Bridge System Safety and Redundancy (2014)

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9 1.1 Background Structural redundancy is defined as the ability of a struc- tural system to continue to carry load after the failure of one or several structural components. Although this concept is well understood, no consensus is currently available on non- subjective measures engineers should use to quantify struc- tural redundancy and how to apply such measures to design adequately redundant bridges. In an attempt to overcome this gap, the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (2012) propose to con- sider redundancy during bridge design by using load modi- fiers that reflect the ductility, redundancy, and operational importance of the structure (Frangopol and Nakib, 1991). However, the values of the load modifiers provided in the AASHTO LRFD were determined by judgment rather than through a calibration process. Furthermore, the LRFD spec- ifications do not provide clear guidance on how to select the ductility or the redundancy modifiers. Following several years of research, NCHRP Report 406: Redundancy in Highway Bridge Superstructures by Ghosn and Moses (1998) and NCHRP Report 458: Redundancy in Highway Bridge Substructures by Liu, Neuenhoffer, Ghosn, and Moses (2001) proposed non-subjective and quantifi- able measures of redundancy after studying the behavior of typical bridge superstructure and substructure systems beyond the failure of their first components. Accordingly, a quantitative measure of redundancy was defined in terms of the capacity of the system as compared to the capacity of the weakest component. Three different limit states for the system were proposed: (1) collapse of overloaded originally intact systems, (2) exceeding the functionality limit of over- loaded intact systems, (3) collapse of damaged bridges. An intact system is a bridge that was not damaged prior to the initiation of a loading process. Damaged bridges are those that may have been exposed to a damaging event that resulted in the loss or the reduction in the load carrying capacity of a major component. The recent literature often has referred to this limit state as structural robustness. NCHRP Report 406 and NCHRP Report 458 then pro- ceeded to calibrate sets of system factor tables using reliability methods to ensure uniform system performance for different typical bridge configurations, geometrical arrangements, and material and structure types. In NCHRP Report 406, superstruc- ture system factors are applied on the resistance side of the LRFD equation for slab on girder type bridges based on the girder spacing and number of girders in the system. System factor charts were provided for simple-span and continuous steel and pretensioned I-beam superstructures subjected to traffic loads. However, the report did not address box-girder superstructures with sufficient detail to make specific recommendations. As presented in NCHRP Report 406, the system factor tables are intended for use when analyzing the redundancy of the most typical bridge configurations and dimensions. Recogniz- ing that it will not be possible to develop system factor tables to cover all possible bridge configurations or damage scenar- ios and in order to provide the engineers with a method to analyze the redundancy of bridge types and conditions not covered in the available tables, NCHRP Report 406 provides a direct redundancy analysis procedure to generate the system factors using nonlinear analysis and incrementally increasing the design load. In NCHRP Report 458, substructure system factors were provided for confined and unconfined 2- and 4-column piers founded on spread footings, drilled shafts, or piles in vari- ous soil types. A direct redundancy analysis procedure also was provided to cover other substructure configurations. The ability of the superstructure to enhance the substructure’s redundancy was recognized as poor for typical systems where the interaction between the superstructure and substructure relied on support bearings. Specifically, the report did not study integral construction systems. The system factor tables provided in NCHRP Report 406 were subsequently simplified and included in the Load and C H A P T E R 1 Introduction

10 Resistance Factor Rating (LRFR) method in the AASHTO Manual for Bridge Evaluation (MBE) (2011), which also rec- ommended the use of the direct redundancy analysis method for special cases. Since then, several consulting firms (with the cooperation of the Wisconsin Department of Transportation) have applied the recommendations of NCHRP Report 406 for the safety assessment of existing bridges (Hubbard, Shkurti, and Price, 2004; Milwaukee Transportation Partners, 2005). More recently, Hunley and Harik (2012) performed detailed analyses of box-girder bridges with different configurations to study the effect of external bracing on straight and curved steel box-girder bridges using the methods and criteria pro- posed in NCHRP Report 406. These studies and other similar investigations have applied variations of the NCHRP Report 406 method to analyze different bridge damage scenarios. The influence of different diaphragm configurations on improving the redundancy of box-girder bridges under various bridge geometric and damage conditions was analyzed on a case-by- case basis as was the intent of NCHRP Report 406. Hunley and Harik (2007) state that “the approach pro- posed in NCHRP Report 406 has gained acceptance from agencies and bridge designers on several projects.” The work of NCHRP Report 406 also has been well received in Europe where it was included in a set of recommended guidelines for evaluating the safety of existing railway bridges (Guideline for Load and Resistance Assessment of Existing European Rail- way Bridges, 2007). The method proposed in NCHRP Report 406 and NCHRP Report 458 or variations on the method also have been adopted by several research studies throughout the world to analyze the redundancy of bridge structural systems (Hunley and Harik, 2012; Mohammadkhani-Shali, 2007; Imhof, 2004; Casas and Wisniewski, 2005). As part of NCHRP Report 458, the authors presented the recommendations of NCHRP Report 406 and NCHRP Report 458 in a format that would be implementable in the AASHTO LRFD and LRFR. The format addressed redundancy in a com- prehensive compatible set of specifications that covered bridge superstructures and substructures independently. However, the format has not been implemented in the LRFD specifi- cations pending more investigation to simplify the format, increase the range of applicability of the system factors, and further confirm the validity and practicality of the approach. 1.2 Research Objectives In summary, the framework and methods of NCHRP Report 406 have led to the development of non-subjective and quantifiable measures of bridge redundancy that have been successfully applied to provide system factor tables for a range of bridge superstructure and substructure configura- tions. A variation of the NCHRP Report 406 recommendations also has been adopted as part of the AASHTO LRFR for the safety evaluation of highway bridges. The direct redundancy analysis method proposed in NCHRP Report 406 to analyze configurations and damage scenarios not considered in the report has been successfully applied by engineering firms, bridge agencies, and researchers to analyze the redundancy of different types of bridges subjected to various types of damage scenarios. However, NCHRP Report 406 and NCHRP Report 458 did not provide system factors for some bridge system and subsystem configurations that have become more popular in recent years. Also, the report did not verify the applicability of the factors for analyzing the combined system, including the interaction between the superstructure and substructure. Hence, the objectives of this research study are to 1. Review the state of the art as well as the state of practice on the subject of structural redundancy to assess the method proposed in NCHRP Report 406 and compare it to alterna- tive approaches for quantifying structural redundancy and considering redundancy during the design of new bridges and the evaluation of existing bridges. 2. Verify the applicability of the NCHRP Report 406 method and investigate the validity of the results in NCHRP Reports 406 and 458 for analyzing the redundancy of complete bridge systems combining superstructure and substructure interaction. 3. Extend the system factors to cover common bridge config- urations including those not addressed in NCHRP Report 406 and NCHRP Report 458. 4. Consolidate the recommendations made in NCHRP Reports 406 and 458 into a format that can be incorpo- rated into the AASHTO LRFD and LRFR specifications. 5. Illustrate how the recommended approach and system factors can be applied in bridge engineering practice. This report summarizes the findings of the study and verifies and complements the results presented in NCHRP Report 406 and NCHRP Report 458. The summaries serve to develop a set of specifications that would be implementable in the AASHTO LRFD and LRFR specifications. 1.3 Report Outline This report presents the findings of the NCHRP Proj- ect 12-86 research and uses the results of new analyses to verify and complement the results presented in NCHRP Reports 406 and 458. This report is divided into the follow- ing six chapters. • Chapter 1 provides the background for this study and sum- marizes its objectives. • Chapter 2 presents a review of the general concepts of bridge redundancy and the method adopted to quantify the redun- dancy of bridge systems and subsystems.

11 • Chapter 3 addresses the redundancy of bridge systems sub- jected to lateral loads when bridge safety is evaluated using displacement-based criteria. • Chapter 4 addresses the redundancy of bridge systems subjected to lateral loads when bridge safety is evaluated using the traditional force-based method. • Chapter 5 addresses the redundancy of bridge systems subjected to vertical loads. • Chapter 6 gives the conclusions of this study. • Appendices are not included herein but are available on the TRB website and can be found by searching for NCHRP Report 776. Appendix A gives a proposed set of specifi- cations to design bridge members based on the level of bridge redundancy. Appendix B provides examples illus- trating the application of the system factors and the direct analy sis method for evaluating the redundancy of bridges. Appendix C gives a review of the literature and state of practice. Appendix D gives the summary of the models and the results for the analysis of different types of bridge superstructures. References AASHTO (2012) LRFD Bridge Design Specifications. 6th ed, Washington, D.C. AASHTO MBE-2-M (2011) Manual for Bridge Evaluation, 2nd ed, Washington, D.C. Casas, J. R. and Wisniewski, D. F. (2005) Safety Formats and Required Safety Levels—Background document, WP4-G-R-01, Sustainable Bridges—VI Framework Program. Brussels, Belgium. Frangopol, D. M. and Nakib, R. (1991), “Redundancy in Highway Bridges,” Engineering Journal, American Institute of Steel Construction, 28(1), 45–50. Ghosn, M. and Moses, F. (1998) NCHRP Report 406: Redundancy in Highway Bridge Superstructures, Transportation Research Board, National Research Council, Washington, D.C. Guideline for Load and Resistance Assessment of Existing European Railway Bridges (2007) http://www.sustainablebridges.net/main. php/SB4.2_Guideline_LRA.pdf?fileitem=28868626 Hubbard, F., Shkurti, T., and Price, K. D. (2004) “Marquette Interchange Reconstruction: HPS Twin Box Girder Ramps,” International Bridge Conference, Pittsburgh, PA. Hunley, T. and Harik, I. (2007) “Redundancy of Twin Steel Box Girder Bridges,” World Steel Bridge Symposium, National Steel Bridge Alliance, New Orleans, LA. Hunley, T. C. and Harik, I. E. (2012) “Structural Redundancy Evaluation of Steel Tub Girder Bridges,” Journal of Bridge Engineering, 17(3), May 1, 2012. Imhof, D. (2004) “Risk Assessment of Existing Bridge Structures,” Ph.D. diss., King’s College, Cambridge University, England. Liu, D., et al. (2001) NCHRP Report 458: Redundancy in Highway Bridge Substructures, Transportation Research Board, National Research Council, Washington, D.C. Milwaukee Transportation Partners (2005) “Redundancy of Box Girder Steel Bridges—A Study for the Marquette Interchange HPS Twin Box Girder Structures,” Project ID 1060-05-1222. Mohammadkhani-Shali, Soheil (2007) “Study of System Redundancy in Bridges: Failure Mechanism Analysis Using Response Surface Methods,” PhD thesis, Ecole National des Ponts et Chausses, ENPC, Paris, France.