Method for Considering Structural Irregularity in Seismic-Resistant Design

The seismic design of structures with structural irregularities presents a significant challenge due to the complexity of their dynamic response and the lack of rational criteria in current regulations. Existing codes, such as NTC Sismo 2023, penalize irregular structures by reducing allowable interstory drift limits; however, this approach is not grounded in structural dynamics principles. As a result, seismic demand may be overestimated or underestimated, affecting both structural efficiency and safety. This study proposes a new penalization method for irregular structures based on dynamic properties. The method adjusts floor accelerations and interstory drift demands by applying a penalization factor to results obtained from modal response spectrum (MRS) analysis. It was developed through the analysis of 23 buildings designed in Mexico with varying levels of irregularity, using dynamic properties derived from MRS analyses. The proposed methodology is validated through nonlinear dynamic time-history analysis (NDTHA) of three irregular buildings subjected to pairs of earthquake records representative of Mexico City seismicity. Results indicate that a penalization factor based on the ratio of the first-mode modal mass to the total structural mass shows strong correlation with seismic responses observed in nonlinear analyses. Finally, the feasibility of incorporating this methodology into seismic design codes is discussed, with the aim of achieving a more rational treatment of structural irregularity, reducing reliance on empirical penalization factors, and improving earthquake-resistant design procedures