I terremoti del Maggio 2012 in Emilia. Potenziale distruttivo del moto del terreno

The seismic sequence that struck the Emilia Region in 2012 has been recorded by a number of instruments of the Italian strong motion network, managed by the Department of Civil Protection. In this study, the accelerograms recorded by the stations located within 25 km from the epicentres are analysed from an engineering standpoint. Different intensity measures and acceleration, displacement and energy response spectra are calculated. A comparison with the accelerograms recorded during L'Aquila 2009 earthquake is also performed. The study shows that that the strong motion was rather severe in the near fault, especially in terms of horizontal displacements and vertical accelerations, and highlights the presence of a rather long pulse in the N-S components of the near-fault records. This is probably related to the N-S direction being approximately normal to the causative faults. The comparison with L’Aquila earthquake proves that, even though the acceleration-related measures were stronger in L'Aquila than in Emilia, the opposite occurs for the displacement demand

Characterization of the dynamic response of structures to damaging pulse-type near-fault ground motions

The presence of long-period pulses in near-fault records can be considered as an important factor in causing damage due to the transmission of large amounts of energy to the structures in a very short time. Under such circumstances high-energy dissipation demands usually occur, which are likely to concentrate in the weakest parts of the structure. The maximum nonlinear response or collapse often happens at the onset of directivity pulse and fling, and this time is not predicted by the natural structural vibration periods. Nonlinear response leading to collapse may in most cases occur only during one large amplitude pulse of displacement. From the study of the response of both linear and nonlinear SDOF systems, the effects of these distinctive long-period pulses have been assessed by means of : (i) synthetic parameters directly derived from the strong ground motion records, and (ii) elastic and inelastic spectra of both conventional and energy-based seismic demand parameters. SDOF systems have first been subjected to records obtained during recent earthquakes in near-fault areas in forward directivity conditions. The results indicate that long duration pulses strongly affect the inelastic response, with very high energy and displacement demands which may be several times larger than the limit values specified by the majority of codes. In addition, from the recognition of the fundamental importance of velocity and energy-based parameters in the characterization of near-fault signals, idealized pulses equivalent to near-fault signals have been defined on account of such parameters. Equivalent pulses are capable of representing the salient observed features of the response to near-fault recorded ground motions