La modellazione delle strutture isolate in presenza di sistemi di isolamento nonlineare e l’analisi dinamica non lineare semplificata secondo l’OPCM 3274/3431 (punto 10.7.6)

In addition to the conventional time history analysis for base isolated structures, the Italian building code OPC 3274/3431 allows a simplified nonlinear dynamic analysis, described in section 10.7.6. Although the analysis methods and criteria selection sections of the Italian code are generally very similar to EC8 and other international codes, these codes do not allow such a simplified procedure. This method appears theoretically inconsistent due to an unjustified decoupling of the nonlinear equations of motions of the superstructure and of the isolation system, since the nonlinear analysis is carried out treating the structure as a rigid SDOF system. The experimental tests performed on a full-scale building constructed at Rapolla (Potenza, Italy), pointed out some important effects of energy exchange from the first isolated mode towards the higher modes, due to the nonlinearity of the isolation system. This results in an increased contribution of higher modes to the overall structural response, as already evidenced in some of the principal studies in literature and also translated into practice by the international building codes and guidelines. Some nonlinear seismic response analyses have been carried out in order to show the inconsistency of the simplified analysis of the OPC 3274/3431. This procedure considerably underestimates the forces in the structure even for practical cases of optimal design of the isolation systems.