Structural fragility of T-joint connections in large-scale piping systems using equivalent elastic time-history simulations

The main focus of the paper is to evaluate seismic fragility of nonlinear connections in large piping systems. Piping systems in general exhibit damage due to excessive inelastic deformation at discrete locations such as T-joints, elbows, valves, etc., and low ductility corresponding to failure. Since the supporting structure remains primarily elastic, this study evaluates the effectiveness of using linearization techniques to describe the localized nonlinearities with equivalent elastic properties. A concept called “Equivalent Elastic Limit State” is introduced. A large number of representative nonstructural systems which are essentially linear but characterized by localized fragile nonlinearities are studied to propose a model for computing seismic fragility curves using only elastic time-history simulations. It is shown that the seismic fragility evaluated using elastic analyses is close to the actual fragility. Fragility estimated using this approach can be used as a very reliable prior estimate within Bayesian fragility models so that the posterior estimate of fragility curve can be obtained using a very few number of nonlinear time-history analyses. The efficacy of the model is illustrated for seismic fragility analysis of a full-scale piping system in a hospital with fragile 2″ T-joint connections.