Influence of the Rotational Mass Inertia on the Torsional Seismic Response

Structures with uneven distributions of mass and stiffness (in the elastic range of behavior) and of mass and strength (in the inelastic range of behavior) are subjected to general torsion. These structures are called plan irregular ones. From a static elastic point of view general torsion may be described as a rotational movement around the center of stiffness, due to a given eccentricity (a distance between the center of stiffness, CR and the center of mass, CM). In order to describe more accurate the seismic behavior of plan irregular structures, at least a dynamic elastic point of view is needed. Under dynamic seismic input rotational mass inertia tries to overcome the rotation generated by the given eccentricity. Therefor a rotational inertial moment appears, as counterpart for the rotation around the CR. When moving to the nonlinear range of behavior, the rotational movement around CM takes place as counterpart of the rotational movement around the center of strength, CF. The authors aim to investigate how the rotational moment, due to mass inertia, changes for different structural layouts and seismic inputs. Because the rotational inertial moment is a measure for the elastic as well as for the inelastic structural behavior, it turns out to be a consistent way of describing the seismic response of plan irregular structures.