Dynamic Response and Seismic Performance of EC8 Conforming RC Frames with Stiffness Irregularity in Elevation

Structural irregularity in elevation may compromise the seismic response of buildings. Hence, to obtain a seismic performance of irregular buildings not lower than that of their regular counterpart, most seismic codes require that a reduced behaviour factor should be used for their design. These provisions appear to be not well supported by the literature. Indeed, most of the studies were devoted to judging the effectiveness of nonlinear static methods of analysis in evaluating the seismic response of regular and irregular structures. Only a few studies have focused on the detrimental impact that the irregularity in elevation may have on the seismic response of code-conforming seismic-resistant building structures. This paper analyses RC frames with different types and levels of irregularity in the distribution of stiffness along the height. Their seismic response is evaluated by nonlinear dynamic analysis. The seismic performance of the irregular frames is compared with that of the regular one. The results obtained show that the capacity design criteria given in Eurocode 8 make the storey collapse mechanism unlikely to occur also for frames with irregular distribution of stiffness. This leads to a suitable seismic performance even if the unreduced behaviour factor is used for their design.