The role of intensity measures on the accuracy of seismic fragilities

Seismic fragilities are the probability that structural response of a system overcomes specified limit values for given seismic intensity measures. These curves are frequently defined as functions of single/multiple ordinates of the pseudo-acceleration response spectrum. Recently it was reported that this approach can lead to inaccurate estimation of the structural performance for complex non-linear multi-degree of freedom systems. This preliminary paper investigates the accuracy of fragility curves for an actual building struck by the 2016 Italian earthquake. Numerical analyses of a multi-degree of freedom structural system subjected to this earthquake are performed, considering both linear and non-linear behaviour, and used to construct scatter plots of intensity measures versus the corresponding actual maximum of the response time history. Finally, an example of the fragility curves estimated using different intensity measures is presented for both linear and nonlinear structural behaviour. It is shown that these fragilities have very large variability depending on the selected scalar or vector-valued intensity measure.