3D Pushover Analysis for Evaluating Torsional Effect of RC Structures

In the recent years, nonlinear static analyses received a great deal of research attention within the earthquake engineering community. Their main goal is to describe the nonlinear capacity of the structure when subjected to horizontal loading with a reduced computational effort with respect to nonlinear dynamic analyses. For 2D frame structures many studies have been performed in order to validate different pushover techniques by comparison with results from dynamic analyses and a good match has been found. On the contrary, few methods to perform pushover analyses for irregular structures (3D frame structures) can be found. Definition and assessment of 3D pushover methods are much more complex than 2D analyses for several reasons. First of all, a correct horizontal loading distribution both over the frame height and the floors must be defined in order to take torsional effect into account. Secondly parameters of structural response (maximum displacement and maximum rotation of roof floor, or relative rotation/displacement between floors, and so on) to be predicted by the nonlinear static analyses must be preliminary defined. The second point is fundamental to evaluate the accuracy of different methodologies. In the present work, a new pushover procedure for 3D RC structures, named Force/Torque pushover (FTP) is proposed; a set of force and torque distributions is selected, in order to predict the most severe configurations the structure may undergo during the earthquake. Three different worst-case performance conditions for the structure have been selected, corresponding to the attainment of i) the maximum displacement δmax, ii) the maximum rotation θmax, iii) the maximum strain in concrete core εcu. In the present work, the latter of these conditions has been chosen as the limit state, since it may represent the failure condition for a given structural element. In order to validate the proposed technique, comparison between results from a series of incremental dynamic analyses and pushover analyses are carried out with an accurate selection of representative parameters of structural response