Three-Dimensional Inelastic Response of an RC Building during the Northridge Earthquake

The three-dimensional inelastic earthquake response of a seven-story reinforced-concrete building during the 1994 Northridge earthquake is studied herein. The objectives of this investigation are as follows: (1) to understand the inelastic behavior of the building using recorded motions; and (2) to propose a simplified inelastic model that could explain the lateral-torsional coupling observed in this nominally symmetric building. Because several two-dimensional inelastic models of the building have been reported by other researchers, this paper focuses on the three-dimensional behavior of the structure. Response results of a simplified inelastic stick model that uses the story-shear and torque surfaces are compared with the results obtained from a conventional elastic three-dimensional building model. These results suggest that damage in the building occurred in the first few cycles of the response, and that the building showed markedly inelastic torsional behavior in spite of its nominal symmetry in plan. Such torsional behavior could also occur in other symmetric-plan buildings with strong perimeter frames, and constitutes a rather new phenomenon that should be studied further. It is proposed herein that such behavior be foreseen in design by using the concept of the ultimate story-shear and torque surface.