Impact of epistemic and aleatory uncertainties on the seismic reliability assessment of existing structures

Seismic reliability assessment of structural systems is commonly done via the convolution of seismic hazard and structural fragility to get resulting failure rate, and later compute seismic reliability index. Within this frame, many uncertainty sources have to be handled by the risk analyst, who is asked to make some reasonable assumptions in order to carry out his/her seismic safety quantification. However, slight changes in the definition of input parameters and/or the use of different empirical or analytical models can strongly impact the final reliability outcomes. The present work aims therefore to investigate the impact of different sources of uncertainties by comparing together key parameters to be defined in both hazard and fragility analysis, considering a casestudy represented by an existing multi-span steel-concrete composite bridge to better understand which of these sources is more impacting on the final estimates. Results show how uncertainties associated to hazard and fragility are as a whole of the same order of magnitude and thus how both uncertainty sources have to be properly considered. In addition, a detailed sensitivity analysis was carried out for ranking the uncertainty sources in relation to their impact on the overall reliability index dispersion. Results showed that the choice of a suitable Ground Motion Prediction Equation (GMPE) model, the non-linear modelling technique and the number of the accelerometric records adopted for the structural analysis seem to be the factors that mostly contribute to the seismic reliability index variability, representing the uncertainty related to these parameters the majority of the overall reliability index dispersion.