Nonlinear behavior of posttensioned hybrid coupled wall subassemblages

This paper discusses the nonlinear load-deformation behavior of coupling beam subassemblages in a new type of hybrid coupled wall system for seismic regions. Coupling of concrete walls is achieved by posttensioning steel beams to the walls using unbonded posttensioning tendons. Different from conventional hybrid coupled wall systems, the coupling beams of the new system are not embedded into the walls. One of the most important advantages of the new system is that it can be used to couple existing walls as a part of a strengthening and retrofit scheme. This paper describes an analytical investigation of the nonlinear behavior of coupled wall subassemblages that include a coupling beam and the adjacent wall regions at a floor level. The effect of structural design parameters such as the amount of posttensioning, beam properties, and wall properties on the behavior of the subassemblages, including the amount of coupling, energy dissipation, and deformation capacity is investigated. A simple design method to estimate the nonlinear load-deformation behavior of the subassemblages is developed. The investigation is based on an analytical model using fiber elements, which is verified by comparing with a finite element model. The results indicate that unbonded posttensioned steel coupling beams with initial stiffness similar to embedded steel beams can be designed to provide stable levels of coupling without experiencing significant damage over large nonlinear cyclic deformations. The amount of coupling between the walls can be controlled by changing the posttensioning force.