An Efficient Approach for Seismic Fragility Assessment with Application to Old Reinforced Concrete Bridges

A procedure for seismic fragility assessment, suitable for application to non ductile RC structures is presented, which is based on the estimate of a response surface that gives the probability of failure of the structure as a function of the random variables that affect the response. The seismic fragility or risk is then evaluated through numerical integration. The method considers different sources of uncertainty: (i) in the seismic input, through the use of different accelerograms for the dynamic analysis; (ii) in the structural response, through the use of a refined nonlinear finite element model; and (iii) in the ultimate state capacity, taking into account the different modes of failure which may occur, for which a random mechanical capacity model is available. Aiming at reducing the number of simulation analyses, the uncertainties on the seismic input and on the mechanical parameters governing the response are treated according to the response surface methodology, while the limit-state randomness is treated explicitly during the simulations. Using the proposed procedure, the seismic safety of two reinforced concrete bridges from Italian highway network, with simply supported deck and either single stem or frame piers, is evaluated. The results are expressed in terms of fragility curves as function of spectral acceleration. The obtained results highlight the influence of material randomness on reliability and the relative importance of seismic input with respect to mechanical and epistemic uncertainty.