Issue 51

M. Guadagnuolo et alii, Frattura ed Integrità Strutturale, 51 (2020) 398-409; DOI: 10.3221/IGF-ESIS.51.29 402 and other structures with large halls [23,24], without intermediate diaphragms [25], towers, bell towers, and other tall and slender structures [26].They are strongly linked to the Italian Building Code [27], recently updated with a new version [28], which contains all the parameters necessary to perform a reliable seismic analysis. To evaluate the seismic safety, three different levels of increasing completeness have been identified, of which the LV1 level concerns the assessment of seismic safety index on a territorial scale, also to establish the priority degree of interventions; this method is based on a limited number of geometric and mechanical parameters or visual tests, being linked only to a simplified type of evaluation; as will be better clarified by the following procedure (Eqns. 1-9) [14]. Same papers compare simplified approaches with more sophisticated procedures [29, 30]; however, retrofitting and strengthening of existing buildings have to be designed through refined non linear analyses [31]. If the seismic safety index estimated using the procedure is greater than the unit, the structure is able to withstand the seismic forces required by the seismic code, on the contrary no. This is useful for highlighting critical situations and establishing a priority for the interventions [27, 28]. The seismic safety index I SP is estimated by the ratio between the return period of the seismic action SLV T of the earthquake which gets the building to reach the ultimate limit state and the expected return period of the earthquake on the site ,  R SLV T .   ,  ,  SLV SP SLV R SLV T I T  (1) and ,  ln(1 ) R R SLV VR V T P    (2) where: R V is the reference period; VR P is probability of exceedance in the reference period. In the same way, an acceleration safety index I SA is computed as the ratio between the peak ground acceleration of the earthquake which gets the building to the limit state of activation SLV a and the peak ground acceleration of the design earthquake ,  g SLV a , related to the site: ,  SLV SA g SLV a I a  (3) ,  g SLV a is the design ground acceleration, corresponding to the assigned return period of the earthquake, related to the subsoil; SLV a is the ground acceleration leading to the achievement of the structure ultimate limit state (SLV), computed as a function of the fundamental period of vibration T1 of the structure [30].   , 1 1 0 e SLV SLV B C s T a T T T S F     (4)   , 1 1 0 e SLV SLV C s T T a S F T    1 C D T T T   (5) where the ordinate value of the elastic response spectrum , e SLV s is: