Demonstration of Torsional Coupling Caused by Closely Spaced Periods—1984 Morgan Hill Earthquake Response of the Santa Clara County Building

The parameters of the dominant modes of vibration of the steel-framed Santa Clara County Office Building in San Jose, California, are determined using “the modal minimization method” for structural identification. The optimal estimates of the model parameters are determined by minimizing a selected measure-of-fit between the responses of the structure and the model. Two types of models are used: (1) A planar linear model with classical damping and (2) A three dimensional linear model consisting of rigid floor decks, where each floor is allowed three degrees of freedom - two orthogonal translations plus a rotation. The Santa Clara County Office Building continued vibrating in a free vibration manner with very low damping, long after the intense part of ground motion had ended. The records of its torsional motion exhibit a strong beating effect which is explained by the strong coupling of torsional and translational modes of vibration. Such a strong coupling of modes of vibration is attributed to the proximity of the value of torsional stiffness to that of translational stiffnesses.