Seismic fragility assessment of reinforced concrete and post-tensioned slab-column connections - reliability-based formulations for storey drift limits

Many design codes, such as ACI 318−19, ACI 352.1R-11 and ASCE 41−17, offer deterministic inter-storey drift limit functions of the gravity shear ratios for flat slab floor systems acting as the primary lateral load-resisting system. However, such deterministic limits are not capable of capturing the inherent aleatory uncertainty in the damage capacities of slab-column connections, inducing unknown failure risks during seismic events. To address this, seismic fragility analysis is conducted to generate fragility curves relating the probability of a connection exceeding a performance level or damage state to the inter-story drift ratio and gravity shear ratio. The developed fragility curves are based on an updated experimental database of 221 interior reinforced concrete and post-tensioned slab-column connections without and with shear reinforcement under combined gravity and lateral loadings. Two damage states are introduced, yielding and failure due to punching shear or flexure. The developed fragility curves are used as basis for generating reliability-based formulations and charts to aid designers in determining the drift limits as a function of the design gravity shear ratio as well as a chosen reliability index. The analysis showed that connections with continuous bottom reinforcement, shear reinforcement, or prestressing have enhanced drift capacities and reliability. Also, higher gravity shear ratios significantly lower the limit drift ratio of both reinforced concrete and post-tensioned slab-column connections without shear reinforcement at a given reliability index value. Comparing the proposed reliability-based drift limits to the deterministic limits in ACI 318−19, ACI 352.1R-11 and ASCE 41−17 revealed that the code limits tend to correspond to a reliability index ranging from about 0 to 2.5 for connections without shear reinforcement; and 3 to 8 for connections with shear reinforcement. The existing design limits for slab-column connections without shear reinforcement are least reliable at high gravity shear ratios with reliability index generally below 1.25. These results signify that the proposed reliability-based limits can prompt more informed risk-based designs, especially for connections without shear reinforcement.