ON THE SITE-AMPLIFICATION AND SOIL-STRUCTURE INTERACTION IN URM STRUCTURES: USE OF FRAGILITY CURVES TO ASSESS THE SIMPLIFIED CODE-APPROACH

The paper investigates the effects of site-amplification and soil-foundation-structure interaction (SFS) on the fragility curves of a real unreinforced masonry (URM) structure. The building is inspired to a school with an irregular T-shape plan and is modelled according to the equivalent frame approach. The structure was ideally placed on a stiff rock outcrop (case FB A) or settled on four different soft soil profiles, including its real foundation subsoil. In the latter cases the base of the structural model was assumed as fixed (FB C) or endowed with springs (CB C) simulating the soil-foundation dynamic impedance. To evaluate the fragility curves, the corresponding structural models were analysed through non-linear dynamic analyses under 136 input motions, propagated in 1D soil models reproducing the selected profiles for FB C and CB C. The results were compared in the paper in terms of: -fragility curves calculated for five damage levels (defined to be conceptually consistent with those of the EMS98 scale) and expressed as a function of different intensity measures of the input motion; - average damage derived as a weighted average of the probability of failure associated to all the damage levels. The comparison among the results of the different models highlights the expected increase of damage due to site effects and a beneficial effect of soil-structure interaction, mainly due to the increment of damping associated to the additional energy dissipated by the soil-foundation system. Finally, the average damage of FB C and CB C models were compared with those obtained from the fragility curves of a fixed base system in which site effects are considered through the conventional coefficients proposed in Codes (e.g. by the Eurocode 8), that neglect SFS. This latter approach is that followed in the current practice. The comparison demonstrates that the Code-based approach underestimates the fragility especially at high damage levels.