to 

grouting techniques. In 1999, two corroded strands were found in a tendon anchorage at an 

expansion joint in a span of Niles Channel segmental bridge. It was concluded that initial 

corrosion resulted from the absence of grout due to accumulated bleed water that separated from 

the grout. This resulted in voids in the grout at the tendon anchorage, which then filled with 

water leaking through the concrete cover at the anchorages. In 2000, a failed external tendon and 

strands in an additional tendon were found in the Mid-Bay Bridge. Additional inspection 

resulted in additional tendons being replaced, mainly near expansion joints. It is believed that 

the nature of the grout mix, coupled with bleed water accumulation, and the filling of voids with 

water during construction may have contributed to the observed tendon corrosion. Also in 2000, 

inspections of the I-75/I-595 Sawgrass Interchange had found efflorescence at some tendon 

anchorages. In fact, during repair processes, it was found that some tendon ducts did not contain 

any grout at all.

Based on these findings, the ASBI Grouting Committee [48], summarized that most segmental 

concrete bridge grouting problems were related to: voids caused by accumulation of bleed water 

at tendon anchorages; salt contaminated water filling ungrouted tendon anchorages due to 

improper sealing of the ducts during construction; and substandard quality of grout installation 

and material. The bridge designer must be aware of these issues and consider how to avoid their 

potential effects during the design of post-tensioned concrete structures.

Additional information related to grouting materials, post-tensioning systems, and construction 

practices can be found in the FHWA publication Post-Tensioning Tendon Installation and 

Grouting Manual [49] and the American Segmental Bridge Institute’s (ASBI) Construction 

Practices Handbook for Concrete Segmental and Cable Supported Bridges [50].

Grout is also used to fill cavities created by the removal of mandrels used for the harping of 

prestressing tendons in prestressed concrete girders. The bridge designer must verify that the 

grout specified is: approved by the governing agency and does not have an adverse affect on the 

performance of the superstructure. On May 20, 2000, at the Charlotte Motor Speedway in 

Charlotte, North Carolina, one span of a four-span, simply supported precast, pretensioned 

concrete pedestrian bridge collapsed, injuring 107 people [51]. Each span consisted of two 

precast, pretensioned double-T beams located side-by-side and connected longitudinally by 

welded shear connectors along the flange. A harped prestressing strand profile was employed, 

using a single hold-down at the midspan. Once the concrete hardened, the mandrel used to 
37

create the harped pattern was removed and the resulting cavity filled with grout. The forensic 

investigation indicated that severe corrosion of the prestressing strands occurred at the midspan 

of the double-T beams at the location of the mandrel hold down, where the cavity was filled with 

grout. The resulting loss in steel cross-sectional area reduced the strength of the superstructure. 

The aggressive corrosion of the strands was apparently caused by excessive additives (15 times 

that recommended) of a grout admixture containing chloride [51]. It should be further noted the 

grout admixture utilized by the contractor was not approved for use in precast concrete 

construction by NCDOT [51].

3.1.1.5 Reinforcement Details

The detailing and subsequent installation of reinforcement will have a significant effect on the 

behavior and long-term performance of a concrete superstructure. The bridge designer must 

develop reinforcement details that can be constructed as the design intends. If details are too 

cumbersome and complicated, it is possible that the detail will not be constructed as intended. 

Improper detailing may cause a weakness in the superstructure, provide a location for crack 

initiation, or create inadequate concrete cover, leaving the reinforcement susceptible to 

corrosion. Furthermore, details with significant reinforcement congestion can lead to poor flow 

and distribution of concrete when it is placed.

The collapse of the de la Concorde overpass shows that the bridge designer must fully consider 

the effects of reinforcement details in a concrete superstructure. An inquiry into the collapse of a 

portion of the overpass in Laval, Quebec, Canada, on September 30, 2006, found that improper 

detailing and installation of the steel reinforcement was a contributing factor to the bridge failure

[52]. The simple single span of the bridge consisted of adjacent box girders, which rested on a 

beam seat. The beam seat was built into the thickened slab that cantilevered from the abutments 

at each end, as shown in Figure 3-4. The commission determined that in the as designed 

structure, the concentration of numerous reinforcement bars on the same plane in the upper part 

of the thickened cantilever slab created a plane of weakness where horizontal cracking could 

occur [52]. Top bars in the thickened slab were not sufficiently anchored. Additionally, the 

improper placement of several reinforcement bars during construction created a much larger 

zone of weakness extending into the thickened slab [52]. 
38

Figure 3-4 Sketch showing the elevation of de la Concorde overpass; the simple central span of 

the bridge consisted of adjacent box girders, which rested on a beam seat that was built into the 

thickened slab that cantilevered from the abutments at each end (taken from [52]).

Similar to reinforcement details, the details associated with prestressing and post-tensioning 

tendons will have a significant effect on the durability of a concrete superstructure. As discussed 

previously, the ASBI Grouting Committee (2000) reported on the corroded post-tensioning 

tendons in Florida. Some of the failures reported were due to grouting techniques as well as 

inadequate detailing of the ducts, especially at the anchorages. In the case of the Niles Channel 

Bridge, it was noted, at some locations, that water leaked through the concrete cover at the 

anchorages, resulting in corrosion of the post-tensioned tendons at this location. The ASBI 

Grouting Committee (2000) highlighted duct detailing issues such as: 

Providing effective sealing of tendons from water ingress in the interval between 

stressing and grouting. 

Providing anchorage details that are protected from water ingress, and showing such 

details on the design plans. 

Providing adequate ducts for internal tendons of concrete structures located in salt water 

environments or exposed to de-icing solutions. 

Adequate details for the post-tensioning ducts and anchorages must be provided by the bridge 

designer. Particular attention must be given to anchorages located at expansion joints, which are 

susceptible to water ingress. Consideration of these issues at the design stage will potentially 

eliminate failures associated with the failure of post-tensioning tendons. Additional information 

regarding reinforcement details, including detailing of post-tensioning tendons and anchorages 

can be found in the Florida Department of Transportation’s Structure Manual, Article 4.5 [53]. 

3.1.1.6 Fire / Extreme Heat

A fire or extreme heat event will cause high thermal gradients in a concrete girder, and as a result 

the surface layers of the concrete girder could expand and eventually spall off the cooler, interior 

portion of the girder. During an extreme heat event, when temperatures approach 800°F to 

1200°F, there can be a significant loss of strength in a concrete girder. The temperature, at 

which a concrete beam will fail, is mainly dependent upon the type of aggregate used. Concretes 

made with carbonate aggregates, such as limestone of dolomite, are relatively unaffected by 
39

temperature until they reach 1200°F to 1300°F, at which time they rapidly lose strength. 

Aggregates such as quartzite, granite, and sandstones undergo plastic change at about 800°F to 

1000°F, which causes a sudden change in volume and spalling of the concrete. Lightweight 

aggregates gradually lose their strength at temperatures above 1200°F [46]. 

Several fires on or below precast concrete girder bridges have occurred in recent years, in which 

the fire did not cause a collapse of the structure, but did necessitate some repairs and/or 

replacement. Several of these events are reported on in articles by Shutt [54] and Stoddard [55]:

On June 20, 2007 near Nashville, Tennessee, a fuel tanker rear-ended a loaded dump 

truck resulting in a fire below a two-span, two-celled, hollow box-beam bridge. Analysis 

showed the bridge endured much heat but sustained little damage [54].

On July 28, 2006 in Parker, Arizona, a fuel tanker crashed on a multi-span bridge that 

consisted of AASHTO Type III precast, prestressed girders. Fuel from the tanker spilled 

onto the bridge, beneath the structure, and through deck drains and expansion joints. The 

fire-damaged girders did not show visible signs of loss of prestress, but experienced

various degrees of spalling. The repair plan included restoring girders where the 

reinforcement was exposed, and adding protective coating to deck to mitigate corrosion

[54].

In December 2002, a railroad tanker collision caused a fire under a prestressed concrete 

girder bridge in Tacoma, Washington. Stoddard [55] describes the inspection, testing, 

and analysis that occurred after the fire. All of the girders in the affected span had 

damage to the corners of the bottom flanges that could easily be removed to expose the 

outermost strands. However there was no noticeable loss of prestress.

On July 12, 2005, a significant fire incident occurred near Ridgefield, Connecticut which caused 

significant damage to an adjacent box-beam bridge. The Connecticut DOT coordinated with the 

FHWA’s Turner Fairbank Research Center to investigate the flexural capacity of the beams 

removed from the bridge after the fire. The results of the investigation showed that the beams 

still had sufficient flexural capacity but the long-term viability of the beams was questionable. 

The visual and petrographic examinations showed that the damage to the bottom flange concrete 

was sufficient to allow pathways through the concrete to the depth of the bottom prestressing 

stands [56]. This could potentially lead to the accelerated deterioration of the bottom row of 

strands due to water ingress and subsequent corrosion. A photo of the bottom face of one of the 

tested beams is provided in Figure 3-5.
40

Figure 3-5 Photo of the bottom face of a tested beam subjected to fire while in service showing 

that the concrete has deteriorated such that sufficient pathways through the concrete to the 

bottom layer of prestressing strands have developed (taken from [56]).

Bridge designers need to be aware of the fact that a fire can occur below a concrete girder 

superstructure. Depending on the importance of the structure, it may be necessary to investigate 

the bridge behavior due to an extreme heat event to ensure that a collapse does not happen. A 

design could be developed that would allow some delay before a collapse would occur, which 

would allow human lives to be saved. 

3.1.1.7 Blast / Terrorist Attack 

See section 2.1.1.6 of the Steel Girder Failure Framework. 

3.1.1.8 Overload 

See section 2.1.1.7 of the Steel Girder Failure Framework. 

3.1.1.9 Vehicle and Vessel Collisions 

See section 2.1.1.8 of the Steel Girder Failure Framework. 

3.1.1.10 Flood/Storm Surge 

A bridge with a small vertical clearance over a waterway could be vulnerable to damage from a 

debris flow and storm surge in a flood. If the vertical clearance is small, it is possible that the 

girders of the structure will cause flood debris to be stopped at the bridge. This debris stoppage 

and water flow could lead to additional lateral loads on the girders that were unanticipated during 

design. Wardhana and Hadipriono [1] found that 16 cases of bridge failure (any type of bridge) 

derived from debris flows in the same year, 1995, resulting from flash flooding in Madison 

County, Virginia. 
41

In August 2005, several concrete girder bridges were damaged or completely destroyed in 

Alabama, Louisiana, and Mississippi during Hurricane Katrina. Padget et al. [57], used data 

from 44 damaged bridges to develop relationships between storm surge elevation, damage level, 

and repair costs. The authors point out that several traditional fixed spans displaced due to a 

combination of buoyant forces and pounding by waves. The US-90 Biloxi-Ocean Springs 

Bridge suffered severe damage due to a combination of storm surge and wind/wave induced

loading [57]. The four-lane, 1.9 mile, multi-span concrete girder bridge had low-lying spans. 

The storm surge caused severe damage to the bearings, and most connections between the deck 

and the pier caps were destroyed, allowing free movement of the spans. In fact, several spans on 

the western half of the bridge became completely unseated and were submerged in the bay, as 

shown in Figure 3-6. In traditional hurricane prone areas, the designer should take measures to 

reduce the likelihood of failure caused by storm surge: the bridge could be designed to a higher 

elevation, provide details such as transverse shear keys to prevent lateral movement, or tie downs 

to prevent upward movement. 

Figure 3-6 Photo showing the spans of the US-90 bridge completely unseated and submerged in 

the water due to storm surge induced loading (taken from [57]).

3.1.1.11 Seismic

See section 2.1.1.9 of the Steel Girder Failure Framework. 

3.1.2 Superstructure - Bearings 

See section 2.1.2 of the Steel Girder Failure Framework. 

3.1.3 Superstructure - Concrete Deck/Railings 

See section 2.1.3 of the Steel Girder Failure Framework. 
42

3.2 Inspection Category 

The fault tree that follows the Inspection category is shown in Figure 3-7. Similar to a steel 

girder bridge, the lack of inspection or inadequate inspection can lead to a failure of a concrete 

girder bridge. Obviously, for a concrete bridge, it is difficult to inspect the steel reinforcement, 

prestressing and/or post-tensioning steel. However, when possible, details should be 

incorporated into the design by the bridge designer to ensure that the bridge can be inspected by 

normal methods. Details that are difficult to inspect may lead to a condition in which 

reinforcement corrosion or concrete cracks go undetected. During the design the bridge designer 

must consider whether certain details need to be inspected. Some details, such as prestressing 

strands and internal post-tensioning tendons, can not be inspected by typical means and methods. 

If future evaluation is needed, provisions like externally accessible strain gauges, or corrosion 

potentiometers, must be built in. External post-tension ducts are more inspectable than internal 

ducts. 

Some concrete girder bridges are much more difficult to inspect than others. For example, 

adjacent prestressed concrete box girder bridges tend to be difficult to inspect. There is not a 

practical manner in which to asses the condition of shear keys and grouting between the adjacent 

box girders. The shear keys and grouting can deteriorate due to the use of deicing salt solutions. 

If the bridge is being designed for an environment in which deicing salts are used, it may be 

necessary to consider the loss of the shear keys for load rating purposes. 

Figure 3-7 Portion of the precast concrete girder bridge fault tree showing the Inspection 

Category only, with three Basic Events provided.

The application of architectural treatments to a concrete girder may make it difficult to inspect a 

suspect area. The presence of architectural treatments will impair sounding techniques (hammer 

tapping, for example) often used to determine if the concrete, mild reinforcement, and/or 

prestressing strands are deteriorated. 

Access holes must be provided in larger concrete box girder bridges, which allow bridge 

inspectors access into the box itself. In a continuous box girder bridge (segmental), access holes 

must also be provided in the internal full depth diaphragms at the supports, which allow bridge 

inspectors to pass through from span to span. Also, access is needed to all cells in multi-cell 

structures. As an example, the Florida Department of Transportation’s Structures Manual, 
43

Article 4.6.1 [53] provides details regarding access in concrete box girders. FDOT recommends 

providing access doors at intervals of no more than 300 feet; access openings should be no less 

than 32 inches wide by 42 inches tall; and access entrances should not be placed over traffic 

lanes or locations that could endanger bridge maintenance personnel or the traveling public.

3.3 Construction Category

The fault tree that follows the Construction category is shown in Figure 3-8. Similar to the steel 

girder bridge framework, this part of the overall fault tree may not necessarily apply to the bridge 

designer, but may be more applicable to a specialty engineer working for a contractor. However, 

it is necessary that the bridge designer is aware of the potential construction issues when 

developing a new bridge design in order to ensure that the bridge is constructible.

Many of the basic events shown in the Construction category of the fault tree in Figure 3-8 are 

similar to the basic events for steel girder bridges. A few items will be highlighted in this 

portion of the framework for concrete girder bridges, however the reader is directed to section 

2.3 for basic events not discussed.

Figure 3-8 Portion of the precast concrete girder bridge fault tree showing the Construction 

Category only, with several Events and Basic Events provided.
44

3.3.1 Girder Erection

The lack of proper temporary bracing during girder erection can lead to a failure of a concrete 

bridge. Tremblay and Mitchell [58] report on a concrete bridge which utilized precast 

pretensioned AASHTO Type V girders that had two spans collapse during construction. On 

June 18, 2000, the four-span Souvenir Boulevard Bridge in Laval, Quebec, had four girders slide 

off their pot bearings in the two interior spans. The girders had been erected and formwork 

placement nearly completed for diaphragms and the deck slab, but the deck was not to be placed 

for a few more days. Guided and non-guided pot bearings were used at several locations 

throughout the structure. The authors concluded that the girders were not adequately braced 

during construction for the pot bearings used [58]. The use of sliding pot bearings coupled with 

insufficient temporary bracing resulted in the girders being in an unstable state of equilibrium,

where collapse could be triggered by a very small load or disturbance [58].

On the August 9, 2007, nine of the eleven precast, prestressed concrete girders in a span fell from 

the piers. The investigation established that the incident could have been prevented by installing 

adequate temporary lateral bracing to stabilize the concrete girders, individually and collectively, 

until permanent reinforced-concrete diaphragms and the deck were completed [59]. 

Furthermore, the investigation pointed out that inadequately braced concrete girders set on 

elastomeric bearing pads can become unstable over time [59].

In concrete girder bridges, especially those that are cast-in-place, the formwork design must be 

considered by the specialty engineer. Improper use of the formwork, or poor design, could lead 

to a failure during bridge construction. ACI 347-04: Guide to Formwork for Concrete [60] 

provides the engineer with guidance for the design and specification of formwork for concrete 

structures. Additional information regarding concrete formwork design can be found in Hurd 

[61]. The reference provides the engineer with formwork material properties, design data, 

construction suggestions, and has guidance relating to structural design details for formwork.

3.3.2 General Construction

The design of temporary support structures must be fully considered by the bridge designer

and/or specialty engineer for a concrete girder project, as appropriate fir a given project. The use 

of improperly placed and/or designed temporary supports will lead to a failure during 

construction, as shown in Figure 3-4. Hart et al. [62] presented the results of an investigation of 

the collapse of a temporary support for a cast-in-place, post-tensioned concrete box girder bridge 

in California. The authors concluded that the temporary support collapsed because it was 

constructed with an excessive initial out-of-plumbness, resulting in an inability of resisting the 

gravity loads above. 

3.4 Fabrication Process Category

The fault tree that follows the Fabrication category is shown in Figure 3-9. This part of the 

overall fault tree will apply to the bridge designer, construction inspector, as well as the specialty 

engineer working for a contractor or erector. As shown in Figure 3-9, there are several basic 
45

events shown that can trigger a failure in a bridge project related to the fabrication of the 

concrete superstructure. 

Some of the basic events shown in the Fabrication portion of the fault tree in Figure 3-5 are 

similar to the basic events for steel girder bridges. A few items will be highlighted in this 

portion of the framework for concrete girder bridges, however the reader is directed to section 

2.4 for basic events not discussed. 

Figure 3-9 Portion of the precast concrete girder bridge fault tree showing the Fabrication 

Category only, with several Basic Events provided.

In some cases, it has been noted that formwork will tend to float or bow during precast concrete 

girder fabrication, especially for box girders. This float or bow will tend to cause the webs on 

one side of the box to be slightly wider than the other side. Although this float or bowing will 

not affect the flexural behavior of the girder itself, it could lead to inadequate concrete cover. It 

was noted in the report regarding the Lakeview Bridge collapse that in some locations, the webs 

of the concrete box differed in thickness by one inch [43]. 

The correct placement of prestressing/post-tensioning strands and/or mild reinforcement must be 

ensured during girder fabrication. Although a difference in the exact location of the 

reinforcement may not affect the girder’s capacity, it could results in a smaller concrete cover 

than given by the design plans. For the precast prestressed concrete box-girder tested for the 

investigation of the Lakeview Bridge collapse, it was noted that the bottom strand was only 3⁄4” 

from the edge of the beam, where the prescribed value per design was more than 1 1⁄2” [43]. A 

condition such as this could lead to more significant spalling due to the reduced concrete cover. 

Similar to steel girder bridges, the shipping and handling of concrete girder field pieces must be 

considered by the bridge designer during the design process. Permits may be required. Longer 

girder field pieces may be too heavy to lift with typical erection cranes and transported by typical 

methods. If longer girder field pieces are used in the design, the bridge designer should contact a 

transporter to verify that the necessary equipment and permit capabilities are available. 
46

4.0 SUBSTRUCTURE FAILURE FRAMEWORK 

A general fault tree for the case of a bridge substructure failure is developed and presented in this 

section. As discussed earlier, failure in this framework refers to a total collapse of the bridge 

system or an event that renders the substructure with a critical defect. Similar to previous 

frameworks, this substructure framework is developed for a general case, as shown in Figure 4-1. 

As shown in Figure 4-1, several causes of substructure failure are related to extreme events, such 

as scour due to flooding, vessel collisions, and earthquake loading. These topics will be 

discussed in this section. However, the reader is referred to NCHRP Report 489: Design of 

Highway Bridges for Extreme Events [63] additional information regarding extreme event 

loading combinations.

Figure 4-1 General substructure fault tree with main categories of Concrete Substructures, 

Steel Substructures and General Substructures, and several basic events shown for each.
47

4.1 General 

4.1.1.1 Scour and Flood 

Scour is defined as the erosion of streambed or bank material from bridge foundations due to 

flowing water, as illustrated in Figure 4-2. A majority of bridge failures in the United States and 

elsewhere are the result of scour [2]. Wardhana and Hadipriono [1] found that 243 bridge 

failures, out of a total of 503 recorded failures, were related to flooding and scour. The 

AASHTO LRFD Bridge Design Specifications [2] require that scour at bridge foundations be 

designed for 100-year flood event or from an overtopping flood of lesser recurrence interval. 

Additionally, the bridge foundations are to be checked for stability for the 500-year flood event 

or from an overtopping flood of lesser recurrence interval. 

Figure 4-2 Sketch showing potential scour locations for a general bridge crossing over a river 

in which the concrete supports are located within the limits of the waterway.

= Potential Scour Location

Normal River 

Elevation

Pier Pier

FHWA Hydraulic Engineering Circular No. 18: Evaluating Scour at Bridges (HEC-18) [64] 

provides guidelines for designing bridges to resist scour, evaluating existing bridges for scour 

vulnerability, inspecting bridges for scour, and improving the estimation scour at bridges. 

Additionally, the HEC-18 document divides the total scour at a bridge foundation into three 

components: 

Long-term aggradation and degradation: elevation changes in the streambed of the 

waterway caused by erosion and deposition of materials. 

Contraction scour: resulting from the removal of material from the channel bed and/or 

banks caused by encroachment of the embankments into the waterway. 

Local scour: removal of material from around the area of the bridge foundation caused by 

acceleration of water flow associated with a flood or high-water event. 

An example of a foundation scour failure related to severe flooding is the collapse of the 

Schoharie Creek Bridge in 1987. The Schoharie Creek Bridge carried the New York State 

Thruway over Schoharie Creek, near Amsterdam, New York. The bridge consisted of five 

simply supported steel spans, supported on two abutments and four concrete piers. Each 

concrete pier consisted of two columns connected at the top with a pier cap beam, and at the 

bottom the columns framed into a concrete plinth on top of a shallow spread footing. Pier 3 had 

failed due to scour deterioration mechanism: the riprap protection around the footing was 

inadequate against the stream bed erosion, and the plinth and footing fractured in tension causing 
48

the pier to collapse [7]. Additionally, the shallow foundations utilized in the pier design more 

than likely contributed to the scour condition. 

The Schoharie Creek Bridge collapse illustrates the importance designing bridges to resist scour 

as well as the need to properly inspect and maintain bridges vulnerable to scour. Pier and 

abutment foundations must be designed taking into account the design scour depth. Scour design 

depth can be determined in accordance with HEC-18 [64] and the AASHTO LRFD Bridge 

Design Specifications [2]. Placing spread footings below the scour design depth or extending 

piles or drilled shafts below the scour design depth can help to reduce the potential for scour. 

For additional information regarding the prevention of scour, the reader is referred to HEC-18

[64], NCHRP Report 587: Countermeasures to Protect Bridge Abutments from Scour [65], and 

NCHRP Report 593: Countermeasures to Protect Bridge Piers from Scour [66]. 

In addition to scour, bridge designers must consider lateral loads caused by significant flooding 

events, as required by the AASHTO LRFD Bridge Design Specifications [2]. Weber et al. [67] 

report on a collapse of a temporary bridge during a flooding event. On May 26, 1989, a steel pile 

bent supporting two spans of a one-lane temporary ridge over the Great Miami River in 

Hamilton County, Ohio collapsed into the river, killing two people. Floating debris in the 

flooded river had struck the pile bent, leading to the collapse. The authors concluded that the 

primary cause of failure of the pile bent was the lack of structural capacity for lateral loadings 

[67]. The steel pile bent had been proportioned for vertical loads, but not necessarily for lateral 

loads, since some of the lateral load cases investigated resulted in a factor of safety less than 1.0. 

The bracing between the H-piles were mainly positioned to resist in-plane buckling of the Hpiles and not necessarily to resist lateral loads resulting from flooding. The collapse of this 

bridge shows that bridge designers must give careful consideration to extreme lateral loads that 

can be placed on substructure units, for permanent and temporary bridges.

4.1.1.2 Vessel and Vehicle Collision

Bridge designers must consider the lateral loads imparted to bridge piers by ships and/or barges 

when the structure being designed crosses a navigable waterway. Similarly, for an overpass 

structure, lateral loads resulting from vehicle or train collisions must be considered when bridge 

piers are located near traffic lanes or a railroad below. Collisions with bridge piers by 

barges/ships, trains, or vehicles that cause a bridge to fail can not only result in the loss of human 

life, but will damage the transportation system and economy, especially for failures occurring on 

major thoroughfares. Wardhana and Hadipriono [1] note that 59 bridge failures, or 12 percent of 

the total number of bridge failures studied, resulted from land and marine vehicle collisions.

There have been several reported incidents of piers weakened or destroyed by vessel or vehicle 

collisions, subsequently causing a bridge failure. On May 9, 1980, during a significant storm, an 

oceanic freighter collided with a pier, causing more than 1200 feet of the southbound steel truss 

structure of the Sunshine Skyway Bridge to collapse into the Tampa Bay. The vessel collision 

resulted in the death of 35 people [68]. The new cable-stayed bridge utilized a main span that 

was 50 percent wider than the original main span that served as the shipping lane. The main 

span piers and some of the approach piers are surrounded by a dolphin protection system, which 

are circular cells of sheet piling filled with rock and/or concrete.
49

On May 28, 1993, a towboat pushing an empty barge collided with a support pier of the eastern 

span of the Highway Route 39 Judge William Seeber Bridge in New Orleans. The impact 

severed a river pier, causing two approach spans and a two-column bent to collapse onto the 

barge and waterway [69]. Two automobiles fell with the bridge, resulting in one fatality and two 

serious injuries. The canal was closed to navigational traffic for 2 days, and the bridge was 

closed to vehicular traffic for 2 months.

On May 26, 2002, a towboat with two barges collided with a pier of the I-40 Bridge in Webbers 

Falls, Oklahoma, killing 14 people and injuring several others [70]. The traffic in both directions 

of the major east-west national corridor was abruptly stopped. Reinforced concrete piers utilized 

spread footings and pier protection was constructed only on the upstream side of the main span, 

and one at each of the main span piers only. The pier that was struck by the barges was an 

unprotected approach span pier.

El-Tawil et al. [71] report on three events that led to the loss of human life and failure of an 

overpass bridge due to a vehicle collision. 

In 1993 a tractor with a bulk-cement-tank semi-trailer traveling on I-65 collided with a 

supporting column of County Road 22 in Evergreen, Alabama. The pier collision 

resulted in two spans of the overpass collapsing onto I-65, killing two drivers who 

collided with the collapsed bridge. 

In 2002, a tractor trailer traveling on I-45 in Dallas, Texas hit a concrete pier column for 

the Highway 14 overpass. The overpass collapsed, killing one individual. 

In 2003, a semi-trailer collided into a median support on a bridge crossing over I-80 near 

Big Springs, Nebraska. The collision caused the overpass to collapse, killing one person 

and severely disrupting traffic on I-80. 

El-Tawil et al. [71] address the issue of vehicular collisions with bridge piers, and present results 

of detailed finite element analyses of various collision scenarios.

Computer analyses may be warranted for complex and/or significant structures that could be 

subject to collision loads. The bridge designer and/or owner must also consider the use of 

protective devices. Fender type systems consisting of rubber, steel, or concrete, or dolphin type 

protection systems can reduce collision force effects in the pier by absorbing the impact energy. 

Protective islands can be effective in vessel collision protection. Crash walls can be designed to 

resist vehicle and train loads for overpass structures. Furthermore, a framed structure such as a 

concrete box girder bridge with an integral pier cap is more likely to distribute a collision load 

throughout the structure than a girder-slab structure.

Article 3.14 of AASHTO LRFD Bridge Design Specification [2] addresses design guidelines for 

vessel collisions. The requirements in the AASHTO LRFD Bridge Design Specifications are 

adapted from the AASHTO Guide Specification and Commentary for Vessel Collision Design of 

Highway Bridges [72], using the Method II risk acceptance alternative. Both of these 

specifications provide designers with guidelines related to the risk of ship and barge collisions 

and the design of pier protection systems. Additionally, the AASHTO [2] and AREMA [73]

Specifications provide guidance to bridge designers regarding required clearance envelopes.
50

4.1.1.3 Blast / Terrorist Attack

Blast loads resulting from explosions and/or terrorist attacks that occur below the bridge deck 

may impart large lateral forces on the substructure units depending on the proximity to piers or 

columns. The forces may cause large displacements, and shear or flexural failures, resulting in a 

failure of the bridge system. The designer, owner, and/or security personnel should perform a 

risk assessment to determine the threats that a particular bridge could be vulnerable to. 

Depending on the vulnerability, importance, and the risk of terrorist activity associated with the 

structure being designed, the designer may need to consider the effects of blast loads on the 

superstructure. The effects of blast loading can typically be investigated through computer 

simulations and the use of finite element analysis procedures. Additional information regarding 

blast loads and terrorist attacks on bridges can be found in Section 2.1 and 3.1 of this document.

4.1.1.4 Seismic

An earthquake will cause vertical, lateral, and/or rotational movements of the substructure that if 

excessive or unaccounted for, can result in collapse or partial collapse. As appropriate for the 

seismic zone, the bridge designer must consider these loadings and effects in the design of the 

bridge system. In addition, the designer must also provide proper for: steel connections, concrete 

column confinement, bearing restraints, shear keys, etc. 

Some typical items, not an all-inclusive list, which bridge designers should consider with regard 

to substructure and seismic design are as follows:

Provide adequate concrete column confinement

Avoid diaphragm abutments that force failure underground, in which earth moving 

equipment may be required for a post earthquake inspection

Provide plastic hinges in areas that may not directly lead to a collapse of the structure

The use of approach slabs in case of settling of the substructure.

Additional information and references regarding seismic design in highway bridges is provided 

in section 2.1.1.9 of this document.

4.1.1.5 Other Substructure Failure Events

In addition to the above events, there are other failure events a bridge designer must consider in 

the design of substructure units, as shown in Figure 4-1. A soil-bearing failure that results in 

vertical and/or rotational movement of the substructure unit can cause a bridge to fail or develop 

a critical defect. If settlement or movement is a known possibility, the bridge designer should 

consider including the force effects this movement will have on the bridge system with all 

service and strength limit states.
51

4.2 Concrete Substructure

4.2.1.1 Reinforcement Corrosion

Bridge substructure typically consists of reinforced concrete columns and pier caps, concrete 

footings, steel piling, concrete shafts, or post-tensioned elements. Deterioration (corrosion) of 

the reinforcement caused by the combination of water and deicing salts can cause a failure of the 

bridge substructure. The corrosion of the reinforcement will cause the concrete to crack and 

spall, furthering the possibility of a failure. 

Corrosion of the reinforcement can result from inadequate drainage of the superstructure. For 

example, inappropriately detailed drainage components can often become clogged and not allow 

the removal of water and deicing salts from the superstructure. The water and deicing salts can 

then escape the drainage piping, and flow along a portion of the substructure, creating a 

corrosive environment. 

Substructure units that are part of an overpass structure are prone to deicing salt-spray because 

the substructure units are located in what is commonly referred to as a splash-zone. Once 

deicing salts are applied on the roadway being crossed by the superstructure, traffic passing 

through will cause the salt and water combination to become airborne, with some of the deicing 

salt/water solution splashing onto the nearby substructure. This solution will help to create an 

environment suitable for reinforcement corrosion. 

Additionally, substructure units located in a coastal environment can be subject to ocean salt 

spray. Depending on the aggressiveness of the oceanic environment, the bridge designer may 

need to specify a type of corrosion inhibitor to be used in the substructure units. Adequate 

drainage elements and the consideration of deicing salt spray, or oceanic salt spray, will help to 

prevent corrosion of the concrete reinforcement, and subsequent deterioration of the concrete.

4.2.1.2 Thermal, Creep, Shrinkage

In concrete substructure units, forces related to temperature changes, creep, and shrinkage could 

have a significant effect on the behavior of substructure and the entire bridge system. For 

example, short multi-column bents where the columns are tied together at the top with a cap 

beam can have significant component force effects resulting from thermal, creep, and shrinkage 

loadings. If these force effects are not considered during design, the substructure can collapse or 

develop a critical defect. The bridge designer must consider the effects of temperature change, 

creep, and shrinkage in the design of reinforcement concrete substructure units, especially when 

the units become large.

4.3 Steel Substructure

Similar to concrete substructure units, steel substructure units are susceptible to corrosion caused 

by salt spray from deicing salts or a coastal environment. The bridge designer must be aware of 

this possibility given the location of the structure, as specific the proper corrosion protection.
52

Furthermore, steel substructure units may need to be investigated for fatigue and fracture. Given 

that steel substructure units are often non-redundant, a box-girder cross beam for example, 

members are typically designated on the design plans as fracture critical elements, and must be 

tested as such. Further information regarding fatigue and fracture in steel elements can be found 

in section 2.1.1.2 of this document.
53

5.0 COMPLEX BRIDGE FAILURES – LESSONS LEARNED

For complex bridges, such as trusses, arches, suspension, and cable-stayed, a fault tree will not 

be presented. Many of the potential failure mechanisms previously discussed in sections 2, 3,

and 4 apply to these bridge types. For example, failure events such as scour; corrosion; fatigue 

and fracture; earthquake, blast and terrorist activity; detailing and construction issues; and vessel 

collisions should be investigated for complex bridges when applicable. Instead of repeating 

these events and others, a “lessons learned” approach is employed for these complex bridges. 

Specific failures and the lessons learned from those failures will be presented and discussed. It is 

anticipated that the discussion of these failures, and the lessons learned, will aid in the prevention 

of similar failures in the future. 

5.1 Steel Truss Bridges

5.1.1 Quebec Bridge, 1907

One of the more well known failures of a steel truss bridge is the collapse of the Quebec Bridge 

on August 29, 1907, during construction, killing 75 workers. The Quebec Bridge was a threespan cantilever truss, with an 1,800 ft center span. Pearson and Delatte [74] provide a detailed 

account of the Quebec Bridge failure, including events that led up to the failure, highlights of the 

commission’s report, causes of the failure, and ethical aspects. A distinguished panel was 

assembled to investigate the collapse of the Quebec Bridge. They found that the main cause of 

the bridge’s failure was the improper design of the latticing of the compression chords. The 

allowable stresses used to design the members were considered unusually high for the time 

period. It was determined that the compression members did not have adequate stiffness to resist 

buckling. Additionally, member stresses were not recalculated and checked once the center span 

length was increased by 200 ft during the design phase, resulting in several overstressed 

members. 

Pearson and Delatte [74] also highlight the failure of the engineer to recognize that a serious 

problem was developing as the structure was being erected. As the bridge was erected, 

ironworkers noticed significant midpoint displacements in some of the truss compression 

members. The lead engineers believed these deflections were small and not problematic, and 

were the result of some unknown preexisting condition, or hit by another component during 

erection. However, these noticeable displacements were the warning signs that something was 

wrong, and in this case, the start of compression member buckling.

As a result of the Quebec Bridge collapse, research regarding column buckling was initiated. 

The collapse of the bridge demonstrates the need to perform thorough calculations, especially 

when there is a change in bridge geometry and/or size. Furthermore, the collapse demonstrates 

the importance of communication during erection. If problems are noticed in the field by 

workers, supervisors, or inspectors, the problems need to be investigated immediately by the 

engineers. A successful bridge project, from design to final construction, requires the entire 

team to be working together and communicating.
54

5.1.2 Yadkin River Bridge, 1975

On February 23, 1975, the steel through-truss span of the Yadkin River Bridge collapsed in 

Siloam, North Carolina. One vehicle was on the bridge when it collapsed, and six additional

vehicles drove off the approach spans after the main span truss had collapsed. The collapsed 

resulted in four deaths, and 16 injuries [75]. The Yadkin River Bridge was a 225 ft simple span 

steel truss, with the two truss planes 13 ft apart. The truss had a depth of 36.5 ft at midspan. The 

bottom chord consisted of eyebars; the top chord and end posts consisted of two channel sections 

with cover plates and lacing; vertical members were typically channels with lacing; diagonals 

were typically square bars used singly or in pairs; top and bottom laterals and sway frame 

bracing were round bars; and the lateral struts and portal members were single or double angle 

struts. 

An investigation completed after the collapse concluded that the failure was due to a vehicular 

collision with one of the truss’s end posts [76]. The resulting redistribution of dead load caused 

the failure of additional members, resulting in a torsional rotation of the structure. This rotation 

combined with the loss of upper laterals, allowed the top chord to buckle, resulting in the 

collapse of the bridge. The investigation also found that the truss members were adequately 

designed for their time period, and the corrosion that did exist on the members had no practical 

effect on the load-carrying capacity, and the physical and chemical properties of the steel were 

typical for the time period.

The collapse of the Yadkin River Bridge demonstrates the need for bridge designers to consider 

the loss of a single member during the design of a steel truss. The bridge designer must verify

that there is enough redundancy built into the structure. Computer simulations, via finite element 

analyses, can be performed by the designer to investigate the behavior of a steel truss bridge after 

the loss of a single member. Several member loss scenarios should be investigated to verify that 

the structure is adequately redundant.

5.1.3 Minneapolis I-35W Bridge, 2007

On August 1, 2007, the eight-lane, three-span, steel deck truss I-35W Bridge in Minneapolis, 

Minnesota, collapsed into the Mississippi River, killing 13 people. The bridge was constructed 

in 1967, the main span of the bridge was 456 feet, and it carried approximately 140,000 vehicles 

daily through downtown Minneapolis. The collapse of the structure not only resulted in the loss 

of life, but created a major disruption of commerce. The bridge had been rated as structurally 

deficient since 1990, and had undergone annual inspections by the Minnesota Department of 

Transportation since 1993. The most recent inspection, complete in June 2006, indicated that 

there was cracking and fatigue problems, causing the bridge to receive a sufficiency rating of 50 

percent on a scale of 0 to 100 percent [77]. 

At the time of the collapse, the I-35W bridge surface was being repaired requiring the closure of 

some traffic lanes. In addition, there was construction equipment and materials on the bridge

when it collapsed. This led to the FHWA issuing a technical advisory regarding construction 

loads on bridges, Technical Advisory 5140.28 [78], since the NTSB had identified the 

construction loading on the bridge as a possible contributor to the collapse. The issuance of the 
55

technical advisory highlights the importance that the consideration of construction loads can 

have on a bridge project. The specialty engineer, working for a contractor on a resurfacing or 

complete rehabilitation bridge project, must consider the effects that construction loading will 

have on the structure. The specialty engineer must check that members are not overstressed or 

displacements are too large when construction loads are applied. Additionally, it must be 

verified that the structure and its components do not experience any instability during repair 

and/or rehabilitation operations. 

At the time of this writing (December 2008), the National Transportation Safety Board (NSTSB)

was conducting an investigation into the collapse and issued an interim report regarding findings 

of the investigation. The investigation discovered that the original design process led to an error 

in the sizing of some of the gusset plates in the main span trusses. The results indicate that eight

of the 112 gusset plates were undersized and did not provide the factor of safety to be expected 

in a properly designed bridge. At these locations the gusset plates were approximately half of 

the thickness required. The results and calculations associated with this investigation can be 

found in the FHWA’s interim report authored by Holt and Hartmann [79]. 

The findings of the interim investigation led to the NTSB issuing a safety recommendation to the 

FHWA, stating that “for all non-load-path-redundant steel truss bridges within the National 

Bridge Inventory, required that bridge owners conduct load capacity calculations to verify that 

the stress levels in all structural elements, including gusset plates, remain within applicable 

requirements whenever planned modifications or operation changes may significantly increase,” 

[80]. 

Furthermore, the NTSB held a public meeting in November of 2008, and issued a synopsis of the 

Safety Board’s final report, which was still under review at the time of this writing (December 

2008). In the synopsis, the NTSB determined that the probable cause of the collapse was due to 

the inadequate load capacity of the gusset plates at node U10, due to a design error, and which 

failed under a combination of (1) substantial increases in the weight of the bridge resulting from 

previous modifications, and (2) the traffic and concentrated construction loads on the bridge on 

the day of the collapse [81]. The NTSB further stated that: (1) the failure of the original design 

firm’s quality control procedures to ensure appropriate main truss gusset plate calculations were 

performed and inadequate review by the State and Federal transportation officials contributed to 

the design error, and (2) the generally accepted practice among transportation officials of giving 

inadequate attention to gusset plates during inspections and excluding gusset plats in load rating 

analyses contributed to the bridge collapse.

It is imperative in rehabilitation projects that the bridge designer investigates the 

capacity/demand of all members, including gusset plates, whenever there is a significant change 

in loading, an increase in deck thickness for example, or changes in operational use such as 

additional traffic lanes or vehicle types. Guidance for the load rating evaluation of gusset plates 

in truss bridges has been provided by the FHWA [82] for rating in accordance with the Load and 

Resistance Factor Rating Method (LRFR).
56

5.2 Suspension Bridges

5.2.1 Tacoma Narrows Bridge Collapse, 1940

Probably the most famous bridge failure is the collapse of the Tacoma Narrows Suspension 

Bridge on November 7, 1940, over the Puget Sound in Washington State. The bridge opened on 

July 1, 1940, had a total length of 5,000 ft, and a center span length of 2,800 ft. The Tacoma 

Narrows Bridge was the first suspension bridge to use steel plate girders to support the roadway

while previous bridges typically used steel trusses to support the roadway. The plate girders 

used in the bridge were 8 ft deep. The dead load and stiffness of the bridge were much less than 

other suspension bridges built previously.

Shortly after the bridge opened it was discovered that under relatively mild wind conditions, the 

bridge deck oscillated severely. These vertical oscillations were a direct result of the 

slenderness, low self-weight, and low dampening ability of the bridge. On the day of the 

collapse, the bridge was closed after measuring a constant wind speed of 42 mph. The bridge 

deck was experiencing 38 oscillations per minute with a vertical amplitude of 3 ft [83]. The 

collapse was initiated by a failure in the center north suspender, followed by more severe vertical 

oscillations, then one of the girders buckled in the middle of the bridge, suspender cables failed, 

and sections of the main span fell progressively from the center to the towers. The Tacoma 

Narrows Bridge collapse is well documented and additional information can be found at a 

website sponsored by the University of Washington Libraries, 

www.lib.washington.edu/specialcoll/exhibits/tnb/ [84].

The collapse of the Tacoma Narrows Bridge initiated the consideration of aerodynamics in longspan bridge design. Wind not only causes static loads on a bridge, but results in a special 

dynamic behavior as well. The collapse shows bridge designers the importance of stiffness, 

rigidity, torsional resistance, and dampening in suspension bridges as they relate to wind. 

Addressing these issues can be accomplished through wind tunnel testing, and computer 

modeling that integrates the wind tunnel test data. 

5.2.2 Silver Bridge, 1967

The Silver Bridge, which spanned the Ohio River between Point Pleasant, West Virginia, and 

Kanauga, Ohio, was the first eyebar suspension bridge built in the United States. The bridge, 

completed in 1928, had a center span length of 700 ft and end spans of 380 ft each. The 

suspension chain on each side of the bridge consisted of two eyebars having a thickness of 2 in. 

and width of 12 in. [85].

On December 15, 1967, the bridge collapsed during the evening rush hour, resulting in 46 

fatalities, and nine injuries [86]. The NTSB concluded that the collapsed was initiated by a 

cleavage fracture in the lower limb of an eye bar on the north chain, on the Ohio side span, at the 

first chain joint away from the tower. The fracture was then followed by a ductile failure in the 

upper limb of the same eyebar, separating the eyebar from the chain. Immediately following this 

loss of this eyebar, the “sister” eyebar failed, resulting in a complete separation in the north chain 

and the onset of the collapse. The NTSB believed that the initial cleavage fracture was caused 

by the development of a critical size flaw over the 40-year life of the structure as a result of the 
57

combined action of stress corrosion and corrosion fatigue. It was noted by Lichtenstein that the 

eye of the eyebars, where the pin fits in, was elongated by one-eighth of an inch for ease of 

erection, and thus allowing a space for corrosion to develop [85]. The detail did not allow for 

visual inspection, without the dismantling of the joint.

The collapse of the Silver Bridge led to the initiation of routine inspections for all bridges, 

through the approval of the National Bridge Inspection Standards (NBIS) in 1968 by the U.S. 

Congress. The collapse also prompted research into metal fatigue, bridge management, and 

nondestructive inspection methods [87].

The failure of the Silver Bridge also shows the need to consider redundancy in design. With 

only two eyebars being used along the suspension chain, the failure of one eyebar resulted in a 

significant increase in loading on the adjacent eyebar and subsequent failure of the chain. The 

use of additional bars may have provided an increased redundancy since the load would have 

been transferred to more than just one adjacent eyebar. Furthermore, the collapse also shows the 

importance of details and connections, and the required ability to inspect them. Bridge designers 

have learned many lessons from the Silver Bridge collapse and these lessons must be 

continuously applied in future bridge designs.

5.3 Cable-Stayed Bridges

Although there have been no significant failures of cable-stayed bridges reported in the United 

States, there are failure events that the bridge designer must take into account during design. 

Similar to suspension bridges, cable-stayed bridges must be checked for aerodynamic effects to 

ensure that the bridge decks do not experience significant oscillations. Computer modeling and 

associated wind tunnel testing are typical requirements for cable-stayed bridges. Additionally, as 

reported on by Kumarasena et al. [88], serviceability problems related to large amplitude 

vibrations of stay cables under certain wind and rain conditions have been observed. In their 

report, Kumarasena et al. (2005) provide design guidelines for the mitigation of wind induced 

vibrations of stay cables that can be used by bridge designers experienced with cable-stayed 

bridge design and wind engineering.

Another potential failure mechanism that should be recognized by the bridge designer for cablestayed bridges is the potential for corrosion of the stay cables. Hamilton et al. [89] provide an 

extensive survey of the stay-cable protection methods and experimental findings regarding 

durability testing of large-scale grouted stay specimens subjected to alternate wet and dry cycles 

with salt solution. Variables in the experimental study included temporary corrosion protection, 

galvanized strands, epoxy coated strands, and greased and sheathed strands. Additionally, 

Hamilton et al. [90] also report on corrosion testing of stay cables encased in grout, in which 

grout mixes were tested with various commonly used admixtures to optimize antibleed, 

viscosity, and corrosion protection.
58

6.0 CONCLUSIONS

This framework has been developed to advise bridge designers to consider potential failure 

scenarios during the design process. General fault trees describe potential contributory elements 

that designers can use to address potential sources of failure during the design process have been 

provided for several common bridge types. 

This framework serves as a general checklist of issues that should be given attention by a bridge 

designer during the design process in order to minimize potential failures during the service life 

and/or construction of the specific bridge being designed. Fault trees allow the designer to 

graphically see various failure combinations and failure paths. The framework can help a bridge 

designer and/or owner to determine whether additional analyses investigating potential failures 

are warranted. 
59

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