Relevance of torsional effects on the seismic assessment of an old RC frame-wall building in Lisbon

Torsional effects may significantly modify the seismic response of a building and the distribution of damage throughout the structure. In this paper, the torsional response of a case study Reinforced Concrete (RC) wall-frame building built between 1960 and 1980 is presented. The building was constructed without considering appropriate seismic design criteria and without taking into account the forces induced by torsional vibrations. A numerical investigation on the seismic behaviour of the case-study building is carried out by means of nonlinear static pushover analyses performed on a 3-D numerical model of the building; the Extended N2 method is adopted to take into account the influence of higher mode effects in plan and in elevation. The results are then compared with the ones obtained from nonlinear Time-History (TH) analysis. It is concluded that the shear demand evaluated according to the Extended N2 method provides a conservative estimate in comparison to TH analysis. Thus, an improved procedure is proposed for the application of the Extended N2 method in the evaluation of shear demand.