Closed-form seismic fragility estimates, sensitivity analysis and importance measures for reinforced concrete columns in two-column bents

Closed-form seismic fragility estimates are developed for reinforced concrete (RC) columns in bridges with two-column bents. Deformation and shear modes of failure are considered. The closed-form solutions incorporate the important uncertainties associated with both structural properties and ground motion characteristics. Probabilistic capacity and demand models for RC columns in two-column bents are used for the fragility formulation. Sensitivity and importance measures are computed for the parameters and random variables, respectively, included in the limit state function expressed in terms of probabilistic capacity and demand models. The sensitivity measures suggest that the vulnerability of RC columns in two-column bents can be effectively improved by using high strength reinforcement for the column confinement, reducing the spacing between confining reinforcement, and limiting the use of high strength concrete. The importance measures suggest that the random errors in the probabilistic capacity and demand models represent the principal sources of uncertainty. Thus, an approximate closed-form solution for a fragility estimation of a RC column can be developed by considering only the uncertainty in the random errors of the capacity and demand models. Only a marginal difference exists between the closed-form fragility estimates and the corresponding predictive fragility estimates that include all uncertainties.