Progressive collapse fragility models of RC framed buildings based on pushdown analysis

Partial or total progressive collapse under abnormal loading conditions (e.g. deliberate terrorist attacks, uncontrolled gas releases, and vehicle or aircraft impacts) is one of the most vivid examples of low probability-high consequence (LPHC) event that may occur in the lifetime of a structure. Despite this, structural safety for extreme loads that may lead to disproportionate (or progressive) collapse has been probabilistically assessed and controlled in a few cases, thus neglecting uncertainties in loads and system capacity. As such, this paper moves from a deterministic to a probabilistic framework, proposing fragility models at multiple damage states for low-rise reinforced concrete (RC) framed bare structures which may be applied for progressive collapse risk assessment and management. Two building classes representative of structures designed for either gravity loads or earthquake resistance in ac-cordance with current European rules were investigated. Monte Carlo (MC) simulation was used to generate random realizations of two-dimensional (2D) and three-dimensional (3D) structural models. Their fiber-based finite element (FE) representations were developed within an open source platform for nonlinear static pushdown analysis. The output consisted of fragility functions for each damage state of interest. Such fragility models were then compared to those derived through incremental dynamic analysis (IDA) in a previous study. IDA-based and pushdown-based capacities were additionally used to propose regression models for quick estimation of dynamic amplification factor (DAF) at a given displacement/drift target. The analysis results show a significant influence of both seismic design/detailing and secondary beams on robustness of the case-study building classes