Effects of the extended N2 method on non-linear static procedures of reinforced concrete frame structures

This paper intends to contribute to the validation of the extended N2 nonlinear static method of seismic analysis through the study of two existing reinforced concrete frames. The scope is to further investigate the limitations of the original N2 method in predicting the seismic response of plan asymmetric buildings and to assess the higher accuracy of the extended N2 method. The two case study buildings are representative of the Italian construction between the '70s and '90s. One is designed for gravity loads only, the other is designed according to an old seismic code. Two modelling strategies are used, one with distributed and the other with lumped plastic hinges. Three eccentricities eCM between centre of mass and plan geometric centre are considered (eCM = 0%, 5% and 15%). The pushover analyses are carried out following the N2 method, the N2 method plus the 100:30 directional combination rule and the extended N2 method. The results are compared with those obtained from nonlinear history analyses. For each building the seismic demand is assessed for three hazard levels. The results indicate that the application of the extended N2 method significantly enhances the shape of the demand prediction, particularly for buildings with higher plan eccentricities. On the other hand, the directional combination does not significantly change the results and should be neglected.