The role of inelastic torsion in the determination of residual deformations

Recent developments in performance-based seismic design and assessment approaches have emphasized the importance of properly assessing and limiting the residual (permanent) deformations typically sustained by a structure after a seismic event, even when designed according to current code provisions. The performance-based design framework for residual deformations, previously developed by the authors for 2-D regular structures, is further extended to the behavior of 3-D irregular (asymmetric in-plan) buildings. The seismic response of a set of single-story systems, comprising of seismic resisting frames, and modeled to represent alternative materials (concrete or steel), is investigated under uni-directional earthquake loading excitations. Different layouts in plan, leading to either torsionally unrestrained or restrained systems, are considered. The influence of varying torsional restraint is investigated to define how residual diaphragm rotations and center-of-mass displacements are affected by changing levels of stiffness and strength, or mass eccentricity. From these findings additions to the previously proposed estimation procedure are made, with a specific example used to validate the suggested changes. Finally, a general example is developed based on currently available methods of evaluating maximum torsion response. It is suggested that such approaches are likely to be insufficient as they do not explicitly define how seismic-resisting elements are influenced by inelastic torsional response.