Seismic code provisions for asymmetric structures: a re-evaluation: Fid-Bau Portal

Seismic code provisions for asymmetric structures: a re-evaluation

Rutenberg, A. / Pekau, O.A.

Abstract Results of a parametric study on earthquake time history response of asymmetric single storey structures with one axis of symmetry modelled as two degree-of-freedom systems are compared with static seismic code provisions. A large number of such systems with a range of mass centre to rigidity centre eccentricities, a range of uncoupled lateral natural frequencies and a number of torsional to lateral frequency ratios $Ω 0$ were subjected to several earthquake acceleration records. Five percent damping was assumed in the two coupled modes. Maximum displacements at several locations along the roof deck were computed, normalized with respect to the symmetric case, averaged, and compared with code oriented static methods. The study shows that the static approach does not give reliable estimates for the response of frames in asymmetric buildings, even when the amplification factors provided by earthquake codes are incorporated into the formulation. In particular, code provisions often underestimate the response of frames located on the side of the rigidity centre away from the mass centre for small to moderate eccentricities when $Ω 0$ ² = 0.5, 1.0; whereas for systems with higher torsional rigidities $(Ω 0$ ² ⩾ 2.0), the static approach appears to yield reasonable results. For members located on the opposite side of the roof, code provisions, including recent revisions, appear to overestimate the response with decreasing frequency ratio.