Bridge System Safety and Redundancy (2014) / Chapter Skim
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1 S U M M A R Y Bridge System Safety and Redundancy This report develops a method to calibrate system factors that can be applied during the design and load capacity evaluation of highway bridges to account for bridge redundancy and system safety. The proposed system factors can be used during the design and safety assessment of bridges subjected to distributed lateral load being evaluated using the displacement-based approach specified in the AASHTO Guide Specifications for LRFD Seismic Bridge Design or the traditional force-based approach.
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2A Dbu target provides a reliability measure of that reserve strength. The reliability index for members of bridge systems subjected to seismic load or other lateral loads has not been determined.
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3 where Mavailable = moment capacity of the connecting elements such as cap beams and pile caps or the reduced moment that can be supported by the column based on the available shear reinforcement, development length, splice, or connection detailing. Details on how to calculate the available moment capacity for a member with weak detailing are available in the FHWA Seismic Retrofitting Manual for Highway Structures, Part 1, Bridges, as Mp column = plastic moment capacity of column, Mu column = ultimate overstrength moment capacity of column calculated using nonlinear sectional capacity analysis programs or conservatively estimated to be 1.15 Mp column, ju = ultimate curvature of the weakest column in the bent, and ju connection = minimum ultimate curvature of the connecting elements.
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4Bridge Systems under Vertical Vehicular Load System factors for multi-beam I-girder and box-girder bridges are proposed for evaluating the redundancy of originally intact systems subjected to vehicular overloads and for damaged bridges that have been previously exposed to local member damage. Redundancy of Originally Intact Systems under Overloads The system factors for I-girder bridges and spread box-girder bridges are provided in Tables S2 and S3 in function of the capacity of the bridge to resist first member failure represented by the variable, LF1, the dead load to resistance ratio of the members and based on the material and geometric properties of the bridge.
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5 The parameter L1 gives the live load applied on the most critical member, which is defined as the member that fails first. It can be calculated as = ×.
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6is represented by the variable, LF1, defined in Equation S6. For the spread box-girder bridges, three different damage scenarios are considered.
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7 of the bracing and diaphragms to transverse moment capacity calculated using Equations S11 or S12. The equivalent transverse moment capacity for cross bracing as defined in the FHWA Steel Bridge Design Handbook: Bracing System Design (2012)
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8General Comment The analyses in NCHRP Report 406 and NCHRP Report 458 concentrated on bridges that closely met the member strength design requirements. The analyses performed in this study considered the redundancy of deficient bridges as well as overdesigned bridges that expand the applicability of the proposed system factors.
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