Torsion in symmetric buildings due to soil‐structure interaction: A discussion on code values of accidental eccentricity factor in seismic design

Soil‐structure interaction (SSI) is often neglected in seismic design because of its inherent complexity. Therefore, SSI‐related research is crucial for ensuring that more significant issues reach engineering practice. This study focused on torsional motion to investigate the accidental eccentricity caused by SSI effect by comparing the proportional relationship between the torsional motion of an SSI system and the translational motion of a fixed‐base oscillator. The SSI model was created using a three‐dimensional rectangular foundation embedded in a layered half‐space with an oscillator mounted on the top. We employed an indirect boundary element method (IBEM) combined with non‐singular Green's functions of distributed loads to calculate the system responses of the SSI. Parametric analysis‐based results in the frequency domain revealed that the accidental eccentricity factor owing to SSI was typically in the range of 0.03–0.15. Additionally, a case study of four buildings subjected to 42 earthquakes revealed that the accidental eccentricity factor exceeded 0.05 in several cases, and 0.2 in some cases. Furthermore, it depended on the excitation spectral shape and dynamic characteristics of the SSI system. Therefore, the coded accidental eccentricity factor value of 0.05 is insufficient for ensuring the safety of buildings from the torsional effect of SSI, and a value of 0.1 is more reasonable.