An approach for minimum rotational response of medium-rise asymmetric structures under seismic excitations

Studied in this article is the elastic and inelastic rotational behavior of asymmetric multi-story buildings under unidirectional and bidirectional excitations in relation to the location of the modal center of rigidity. The concept of this point in mono-symmetric uniform buildings has been outlined by the author in earlier papers, and its significance lies on the property that when its location is within a close distance from the mass axis the torsional response is mitigated. At present, a generalized definition of this point is given for buildings with double eccentricity, using (1) the approach of the continuum medium and (2) the stiffness matrix methodology. To determine the location of modal center of rigidity in practical terms, a few modal quantities are required (element frequencies and effective modal masses), which may be easily obtained by commercial engineering software. To verify the aforesaid property of modal center of rigidity under unidirectional and bidirectional excitations, the rotational response of 10-story wall-frame structures is examined under the ground motions of Erzincan 1992 and Kobe 1995.