Seismic Evaluation and Retrofit of Asymmetric Multi-Story Wood-Frame Building

Abstract

The presented research focuses on large-scale seismic testing under multi-directional ground motion of a three-story high, wood-frame residential building representing late 1960's California construction. Earthquake lateral resistance is provided by plywood shear walls around the perimeter of the building with an open front in the first story for tuck-under parking. Accordingly, the as-built structural configuration is asymmetric in plan and discontinuous in elevation with tendency to twist about a vertical axis and to form a weak story mechanism. The test results confirm this tendency. They also reveal the sensitivity of the response to multi-direction ground motion. Asymmetric damage patterns are induced by the multi-component motions in the walls oriented perpendicular to the open front for the as-built test structure, with or without finish materials. However, the observed damage remained noncritical as far as structural integrity is concerned even for ground accelerations exceeding 120% of that recorded during Northridge earthquake. This is viewed as a consequence of the better construction of the test building compared to actual construction. Investigated retrofit includes adding a welded moment resisting steel frame around of the garage opening and strengthening the diaphragm to header beam connections. The study indicates that the retrofit significantly reduced the maximum story drift in the open front. Moreover, the finish material and the retrofit greatly reduce the maximum rotation of the building about the vertical axis.