Seismic performance of R/C plane frames irregular in elevation

The paper addresses multistorey reinforced concrete (R/C) frame buildings, irregular in elevation. Two ten-storey two-dimensional plane frames with two and four large setbacks in the upper floors respectively, as well as a third one, regular in elevation, have been designed to the provisions of the 2004 Eurocode 8 (EC8) for the high (DCH) and medium (DCM) ductility classes, and the same peak ground acceleration (PGA) and material characteristics. All frames have been subjected to both inelastic static pushover analysis and inelastic dynamic time-history analysis for selected input motions. The assessment of the seismic performance is based on both global and local criteria. It is concluded that the effect of the ductility class on the cost of buildings is negligible, while the seismic performance of all irregular frames appears to be equally satisfactory, not inferior to (and in some cases superior than) that of the regular ones, even for motions twice as strong as the design earthquake. As expected, DCM frames are found to be stronger and less ductile than the corresponding DCH ones. The overstrength of the irregular frames is found to be similar to that of the regular ones, while DCH frames are found to dispose higher overstrength than DCM ones. Pushover analysis seems to underestimate the response quantities in the upper floors of the irregular frames.