Structural Redundancy of Dual and Steel Moment Frame Systems Under Seismic Excitation

The extensive investigation of structural failure after the Northridge and Kobe earthquakes showed poor structural performance due to brittle member behavior and improper design. The lack of ductility capacity and redundancy has become a serious concern. Most reliability and redundancy studies in the past have been limited to ideal simple systems. As a result, structural redundancy under stochastic loads such as earthquakes has not been well understood, which could lead to misunderstandings among structural engineers. In this study, the redundancy of dual systems and special moment resisting frames (SMRF) is investigated in terms of system reliability under SAC ground motions. Major factors considered include structural configuration (number and layout of shear walls and moment resistant frames), ductility capacity, uncertainty in demand and capacity including correlation of member strength. Dual systems of five and ten stories, equal lateral resistance, and different configurations are first investigated. Interaction between walls arid moment frames is considered. Ductile special moment resistant frames (SMRF) of three and nine stories, equal floor area and strength but different numbers of bays and different beam and column sizes are then analyzed. Furthermore, three SMRF systems of 1 x 1, 2 x 2, and 3 x 3 bay, equal strength, and brittle beam-column connections are investigated to examine the effect of ductility capacity and torsion. A uniform-risk redundancy factor is then proposed and compared with the redundancy factor (p) in the NEHRP-97, UBC-97, and IBC2000. The p factor is found to be inconsistent. It overestimates the effect of configuration and underestimates the effects of ductility and torsion.National Science Foundation Grant EEC 97-0178