Redundancy in bridge substructures subjected to lateral loads and calibration of pier system factors

This paper summarizes the findings of the National Cooperative Research Program, NCHRP reports 458 and 776 that presented a procedure to calibrate system factors that account for the level of structural redundancy in typical highway bridge substructures under lateral loads. A proposed set of system factor equations are presented in a format suitable for inclusion in the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design Bridge Design Specifications and the Manual for Bridge Evaluation. In a first step, the study proposes a set of empirical equations to estimate the lateral load carrying capacity of substructure systems. A comparison between the results of a nonlinear pushover analysis and the empirical equation showed an error of about 8.7%. In a second step, a reliability-based approach is used to calibrate system factors that account for the redundancy and ductility in bridge substructures. The proposed approach replaces the subjectively assigned load modifiers in the current AASHTO design equations with properly calibrated equations. The results show that a system factor equal to 0.74 would ensure that the system reliability of a substructure under seismic loads will be 0.50 higher than the reliability level implied in current member-focused design specifications.