Reinforced concrete coupled walls are an efficient structural system for medium to tall buildings that provide significant stiffness and strength against wind and seismic loads. The coupling of the individual wall units is typically provided by short and stiff coupling beams which deflect in double curvature with high shear stresses, and therefore are susceptible to brittle shear failures; refer to Fig. 1. At the same time, in regions of high seismicity, the coupling beams are required to possess large ductility and energy dissipation capacity. However, while a number of experimental programs have been performed to evaluate the ductility and complete cyclic response of coupling beams, (2-12) the boundary conditions used in the tests deviate in an important detail regarding the boundary conditions encountered in real structures: in coupled wall systems, the stiff walls and floor diaphragms restrain the elongation of the beams, and this generates compression in the coupling members influencing their behavior (Fig. 1). Because past experimental studies have almost exclusively neglected this effect, it is not captured by current mechanical models for coupling beams and it is not included in code provisions for the performance-based seismic evaluation of existing buildings. (13-16) Exceptions are a test performed by Adebar et al. (17) and a test by Poudel et al. (18) that featured diagonal reinforcement and elastic axial restraint. Naish et al. (19,20) tested specimens with portions of prestressed and non-prestressed slabs that also provide a certain axial restraint. It was observed by Adebar et al. (17) that the test specimen exhibited approximately twice larger overstrength than expected, indicating possible unfavorable failures in adjacent walls. Poudel et al. (18) reported a 30% increase of shear strength, and Naish et al. (19,20) reported a 20 to 30% increase associated with the bending carried by the slab.
In part motivated by the test by Adebar et al., (17) Barbachyn et al. (21) performed an analytical study to evaluate the effect of axial restraint. They used a nonlinear strut-and-tie model that accounts for two shear mechanisms: tension and compression in the diagonal reinforcement and the concrete, and a truss mechanism involving the stirrups. After performing a parametric study on a set of diagonally reinforced coupling beams, it was concluded that axial restraint can cause premature failure due to splitting and/or crushing of the struts. Furthermore, it was observed that the restraint was more critical for short beams. Considering that these conclusions were reached for beams with diagonal reinforcement that typically ensures enhanced shear behavior, it is important to also study the effect of restraint on conventionally reinforced beams with longitudinal reinforcement and stirrups. Such beams are common in pre-1970s coupled-wall structures and are particularly vulnerable to brittle shear failures, as evidenced by past earthquakes. (22-27)
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