Fragility of Low-Rise Moment Resisting Steel Frame Building Subjected to Crustal and Subduction Earthquakes

Performance-based earthquake engineering (PBEE) requires accurate estimation of the seismic demand on structures. Implementation of PBEE in quantitative evaluation of the performance of a given building is denoted here as Performance- Based Assessment (PBA). The methods used to tackle this task are incremental dynamic analysis (IDA), fragility analysis, and collapse safety verification where both epistemic and aleatoric uncertainties were considered. The purpose of this study was to investigate the performance of low-rise steel moment resisting frame (MRF) office building located in Vancouver, B.C., Canada under earthquake shaking, to identify the maximum interstorey drift and residual interstorey drift, as well as, to derive fragility curves for life safety (LS) and collapse prevention (CP) performance levels. In addition, the proposed equation for predictive maximum residual interstorey drift provided in FEMA P 58-1 (2012) was verified. It is noted that steel moment resisting frame buildings undergoing residual interstorey drift larger than 1.0% hs (where hs is the storey height) are proposed for demolition rather than repair. In this paper, a comparative analysis is conducted in order to investigate the collapse margin safety of studied building subjected to crustal versus subduction ground motions. The case study is a prototype 4-storey moderately ductile MRF office building located on Site Class C in Vancouver B.C., Canada. The building structure was designed according to the 2010 edition of National Building Code of Canada and the steel design standard CSA/S16-09. For simulating the nonlinear response from yielding to failure, a refined plastic hinge model able to consider strength and stiffness degradation caused by the low-cycle fatigue was developed in the OpenSees framework (Open System for Earthquake Engineering Simulation). The proposed model was validated against 16 experimental test results found in the literature. In this study, fragility curves were derived from the incremental dynamic analysis curves (IDA). To assess the damage state on the IDA curves, the maximum interstorey drift and maximum residual interstorey drift were recorded for each level of earthquake intensity. In addition, a damage index was proposed in order to trigger the ductile-fracture induced failure in cross-section fibers of a W-shape beam’s plastic hinge zone. From analyses it was found that the nonlinear response was mostly influenced by the number of loading/ unloading cycles with large amplitudes and the Trifunac duration. Hence, the interstorey drift limit of 5.0% hs proposed for the CP limit state in FEMA 273 (1997) and FEMA 356 (2000) was found to be slightly overestimated when the low-rise MD-MRF building was subjected to subduction ground motions and too conservative in the case of crustal ground motions. The average value of ajusted collapse margin ratio (ACMR) of studied MD-MRF building was larger than the minimum permissible value corresponding to 10% probability of collapse (ACMR10%) under both sets of ground motions; hence it fulfils the collapse safety margin criteria. As expected, larger collapse margin ratio resulted in the case of building subjected to crustal ground motions than subduction records. Regarding the verification of the predicted median residual interstorey drift equation given in FEMA P 58-1 (2012), it was found that for the CP performance level, the median predicted residual interstorey drift was underestimated by about 20% for both crustal and subduction records.