Collapse Resistance of Seismically Resilient Self-Centering Steel Moment Resisting Frame Systems

Abstract eng:
A self-centering moment resisting frame (SC-MRF) is a viable alternative to a conventional MRF with welded beam-column connections for seismic resistant steel frame buildings. An SC-MRF is characterized by gap opening and closing at the beam-column interface under earthquake loading. The beams are post-tensioned to columns by high strength post-tensioning (PT) strands oriented horizontally to provide self-centering forces when gap opening occurs. For the SC-MRF investigated in this research, energy dissipation is provided by beam web friction devices (WFDs) attached to the columns at the beam-column interface. The SC-MRF is a highly resilient system where typically it is designed to meet several seismic performance objectives. These include no damage under the Design Basis Earthquake (DBE), leading to immediate occupancy performance following the DBE. In addition, under the Maximum Considered Earthquake (MCE) the structure is designed to have minimal damage. Prior analytical and experimental research has shown that a SC-MRF can achieve these performance objectives. The design procedure for SC-MRFs uses global seismic performance factors (SPFs) that include the response modification factor (R), the system overstrength factor (Ω0), and deflection amplification factor (Cd). Since this system is new, little is known about the collapse resistance of this type of structural system under seismic ground motions. For an SC-MRF to be accepted in practice, the global SPFs need to be evaluated in accordance with FEMA P695 to establish whether the collapse margin ratio for such a structural system under extreme ground motions is adequate. In this paper incremental dynamic analyses are presented and the probabilistic collapse resistance of SC-MRFs is assessed. The model of the SC-MRF is a hybrid model that combines stress-resultant elements with continuum elements in order to enable complete structural systems to be modeled while capturing the important limit states that can occur, including gap opening at the beam-to-column interface, yielding of PT strands, second order (P-delta) effects due to gravity loads imposed on the gravity load frames, and cyclic local buckling and strength degradation in the beam plastic hinge regions of the SC-MRF. The model of the SC-MRF is presented, along with the incremental dynamic analysis procedure and results. The sensitivity of seismic collapse to design parameters (values of SPFs, post-tensioning force level, design story drift, energy dissipation) is presented, and the implications on design discussed.

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