A simple code-like formula for estimating the torsional effects on structures subjected to earthquake ground motion excitation

Plan asymmetric (eccentric) structures, characterized by non coincident centre of mass and centre of stiffness, when subjected to dynamic excitation, develop a coupled lateral-torsional response that may increase their local peak dynamic response.In order to effectively apply the performance-based design approach to seismic design, there is a growing need for code oriented methodologies aimed at predicting deformation parameter. In this respect, for plan asymmetric structures, estimating maximum displacements at different locations in plan, especially at the perimeter, requires an evaluation of the floor rotations. The ability to predict floor rotations can be also useful to extend simplified procedures of seismic design, such as push-over analyses, to plan irregular structures.In this paper, starting from a closed-form formulation identified in previous re-search works by the authors, an estimation of the maximum rotational response of one-storey asymmetric systems under seismic excitation is obtained and developed with respect to different applications. In detail: (1) a corrective eccentricity for the evaluation of the dynamic response of asymmetric systems through “equivalent” static procedures is identified, (2) a sensitivity analysis is carried out upon the accidental eccentricity, (3) the increase in the peak local displacements due to the eccentricity is evaluated at the corner-point of the side of the system. The results provide useful insight into understanding the torsional behavior of asymmetric systems and may directly used for preliminary design and/or check of results obtained through three-dimensional finite-element modeling of the structural system.