A simplified procedure for seismic collapse probability assessment of RC frame buildings

This paper presents a procedure for simplified collapse probability prediction of RC frame buildings when subjected to ground motion intensities commonly used in designing buildings. For this purpose, seismic collapse probability of a range of RC moment resisting frame buildings are assessed at the design basis earthquake (DBE) and the maximum considered earthquake (MCE) levels of seismic intensity by conducting more than a thousand nonlinear response history analyses (NRHA) using a suite of 40 seismic ground motion records. The 13 buildings included in the investigation have different heights (5-15 storeys) and footprints (3-6 bays), are designed to different target maximum inter-storey drifts (within the code specified limit), and have critical members detailed to sustain different levels of confinement and anti-buckling demands. The NRHA results consistently demonstrate that the design inter-storey drift and the anti-buckling detailing of longitudinal rebars in the critical frame members strongly influence the collapse probability of RC frame buildings. The results also show that while RC frame buildings designed to reach 2.5% (or less) inter-storey drifts and detailed for ductile seismic performance can successfully avoid collapse at DBE, avoidance of collapse at DBE cannot be guaranteed if RC frames are designed to reach a target drift of more than 2% and not detailed for full ductility. At MCE, collapse probability is found to consistently exceed 10% unless the building is designed to incur no more than 1% plastic drift and the frame members have ductile detailing. Based on the NRHA results of the 13 case study buildings, easy-to-use equations relating their seismic collapse probability at MCE with the design drift and a quantifiable measure of ductile detailing are generated and validated by comparing with the collapse probability obtained by NRHA of an independent RC frame building. The proposed equations provide a rapid, yet reasonably reliable tool for performance-based seismic design of RC frame buildings, and enable designers to estimate seismic collapse probability of RC frame buildings without having to conduct cumbersome NRHA.