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 ρ 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.