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CRITICAL MEMBER REMOVAL AND LOAD REDISTRIBUTION OF A DETERIORATED TRUSS BRIDGE
ABSTRACT
Serious questions surrounding the strength and stability of truss bridges and the adequacy of the aging infrastructure have arisen due to the collapses of the I-35W Bridge in Minneapolis, Minnesota and the Dysart Bridge in Cambria County, Pennsylvania. The behavior of truss bridges when a critical member is removed is at the forefront of research. Being able to understand the load redistribution as well as the global response of deteriorated truss bridges when critical members are removed needs to be fully understood to prevent any further collapses from occurring. A pristine and deteriorated model was created to determine the effects of aging and deterioration on the critical member removal and load redistribution response of a truss bridge. Key information in the analysis was the critical member (assumed to fail when it reached its yield capacity or critical buckling load), the critical load that caused the member to yield, the location of the critical load, and the failure sequence once the critical member was removed from the structure. Eight stacked side-by-side HS-20 trucks caused the pristine model to fail while a loading of seven stacked side-by-side HS-20 trucks caused the deteriorated model to fail. The member failure sequence also was different for the pristine model compared to the deteriorated model. Deterioration and aging was found to have a direct impact on the load redistribution and global failure pattern of the truss bridge.
TABLE OF CONTENTS
LIST OF FIGURES ……………………………………………………………………………………….. v
LIST OF TABLES …………………………………………………………………………………………. vi
ACKNOWLEDGEMENTS …………………………………………………………………………….. vii
Chapter 1 Introduction ……………………………………………………………………………………………… 1
1.1 Background …………………………………………………………………………………………………. 1
1.2 Problem Statement ……………………………………………………………………………………….. 5
1.3 Objectives …………………………………………………………………………………………………… 5
1.4 Scope and Task List ……………………………………………………………………………………… 6 1.5 Summary …………………………………………………………………………………………………….. 8
Chapter 2 Literature Review …………………………………………………………………………………….. 10
2.1 Truss Types…………………………………………………………………………………………………. 10
2.2 Progressive Collapse …………………………………………………………………………………….. 12
2.2.1 Zipper-Type Progressive Collapse ………………………………………………………… 12
2.2.2 Instability-Type Progressive Collapse …………………………………………………… 17
2.3 Truss Bridge Experiments …………………………………………………………………………….. 18
2.4 Modeling …………………………………………………………………………………………………….. 21
2.4.1 General Modeling Considerations ………………………………………………………… 22
2.4.2 Progressive Collapse Modeling ……………………………………………………………. 24
2.5 Condition Evaluation ……………………………………………………………………………………. 30 2.6 Summary …………………………………………………………………………………………………….. 32
Chapter 3 Structure Description ………………………………………………………………………………… 33
3.1 Introduction ………………………………………………………………………………………………… 33
3.2 Rock Creek Bridge ………………………………………………………………………………………. 33
3.3 Beech Creek Veterans Memorial Bridge …………………………………………………………. 37
3.4 Beech Creek Veterans Memorial Bridge Condition Evaluation ………………………….. 42
3.4.1 Bearings ……………………………………………………………………………………………. 42 3.4.2 Floor System ……………………………………………………………………………………… 43
3.4.3 Truss ………………………………………………………………………………………………… 46 3.5 Summary …………………………………………………………………………………………………….. 52
Chapter 4 Modeling Procedure ………………………………………………………………………………….. 53
4.1 Rock Creek Bridge Pristine Model …………………………………………………………………. 53
4.2 Model Validation …………………………………………………………………………………………. 56
4.3 Beech Creek Veterans Memorial Bridge Pristine Model …………………………………… 65
4.4 Examination ………………………………………………………………………………………………… 68
4.5 Summary …………………………………………………………………………………………………….. 72 Chapter 5 Critical Member Analysis and Discussion ……………………………………………………. 73
5.1 Introduction ………………………………………………………………………………………………… 73
5.2 Pristine Model……………………………………………………………………………………………… 75
5.2.1 Loading …………………………………………………………………………………………….. 75
5.2.2 Critical Member Identification …………………………………………………………….. 80
5.2.3 Critical Load Location ………………………………………………………………………… 93
5.2.4 Critical Member Removal and Load Redistribution Analysis …………………… 101
5.3 Deteriorated Model ………………………………………………………………………………………. 118
5.3.1 Loading …………………………………………………………………………………………….. 122 5.3.2 Critical Member Identification …………………………………………………………….. 122 5.3.3 Critical Load Location ………………………………………………………………………… 133
5.3.4 Critical Member Removal and Load Redistribution Analysis …………………… 137
5.4 Comparison of Pristine and Deteriorated Model ………………………………………………. 145
5.5 Conclusions ………………………………………………………………………………………………… 150
Chapter 6 Summary and Conclusions ………………………………………………………………………… 152
6.1 Summary …………………………………………………………………………………………………….. 152
6.2 Conclusions ………………………………………………………………………………………………… 153
6.3 Future Research …………………………………………………………………………………………… 157
Bibliography ……………………………………………………………………………………………………………. 158
Appendix A Figures ………………………………………………………………………………………………… 161
Appendix B Tables ………………………………………………………………………………………………….. 167
Chapter 1
Introduction
1.1 Background
The failures of the I-35W Bridge in Minneapolis, Minnesota (NTSB 2008) and the Dysart Bridge in Cambria County, Pennsylvania (Grata 2007) had led to serious questions surrounding the strength and stability of truss bridges and the adequacy of aging infrastructure in the United States. The Research and Innovative Technology Administration (RITA) published the 2007 Condition of U.S. Highway Bridges by State (RITA 2008) and reported that Pennsylvania had approximately 23,500 bridges and of those bridges 44% were structurally deficient or functionally obsolete. Given the aging infrastructure, the need to understand the behavior and load distribution in truss bridges as deterioration increases is imperative. Understanding the behavior of aging truss bridges may help remediation efforts and prevent catastrophic collapses.
Truss bridges were the primary choice in bridge design from the 1870s through the 1930s and were constructed using timber, wrought iron, and steel. Trusses are comprised of many small members that together can support a large amount of weight and span great distances. In theory, the individual members of a simple truss are only subject to tension and compression forces and not bending. Trusses are also classified by the basic design used (Tennessee Department of Transportation 2008). Shown in Figure 1-1 are the most common trusses: the Warren Truss; the Howe Truss; and the Pratt Truss.
(a) (b)
(c)
Figure 1-1. Examples of Truss Bridge Configurations: (a) Warren Truss, (b) Howe Truss, (c)
Pratt Truss
The Warren Truss shown in Figure 1-1a is the most common truss for both simple and continuous truss bridges because of its ease of construction. For smaller spans, no vertical members are used while for longer spans, vertical members are added to provide extra strength. Warren Trusses are typically used for spans between 50 – 100 meters (160 – 330 feet). The Howe Truss shown in Figure 1-1b has all of the diagonals sloped towards the center of the bridge. In theory the diagonal members handle the compressive forces while the vertical members carry the tensile forces. Howe Trusses are typically used for spans between 9 – 45 meters (30 – 150 feet). The Pratt Truss (Figure 1-1c) is identified by its diagonal members who, except for the end panels, all sloped toward the center of the span. All the diagonal members, except for those in the end panel, are theoretically subjected to tension forces only while the shorter vertical members in the center of the span and the top chords handle the compressive forces. Figure 1-2 displays the forces in a typical Pratt Truss. This thesis will focus specifically on Pratt Trusses because the representative area, Pennsylvania Department of Transportation (PennDOT)
District 2-0 in Clearfield, PA, has a large amount of Pratt Truss bridges.
Figure 1-2. Pratt Truss Idealized Force Diagram
Pratt Trusses, and the majority of all other truss bridges, are classified as fracture critical structures and are composed of fracture critical members. A fracture critical member is an element in tension that if fractured would probably cause a portion or the entire bridge to collapse (Dexter et al. 2005). Fracture critical members fail in a brittle manner, which can be characterized as instantaneous and does not allow for a response and remediation of the problem prior to the failure event. This is unlike ductile behavior which takes some time to occur and, subsequently, can provide some warning signs of impending failure.
A serious problem with fracture critical elements and bridges is the resulting vulnerability the problems create for progressive collapse. Progressive collapse of truss bridges can be initiated by either a rupture (tensile failure) or a buckling (compressive failure) of truss members. Progressive collapse is the collapse of all or a large part of a structure precipitated by damage or failure of a relatively small part of that structure (Nair
2003). The phenomenon is of particular concern since progressive collapse is often (though not always) disproportionate, i.e., the collapse is out of proportion (rupture or buckling of one member can lead to the collapse of the entire span of the truss) to the event that triggers it. Thus, in structures or bridges susceptible to progressive collapse, small events can have catastrophic consequences.
1.2 Problem Statement
The load redistribution in truss bridges when a critical member is removed has come to the forefront of research since the failure of the Minnesota I-35 Bridge. The load redistribution as well as the response of deteriorated truss structures that are susceptible to progressive collapse needs to be fully understood to prevent any further collapses from occurring. This study evaluates the response and load redistribution of pristine and deteriorated Pratt Truss bridges. It will provide a methodology for agencies to create a simple model of a Pratt Truss bridge to obtain information on the current condition of the bridge and the ability to determine the critical member in the bridge.
1.3 Objectives
The primary objectives of this study were to: (1) understand the behavior of Pratt Trusses when a critical member is removed; (2) investigate load redistribution to remaining members; and (3) observe their behavior and the susceptibility of the entire structure to collapse. Lower chord members and their connections, located in the “splash zone” and, more susceptible to deterioration through contact with water and deicing salts, were selected as the members that were studied. A secondary objective was to determine if aging and deterioration affected the load redistribution of truss members and global behavior of the truss bridge by studying bridges built in the early 1930s.
1.4 Scope and Task List
This study investigated the performance of a Pratt Truss when a critical member is removed. The study also examined load redistribution that occurred when the critical member was removed and also if aging and deterioration played a role in the global failure mode of the truss bridge. The behavior of the connections was also explored by creating a sub-model of deteriorated connections to determine if deterioration played a role in the global load distribution in the truss. This study did not look at the influence of fatigue on the members of the truss.
This investigation included both analytical and field evaluation components. The analytical component developed models using SAP2000 that accurately predicted the response of a Pratt Truss bridge as well as modeled the progressive collapse and deterioration of an actual Pratt Truss Bridge located in central Pennsylvania. The first bridge, the Rock Creek Bridge, was from a previous project and had reported experimental and analytical results (Azizinamini et al. 1997). The other Pratt Truss
Bridge modeled was chosen from a group of representative structures in central Pennsylvania.
The field evaluation component involved condition evaluations of the
Pennsylvania Bridge to determine the amount of deterioration that had occurred.
Condition evaluations followed the NCHRP Synthesis Report 354: Inspection and
Management of Bridges with Fracture-Critical Details (2005a) and the American
Association of State Highway and Transportation Officials (AASHTO) Manual for
Condition Evaluation of Bridges (AASHTO 2003). Previous rehabilitation and the extent of deterioration were provided from old inspection reports. Truss members were evaluated for deterioration and connections were inspected to determine their level of deterioration and actual level of restraint. Computer models were updated to represent each bridge’s current condition and modeled to determine if deterioration plays a role in the final behavior of truss bridges.
Tasks completed in association with the research project included:
- Literature search of all relevant material pertaining to this study. The search included: Pratt Trusses, modeling trusses and progressive collapse, dynamic response of truss members due to the initial progressive collapse, and condition evaluations to account for aging and deterioration.
- Modeled two truss bridges, the Rock Creek Bridge and a selected Pennsylvania Pratt Truss using SAP2000. The models were initially to be constructed from design plans and, as such, will not account for deterioration. All computer models were three dimensional and incorporated material and geometric nonlinearities.
- Validated the Rock Creek Bridge analytical model by comparing it to laboratory test results reported by Azizinamini et al. (1997).
- Based on research by Ghosn and Moses (1998) determined the critical member by applying an incremental analysis load until a member reached its failure point or buckles. This member was then considered the most critical member and was removed from the model. The analysis was rerun using same incremental approach, to determine where the load was being redistributed. Removal of the member simulated the member being incapable of carrying any additional load.
- Completion of a condition evaluation of the selected Pennsylvania Pratt
Truss. The condition evaluation followed procedures outlined in NCHRP Synthesis Report 354: Inspection and Management of Bridges with
Fracture-Critical Details (2005a) and the American Association of State
Highway and Transportation Officials Manual for Condition Evaluation of Bridges (AASHTO 2003).
- From the condition evaluation aging and deterioration were accounted for, in the models, by changing cross sectional properties along the length of the members and by changing connections to realistically model the behavior of the bridge.
- Compared how the pristine models behaved with respect to the deteriorated models by examining critical members and load
redistribution. Also compared the final failure mode of the pristine model to the deteriorated model.
1.5 Summary
Since trusses are one of the oldest types of bridges in the United States, understanding the behavior of this aging infrastructure can help determine the need for rehabilitation and assess the risk of failure. Recent collapses of the I-35W Bridge and the
Dysart Bridge have highlighted the importance of understanding how loads are redistributed when critical members are removed by either deterioration, an extreme loading event or from a collision. This investigation studied the progressive collapse behavior of two Pratt Truss bridges and the effects of aging and deterioration on each bridge’s collapse load and mode.
