Kurama and Shen 2000
The authors use an analytical study to assess the seismic behavior and design of unbonded post tensioned hybrid coupled walls. Concrete walls are coupled using steel beams and unbonded post-tensioning, without embedding the beams into the walls, and then subjected to monotonic and cyclic loading. The objective was developing a type of hybrid coupled wall system using unbounded post-tensioning, without embedding the beams into the walls. It was expected the lateral stiffness and ultimate strength of unbonded post-tensioned hybrid coupled walls would be similar to walls with embedded steel coupling beams. Moreover, in the design, the system is intended to soften and undergo large nonlinear rotations without significant permanent residual rotations in the beams or walls. In the paper, behavior of unbonded post tensioned coupling beams under monotonic loading and cyclic loading were presented. After that, beam rotation behavior was explained.
System Concept
PT strands and angles were used in the connections of concrete beams to walls. The post-tensioning force was provided by multi-strand tendons. These were placed on both sides of the beam web without contacting the beam. In order to prevent bond between the PT steel and the concrete inside the wall, PT steel was placed into the oversize ducts. By this method, the post-tensioned steel is anchored to the coupled wall system only at the two end locations. The beam-to-wall connection regions included top and bottom seat angles bolted to the beam flanges and to steel plates embedded inside the walls. The purpose of these angles is to yield and provide energy dissipation during an earthquake. These yielding angles could be replaced after the earthquake. Yielding is a desired behavior for the angles when a gap forms between the beam and wall. This gap allows the beam to undergo large nonlinear rotations with little permanent residual rotations. As the walls displace laterally, the tensile forces in the post-tensioned steel increase and resist opening of the gap. Upon unloading, the post-tensioning steel provides a restoring force that tends to close the gaps.
Analytical Study
An analytical model was developed to investigate the behavior of unbonded post tensioned hybrid coupled walls under seismic loads and to conduct nonlinear static and nonlinear dynamic time-history analyses of multi-story unbonded post-tensioned hybrid coupled walls. The DRAIN-2DX Program was used for the analytical study. In this part of the paper, the modeling approach for beams, walls and the connections were explained. After that, behavior of the unbonded post tensioned coupling beams under monotonic loading and cyclic loading were presented. Moment rotation behavior of the beam and the effect of the gap opening on the stiffness of the beam were explained. According to analytical results, the unbonded post-tensioned steel coupling beams with stiffness and ultimate strength similar to embedded steel coupling beams can be designed to soften and undergo large nonlinear rotations of up to 7.5 percent without significant permanent residual rotations upon unloading.
Reference
Kurama, Y.C. and Shen Q. (2000). “Lateral Load Behavior of Unbonded Post-Tensioned Hybrid Coupled Walls,” Report from University of Notre Dame, Civil Engineering and Geological Sciences.