PSI - Issue 44

Romina Sisti et al. / Procedia Structural Integrity 44 (2023) 1116–1123 R. Sist et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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assumed according to the values provided by the Italian code (NTC 2018) for the masonry typologies detected in Palazzo Comunale, while other material properties were obtained from the original project documents. Due to the limited knowledge of the structural characteristics, all the mechanical parameters were reduced by a confidence factor of 1.35, according to Italian seismic code provisions. The results obtained from these two models were compared with the damage observed after the 1997 and 2016 earthquakes, respectively. To investigate the global response of the building, a nonlinear static analysis was carried out. The analysis considers only the in-plane behavior of masonry panels. The model was loaded with a distribution of in-plane actions, which were monotonically increased to simulate the effects of the inertia forces. Two systems of lateral forces distributions were applied, not simultaneously, in order to simulate the behavior of the undamaged structure (first distribution: the forces are proportional to the displacements of the first modal shape) and the configuration of the structure near the collapse (second distribution: the forces are proportional to the inertia masses). As control node it was chosen the average displacement of the nodes at the roof level. Fig. 9 summarizes the main results of the analysis related to the two models, showing the minimum values of the seismic safety index (αLS), defined as the ratio between the maximum value of seismic acceleration corresponding to the attainment of the life safety ultimate limit state (LS - Life Safety) and the expected demand acceleration given by the Italian code. Before renovation works, Palazzo Comunale exhibits a very poor performance, with a seismic safety index equal to 0,35 as a result of a shear progressive failure of the piers. The 1999 renovation allowed an 84% increase in the safety index, up to 0.65 (capacity peak ground acceleration of 1.41 m/s 2 with respect to the design domand given by the Italian code of 2,16 m/s 2 ) mainly achieved thanks to the improvement of masonry quality and the reduction of the irregularities. In both cases it was possible to calibrate the models in order to obtain a good agreement between the damage pattern simulated by the software and the damage patterns detected after the seismic events. Considering that the achievement of a capacity equal to 65% of the design actions was exactly the design criterion adopted in 1999 project, the results obtained with modern analyses do not diverge from those obtained with the calculation methods available at the beginning of this century. Furthermore, considering that the seismic capacity (1.41 m/s 2 ) was very close to the maximum value of peak ground acceleration occurred during the 2016 seismic sequence (1.6 m/s 2 ), there is a match between the damage observed after the earthquake and that expected according to the definition of the life safety performance level. The out-of-plane response of the walls were analyzed verifying the activation of local collapse mechanisms by means the macro-element approach based on the theory of limit analysis. Each mechanism involves only single masonry panel or an isolated portion of the building which is assumed structurally independent from the remaining parts of the building and modelled as sets of rigid blocks, connected with hinges. Fig. 10 shows the seven mechanisms analyzed, defined according to construction details, such as existing wall-to-wall connections or wall-to-floor connections or structural discontinuities, and reports for each mechanism the seismic capacity for the LS performance level and the relevant seismic safety index. Before renovation many mechanisms were characterized by a safety index lower than 1 while after renovation works, all the safety indices result greater than 1. Thus the activation of all mechanisms was prevented by the introduction of steel tie rods, thus resulting as an effective strengthening intervention to avoid out-of-plane mechanisms. The strengthening solution is also a low-cost technique, reversible and can be implemented in monumental buildings without altering their characteristics.

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Fig. 8. Identification of Palazzo Comunale unit. b) Geometrical model developed on 3 MURI software.