Evaluation of Seismic Design Provisions for Torsionally Irregular Buildings

摘要

This study evaluates ASCE/SEI 7-16 design requirements for seismic-induced torsion in buildings. Modifications to ASCE/SEI 7-16 torsion design requirements are also proposed. These modifications address deficiencies that are identified in the ASCE/SEI 7-16 provisions while also reducing unnecessary sources of conservatism. Collapse resistance is the primary metric used for measuring seismic performance, consistent with the stated goals of ASCE/SEI 7-16. Early findings of this study are published in a prior work, DeBock and Valley (2018). This paper similarly describes the analysis methods and summarizes the early findings, but then focuses more on recent findings. Thus, the two works are complimentary, but able to be read independently. The study shows that ASCE/SEI 7-16 torsion design provisions are generally conservative for most building configurations. However, buildings that rely heavily on lines of lateral resistance orthogonal to the design earthquake force to resist torsion moments tend to have lower performance; such designs can be improved by considering the effects of the orthogonal ground motion component in design (i.e. the 100%-30% orthogonal load combination rule for lateral seismic forces). If this change is implemented, the study finds that the triggers for requiring a redundancy factor of 1.3 can be altered, so that fewer buildings are affected. Furthermore, some buildings with large inherent eccentricity are not classified as torsionally irregular by ASCE/SEI 7-16. Therefore, an additional measure of torsional irregularity is proposed, which is based on the fraction of a story"s total lateral strength on each side of the center of mass. Two different methods of computing design accidental torsion are evaluated: (1) the static procedure (most commonly used in practice, even with modal response spectrum analysis); (2) directly offsetting the model center of mass by 5% of the building plan dimension in each orthogonal direction, which is allowed for modal response spectrum analysis and linear time-history analysis. Demands from the two methods are substantially different; with the latter being unconservative for buildings with long torsional periods. Additionally, the study demonstrates that buildings classified as extremely torsionally irregular need not be prohibited in Seismic Design Categories E and F, as long as the lateral system is proportioned properly; neither should the equivalent lateral force procedure be prohibited on the basis of torsional irregularity. The study is part of a project managed by the Applied Technology Council (ATC) under FEMA contract HSFE60-12-D-0242 task order HSFE60-16-J-0223, entitled "ATC-123: Improving Seismic Design of Buildings with Configuration Irregularities." Any opinions expressed are those of the authors and do not necessarily represent the opinions of ATC or FEMA.