Analytical research on plan torsional irregularity presented at the 2016 SEAOC Conference (Improving Design for Plan Torsion, Korolyk and Wagner) suggests that the treatment of torsional irregularity in ASCE 7-10 poorly represents the correlation with reduction in seismic performance and unduly penalizes structures based on the aspect ratio of plan dimensions without regard for other aspects of the structure, such as lateral strength or ductility. The preceding work also suggests that a simple empirical formula may be used to better relate seismic performance to torsional eccentricity and other critical structural characteristics. The empirical formula fits results of nonlinear Incremental Dynamic Analyses (IDA) of idealized single-story structures with variations in torsional eccentricity, lateral strength, ductility, and plan aspect ratio. This paper presents the application of the preceding work to three structures currently under design in the San Francisco Bay Area in California. Prescriptive linear elastic analysis and design of each structure is conducted in accordance with ASCE 7-10. Each structure is modeled and analyzed using nonlinear IDA to establish expected seismic performance. The results are compared with the structural performance implied by ASCE 7-10 and what is predicted by the empirical formula presented in the previous research. Suggestions for improvement to the empirical formula and methodology and to future versions of ASCE 7 are presented, and commentary on quantifying torsional irregularity is given. From the IDA results, certain key observations are evident: (1) the controlling mechanism of the lateral system is important, (2) accidental torsional eccentricity makes little impact in expected seismic performance. (3) lateral systems with different deformation modes acting in parallel lead to twisting during ground shaking, but this twisting may not be detrimental, and (4) the empirical formula from K&W, 2016 is adaptable to adequately match the response the case study buildings.
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