Fragility curves for typical steel frames with semi-rigid saddle connections

Steel structures with “saddle” type beam–column connections represent a popular construction practice in Iran. In the present paper, the seismic performance and the vulnerability of such structures are investigated. The seismic responses of three-story and five-story structures are modeled according to the nonlinear incremental dynamic analysis (IDA) procedure considering various frame configurations. Typical frames representing unbraced frames with masonry infill walls, braced frames with concentric bracings and braced frames with masonry infill walls were studied under the effect of 44 real ground motion records. Fragility curves were developed for these structures and the probability of exceedance at immediate occupancy (IO), life safety (LS) and collapse prevention (CP) performance limits was assessed for two seismic hazard scenarios (475 and 2475years return period). For the 2% probability of exceedance in 50years case, the probability of exceeding the CP performance limit is 90%, 74% and 56% for the 3-story structure consisting of unbraced frames with masonry infill, braced frames with concentric braces and braced frames with masonry infill, respectively. The corresponding values for the 5-story models are 94%, 69% and 62%. These alarming high values suggest that the seismic retrofit of such existing structures is of crucial importance. •Traditional steel structures with semi-rigid connections are severely vulnerable to earthquake loads•For the 2% probability of exceedance in 50 years case, the probability of exceeding the CP performance limit is 90%, 74% and 56% for the 3-story structure consisting of unbraced frames with masonry infill, braced frames with concentric braces and braced frames with masonry infill, respectively.•The corresponding values (for the 2% probability of exceedance in 50 years case) for the 5-story models are 94%, 69% and 62%.•These alarming high values suggest that the seismic retrofit of such existing structures is of crucial importance.