The Seismic Performance of Coupled Reinforced Concrete Walls
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Reinforced concrete core walls are used commonly in modern building construction as the primary lateral-load resisting system. Common core wall configurations include solid walls in one direction and walls with openings in the orthogonal direction to access elevators or to meet architectural requirements. Therefore, in the orthogonal direction, the walls are typically coupled together by reinforced concrete “coupling” beams. A significant amount of research has focused on the design of coupling beams to ensure that they exhibit ductile response through multiple cycles to large drift demands. However, only a few research studies have investigated the seismic behavior of coupled wall systems, and most of these previous studies have considered coupled-walls in low-rise structures. Most coupled walls are used in elevator cores, which are more typical in mid- to high-rise construction. A research study was undertaken by faculty and students at the Universities of Washington and Illinois to specifically investigate this category of structural system. The advanced testing capabilities of the NEES University of Illinois at Urbana-Champaign (UIUC) testing facility permitted unique experimental simulation of this system. The coupled-wall test specimen simulated the bottom three stories of a ten-story building in a region of high seismicity designed with typical geometry and reinforcement. Specialized load-and-boundary-condition boxes (LBCBs), which are part of the UIUC NEES laboratory, were used to simulate demands originating from the upper stories of a ten-story structure subjected to an ASCE 7-05 lateral load distribution and gravity load. Cyclic lateral loading was applied under displacement control until significant loss of lateral load carrying capacity was observed at 2.27% drift. Strength loss resulted from significant core crushing and longitudinal bar buckling in the compressive wall. Experimental data from this test, in combination with data from previous tests of coupled walls, to evaluate the performance of coupled wall systems including elastic and nonlinear modeling approaches as well as current design methods of coupled wall systems. This research indicates that, coupled wall behavior observed in laboratory testing as well as in nonlinear modeling, is inconsistent with current design assumptions and elastic analyses. For significant earthquake demands, current design approaches including elastic analysis tend to neglect the effects on the flexural strengths of the wall of the large compressive axial loads that develop in the compression piers as well as the redistribution of shear demand from the tension wall to the compression wall as a result of these axial loads. This behavior ultimately limits the ductility of the coupled wall system to the deformation capacity of the compression wall pier.
- Civil engineering