Structural Reliability/Redundancy under Earthquakes

Structural reliability/redundancy has become a serious concern in the building industry after the poor performance of some buildings in recent earthquakes. Yet, there is a general lack of thorough understanding of structural redundancy under seismic excitations among practitioners and researchers, which could lead to questionable design recommendations concerning reliability/redundancy. In this study, the redundancies of special moment resisting frames and dual systems are investigated. Major factors considered include structural configuration (number of bays of moment-resistant frames and number and layout of shear walls), ductility capacity, uncertainty in demand and capacity, interaction between walls and moment frames, and three-dimensional motions. Responses under uniform-hazard ground motions and reliabilities of conceptual structural models of equal total lateral strength but different configurations and ductility capacities are compared. The reliability/redundancy is found to be dependent only moderately on the structural configuration. The effect of ductility capacity and three-dimensional motions, on the other hand, could produce larger differences. A uniform-risk redundancy factor is then developed for design and compared with the reliability/redundancy factor in current codes. The latter is found to be inconsistent. It generally overestimates the effect of system configuration and underestimates effects of ductility capacity and three-dimensional motions.