NCHRP Web Document 46 (Project 20-7[133]): Contractor’s Final Report Improved Live Load Deflection Criteria for Steel Bridges Prepared for: National Cooperative Highway Research Program Transportation Research Board of the National Academies Submitted by: Charles W. Roeder University of Washington Seattle, Washington Karl Barth University of West Virginia Morgantown, West Virginia Adam Bergman University of Washington Seattle, Washington November 2002 ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB. Contents Summary Acknowledgments Chapter 1 - Introduction 1 1.1. Problem Statement 1 1.2. Directions of Research 2 1.3. Report Content and Organization 3 Chapter 2 - Literature Review 5 2.1. Overview and Historical Perspective 5 2.2. Efect of Bridge Deflections on Structural Performance 9 2.3. Efect of Bridge Deflection on Superstructure Bridge Vibration 15 2.3.1. Human Response to Vibration 15 2.3.2. Field Studies 19 2.3.3. Analytical Studies 23 2.4. Alternate Live-Load Deflection Serviceability Criteria 27 2.4.1. Canadian Standards and Ontario Highway Bridge Code 27 2.4.2. Codes and Specifications of Other Countries 29 2.4.3. W right and W alker Study 30 2.5. Sumary 31 Chapter 3 - Survey of Profesional Practice 3 3.1. Description of the Survey 33 3.2. Results of Survey 34 3.3. Bridges for Further Study 37 Chapter 4 - Evaluation of the Variation in Practice 41 4.1. Introduction and Purpose 41 4.2. Program Operation 42 4.3. Aplication of the Deflection Limits 45 4.4. Consequences of These Results 50 Chapter 5 - Evaluation of Bridges Damaged by Deflection 53 5.1. Introduction 53 5.2. Analysis Methods 54 5.3. Discusion of Damaged Bridge Results 56 5.3.1. Plate Girders with Damaged W ebs at Diaphragm Conections 59 5.3.2. Bridges with Damage in Stringer Floorbeam Connections 66 5.3.3. Bridges with Deck Damage 72 5.3.4. Stel Box Girder Damage 74 5.3.5. Trus Superstructure Damage 76 5.4. Sumary and Discusion 7 Chapter 6 - Evaluation of Existing Plate Girder Bridges 81 6.1. Introduction 81 6.2. Analysis Methods 81 6.3. Description of Bridges 82 6.4. Analysis Results 89 6.4.1. Comparison with AASHTO Standard Specifications 90 6.4.2. Comparison to W alker and W right Recommendations 91 6.4.3. Comparison with the Ontario Highway Bridge Design Code 92 6.5. Concluding Remarks 94 Chapter 7 - Parametric Design Study 97 7.1. Introduction 97 7.2. Methodology 98 7.3. Design Parameters 9 7.4. Results 102 7.4.1. Effect of Variations in Geometric and M aterial Properties 102 7.4.2. Comparison of Re-Designs 107 7.4.3. Comparison with Alternate Criteria 109 7.4.4. Comparison of LFD and LRFD 1 7.5. Final Remarks 12 Chapter 8 - Sumary, Conclusions and Recomendations 13 8.1. Sumary 13 8.2. Conclusions and Recomendations 15 8.2.1. Conclusions 116 8.2.2. Recommended Changes to AASHTO Specifications 18 8.3. Recomendations for Further Study 120 References 123 Apendix A - Sample Survey and Sumarized State by State Results 129 Summary This research has examined the AASHTO live-load deflection limit for steel L bridges. The AASHTO Standard Specification limits live-load deflections to for 800 L ordinary bridges and for bridges in urban areas that are subject to pedestrian use. 1000 This limit is also incorporated in the AASHTO LRFD Specifications in the form of an optional serviceability criteria. This limit has not been a controlling factor in most past bridge designs, but it will play a greater role in the design of bridges built with new HPS 70W steel. This study documented the role of the AASHTO live-load deflection limit of steel bridge design, determined whether the limit has beneficial effects on serviceability and performance, and established whether the deflection limit was needed. Limited time and funding was provided for this study, but an ultimate goal was to establish recommendations for new design provisions that would assure serviceability, good structural performance and economy in design and construction. A literature review was completed to establish the origin and justification for this deflection limit. This review examined numerous papers and reports, and a comprehensive reference list is provided. The work shows that the existing AASHTO deflection limit was initially instituted to control bridge vibration, but deflection limits are not a good method for controlling bridge vibration. Alternate design methods are presented. A survey of state bridge engineers was simultaneously completed to examine how these deflection limits are actually applied in bridge design. The survey also identified bridges that were candidates for further study on this research issue. Candidate bridges either: • failed to meet the existing deflection limits, • exhibit structural damage that was attributable to excessive bridge deflection, • were designed with HPS 70W steel, or • had pedestrian or vehicle occupant comfort concerns due to bridge vibration. The survey showed wide variation in the application of the deflection limit in the various states, and so a parameter study was completed to establish the consequences of this variation on bridge design. The effect of different load patterns, load magnitudes, deflection limits, bridge span length, bridge continuity, and other factors were examined. There is wide variation in the application of the existing deflection limit, because of the variation in the actual deflection limits, the variation in the load magnitude and load pattern used to calculate the deflection, the application of load factors and lane load distribution factors, and other effects. The difference between the least restrictive and most restrictive deflection limit may exceed 1,000%. The load pattern and magnitude have a big impact on this variation. Some states use truck loads, some use distributed lane loads, and some use combinations of the above. Truck loads provide the largest deflection for short span bridges. Distributed lane loads provide the largest deflections for long span bridges. The survey identified a number of bridges which were experiencing structural damage and reduced service life associated with bridge deflections. Design drawings, inspection reports, photographs, and other information was collected on these bridges. They were grouped and analyzed to: • determine whether the damage was truly caused by bridge deflections, • determine whether the AASHTO live-load deflection limit played a role in controlling or preventing this damage, and • examine alternate methods of controlling or preventing this damage. This analysis showed that a substantial number of bridges are damaged by bridge deformation. This deformation is related to bridge deflection. The deformations that cause the damage are relative deflections between adjacent members, local rotations and deformations, deformation induced by bridge skew and curvature, and similar concerns. L None of these deformations are checked with the existing live-load deflection limit. 800 Additional analyses were performed to examine how the deflection limit interacts L with bridge vibration, the span-to-depth ( ) ratio and other design parameters. The D study examined the effect these parameters on the economy and performance of bridge design. The AASHTO live-load deflection limit is less likely to influence the design of L bridges with small ratios and is more likely to control the superstructure member sizes D L as the ratio increases. Application of the deflection limit with truck load only shows D that the existing AASHTO deflection limits will have a significant economic impact on some steel I-girder bridges built from HPS 70W steel. Simple span bridges are more frequently affected by this limit than continuous bridges. However, continuous bridges are also likely to be more frequently affected by existing deflection limits if the span length, L, is taken as the true span length rather than the distance between inflection points in the application of the deflection limit. The study shows that many bridges the satisfy the existing deflection limit are likely to provide poor vibration performance, while other bridges failing the existing deflection limit will provide good comfort characteristics. Lastly, this report summarizes major findings and presents proposed design recommendations and further research requirements. Acknowledgments This research report describes a cooperative research study completed at the University of Washington and West Virginia University. Funding for this work is provided by the National Cooperative Research Program under NCHRP 20-07/133 and by the American Iron and Steel Institute through project entitled "Vibration and Deflection Criteria for Steel Bridges." The authors gratefully acknowledge this support. Chapter 1 Introduction 1.1. Problem Statement (AASHTO, 1996) The AASHTO Standard Specification limits live-load deflections to L L for ordinary bridges and for bridges in urban areas that are subject to 800 1000 pedestrian use. These limits are required for steel, prestressed and reinforced concrete, and other bridge superstructure types. Bridges designed by the AASHTO LRFD (AASHTO, 1998) Specification have an optional deflection limit. The specifications and the LRFD commentary do not provide detailed explanations or justification for these limits. Historically, the deflection limit has not affected a significant range of bridge designs. However, recent introduction of high performance steel (HPS) may change this fact. HPS has a higher yield stress than other steels commonly used in bridge design (Fy=70 ksi and higher as opposed to 50 ksi), and the larger yield stress permits smaller cross sections and moments of inertia for bridge members. As a result, deflections may be larger for HPS bridges, and deflection limits are increasingly likely to control the design of bridges built from these new materials. It is therefore necessary to ask: • How the deflection limit affects bridge performance? • Whether the deflection limit is justified or needed? • Whether it achieves its intended purpose? • Whether it benefits the performance of steel bridges? • Whether it affects the economy of steel bridges? This research study was jointly funded under the NCHRP 20-7 program and the American Iron and Steel Institute, and the research was initiated to determine whether the deflection limits for steel bridges are needed or warranted. The study focuses on steel bridges, and the particular goals are - - 1 - • to determine how the deflection limits are employed in steel bridge design in the US, • to determine the rationale behind existing design provisions and to compare AASHTO provisions to design methods used in other countries, • to evaluate the effect of AASHTO and other existing deflection limits on steel bridge design and performance, and evaluate where existing deflection limits prevent damage and reduced service life, • to document any problems that have occurred or are prevented by the existing limits, • and, if problems are found, to evaluate whether the existing limit is the best possible method of achieving the serviceability design objectives. 1.2. Directions of Research The research started in December 2000. The research contract was awarded to the University of Washington (UW). However, early in the study it was noted that a parallel study was in progress at West Virginia University (WVU) with funding from the American Iron and Steel Institute (AISI) and the West Virginia Department of Highways (WVDOH). The WVU study was interested in bridge deflection limits, but it was also concerned with bridge vibrations and the development of improved methods of vibration control. This NCHRP 20-7/133 funding was very limited, and so the work had to be done in a way that will provide the maximum benefit at minimum cost. Further, the similarities of the research justified cooperation and coordination between the two research teams. Cooperation initially was arranged through a conference call between the UW and WVU researchers (Charles Roeder for UW and Karl Barth for WVU) and the responsible - 2 - research program managers (David Beal for NCHRP and Camille Rubeiz for AISI). Cooperation between the two research teams were agreed at that time, and the researchers have had numerous meetings, email exchanges, and conference calls for the duration of this project. The UW issued a subsequent subcontract to WVU to help balance funding with the responsibilities. Through these efforts the researchers have exchanged information throughout the research effort to date. Researchers from both universities are co-authoring this report and all papers resulting from this coordinated effort. The research was divided into 6 tasks. The first task provided an initial review of existing literature and the state of practice for steel bridge deflection control. Task 2 provided an Interim Report, which summarized the results of Task 1, and proposed directions for the work to be completed during Tasks 3, 4, 5 and 6. The Interim Report was prepared in March 2001, and was reviewed by an NCHRP Project Panel as well as being submitted to AISI. The panel provided advice and guidance on the research progress, and this guidance was used to direct the research of Tasks 3, 4, and 5. Tasks 3, 4 and 5 consisted of follow up analysis to examine the deflection limits. The range of variability in the actual professional practice was determined. Bridges, which had reported damage due to excessive deflection or deformation, were analyzed to determine whether deflections could or do prevent this damage. Design studies were completed to determine when and how deflections would affect steel bridge design. Task 6 included preparation of a final report with the summary and recommendations from the research. 1.3. Report Content and Organization This is the Final Report required by Task 6 of the project. It describes the progress made throughout the coordinated project. Chapter 1 of this report has introduced the issues of concern. Chapter 2 summarizes the literature review, and Chapter 3 presents the results of a survey of bridge engineering practice. The work in these first 3 chapters was described in somewhat greater detail in the Interim Report submitted to - 3 -