Nonlinear static analysis of a plan irregular building according to EC8 provisions

The paper deals with the topic of nonlinear static and nonlinear dynamic analyses of multi-storey r/c frame buildings designed according to Eurocode provisions. In particular, the open problem of the extension of N2 method to plan irregular buildings, which makes up for the underestimation of the seismic demand on stiff side of the buildings, is focused. Three methods, which take into account the accidental eccentricity of the mass (due to its uncertainties in the location) are proposed. The main differences between the three nonlinear static analysis methods are: method n.1 combines the displacements of modal response spectrum analysis obtained by the 4 models (associate to 4 positions of the centre mass) by SRSS rule and, for each frame of the multi-storey r/c frame building, the maximum normalized displacement is assumed as reference; method n.2, works on 4 models and only at the end of the procedure combines the results by SRSS rule; method n.3 considers till the end of the procedure the double sign (+ and -) of pushovers. Nonlinear static analyses are carried out using both the “modal” and “uniform” force pattern. The “orthogonal effects”, evaluated by SRSS rule, result to be negligible. The results, in terms of pushover curves, frame top displacements and interstory drifts, are compared with the ones obtained by nonlinear dynamic time-history analyses. The input adopted for nonlinear dynamic analysis (7 natural earthquakes) is selected in order to rigorously satisfy the EC8 provisions.