Accidental Eccentricity in Response Spectrum Analysis: Comparison Between Static Torsional Loading and Dynamic Mass Displacement

Accidental torsion in buildings is a relevant structural phenomenon caused by the discrepancy between the center of mass and the center of rigidity, generating undesired rotational responses under seismic loading. This study aims to compare two approaches used in modal response spectrum analysis to address this phenomenon. The first is the conventional quasi-static procedure, which introduces equivalent torsional moments through an artificial five percent eccentricity in plan. The second is the dynamic procedure, which incorporates this eccentricity by directly displacing the center of mass in the structural model. Eight reinforced concrete buildings with different geometric configurations were modeled using ETABS software. Critical structural response variables were evaluated, including inter-story drift, base shear, and the torsional irregularity index. The results showed significant differences between both procedures, with absolute drift variations of up to ±20 %, base shear increases of up to 12.7 %, and torsional amplification reaching 2.2 % under the dynamic approach. This method enabled the activation of rotational modes that the quasi-static analysis cannot capture, thus providing a more realistic representation of the system's dynamic behavior. It is concluded that the dynamic procedure offers relevant technical advantages for the analysis of structures with potential for accidental torsion. Although the study is based on linear elastic models, its findings provide valuable evidence for seismic-resistant design and the evolution of code-based design criteria.