The influence of accidental torsion on the inelastic dynamic response of buildings during earthquakes

Nonlinear dynamic analysis is becoming an accepted procedure to assess the performance of building structures during earthquakes. Several documents have emerged to provide guidance in terms of mathematical modeling, ground motion selection and scaling, and acceptability of results. While there are some significant differences in these documents, one feature in common is that explicit inclusion of accidental torsion in the nonlinear response history analysis is generally not required. One notable exception is ASCE/SEI 7-16 (2017), which requires performing nonlinear analyses, including accidental torsion when the building has a torsional irregularity. The analysis presented in this paper supports this requirement, but also shows that neglecting accidental torsion in the analysis of torsionally regular buildings can be problematic. Failure to include accidental torsion in nonlinear analysis of torsionally irregular buildings may indicate stable response instead of dynamic instability, or may significantly underpredict deformations and thereby falsely indicate that deformation-based acceptance criteria have been met. Additionally, it is shown that simultaneous application of ground shaking in orthogonal directions is essential, and that the spatial distribution of geometric nonlinearities related to global torsional response must be included directly in the analysis.