Collapse performance of steel self-centering braced frame systems

Current building code requirements in the US are intended to provide life safety for building occupants during a Design Basis Earthquake (DBE) and collapse prevention during a Maximum Considered Earthquake (MCE). The building code identifies specific seismic lateral force resisting systems that may be used in construction. If a new lateral force resisting system is to be added to the building code, values for the global seismic performance factors must be selected and validated. The selected values can be validated through collapse analyses as described in FEMA P695. A new lateral force resisting system is being developed at Lehigh University which is similar to a Concentrically Braced Frame (CBF), but which mitigates the limited ductility and poor seismic behavior that has been observed for conventional CBFs. This new system is known as a Self-Centering CBF (SC-CBF). The SC-CBF increases the ductility capacity of the CBF system by allowing the CBF to rock on its base. Post-tensioning bars run vertically over the height of the SC-CBF to self-center the structure leaving little to no post-earthquake residual drift. This paper examines the collapse performance of the SC-CBF system for multiple prototype structures of various heights to establish the global collapse mode and capacity of SC-CBFs. Results of the incremental dynamic analyses suggest that SC-CBFs have different behavior under high intensity ground motion than conventional CBFs and MRFs. This behavior prevents the rapid growth of roof drift with a small increase in ground motion intensity after a certain intensity level, which is often observed for conventional structures at the collapse limit state, and, instead, it eventually produces an almost linear increase of roof drift with ground motion intensity increase.