PERFORMANCE BASED SEISMIC RETROFIT DESIGN OF BENICIA-MARTINEZ BRIDGE

This paper presents an overview of the performance based seismic retrofit design as implemented in the Benicia-Martinez Bridge. The performance based design emphasizes the characterization of the actual inelastic behavior during the seismic event. These nonlinear components include: seismic isolation bearings; nonlinear behavior of substructure caisson foundations [6 ft (2 m) diameter CISS piles) - flexural yielding and uplifting; and inelastic behavior of approach structure columns. A systematic procedure was developed to evaluate the structure's seismic responses using a combination of global model and various local models of substructure piers. In the global model, the emphasis is placed on the overall characterization of the seismic response of the superstructure system and supporting piers. The foundation below the footing level was approximated using the substructuring method. This allows a number of analyses to be conducted for a variety of retrofit schemes and earthquake events. Once a fundamental understanding of the structural responses was established, a more detailed model was devised to quantify the expected inelastic behaviors, e.g., relative displacement across the isolation bearings, flexural yielding in the steel caisson, amount of uplift (elongation) in the tie-down anchors, etc. These predicted inelastic deformations must be expressed in terms of acceptable material performance not only for safety reasons but also for functional considerations.