Seismic vulnerability of r/c frames with strength irregularity in elevation

A research study on the seismic behaviour of r/c frames with strength irregularities in elevation is presented; in particular, the results of non linear static analyses on r/c “regular” frames are compared to the ones obtained in the case of “irregular” frames. The irregularity is obtained assigning overstrengths either to the beams or to the columns of the “regular” frames (assumed as references). The static analyses are pushed to the “target displacement”, evaluated by the N2 method, introduced by the last EC8 draft and by the new Italian code. The comparison is carried out in terms of interstorey drifts, maximum rotations at the element ends and relating the demand to the capacity in terms of plastic rotations. The results show that the frames characterised by the overstrength assigned to the beams have an irregular behaviour. Finally, an evaluation of the irregularity in elevation criteria proposed by some recent codes is carried out; the analysis allows to conclude that such criteria are not reliable.