رفتار هیدرولیکی تیرهای کوپلینگ بتن تقویت شده مرسوم در دیوار برشی هموار شده با بتن مسلح Hysteretic Behavior of Conventionally Reinforced Concrete Coupling Beams in Reinforced Concrete Coupled Shear Wall

1. Introduction Structural walls can serve as an effective structural system to resist lateral loads, such as earthquakes or winds, in high-rise buildings. Coupling beams that connect these walls, which behave independently at each floor, can improve the building’s lateral resistance capacity. Eurocode 8 (2004) defines a coupling beam as a beam that is designed to reduce the bending moment that acts on the wall by about 25% compared to the case where the wall behaves independently. Figure 1 shows, however, that a greatly amplified nonlinear deformation in the coupling beam is required, even if little deformation occurs in the walls. That is, the coupling beam should not only have the ability to resist the bending moment but should also possess a certain deformation capacity because it plays a role in inducing the ductile behavior of the wall. In this regard, Eurocode 8 (2004) states that a coupling beam can be utilized as the beam in the moment-resistant frame only in those cases where it is dominated by the flexural behavior, that is, when the ratio of the length (l) to the depth (h) is more than 3.0 or the beam can resist the shear force by diagonal reinforcements. According to American Concrete Institute (ACI) code (2014), although a coupling beam can be designed according to the usual beam design method where l/h C 4, as shown in Fig. 2, it can nonetheless be reinforced with diagonal cages in the case of l/h\4. Specifically, a coupling beam with l/h\2 and whose factored shear force (Vu) is greater than 4 k ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fck Acw p is required to be designed with diagonal cages at the center of the span. Where fck is the compressive strength of the concrete and Acw is the cross-sectional area of the web of the beam. The ACI code also requires that the nominal shear strength of the beam should be developed only through diagonal reinforcements and it should be confined sufficiently by transverse reinforcements, as shown in Fig. 3a. Or, the entire beam should be confined by sufficient transverse reinforcement, as shown in Fig. 3b, so that the diagonal reinforcements cannot undergo compressive buckling. The basic concept behind reinforcements for coupling beams for a special structural wall is to confine the beam using closely spaced transverse reinforcements to prevent compressive buckling of the diagonal reinforcements. Accordingly, the currently used codes (ACI 2014; Eurocode 8 2004; Korea Concrete Institute (KCI) 2012) for different areas of the world stipulate that at least four diagonal reinforcements should be placed and laterally confined. Such construction details, however, make actual construction difficult at field sites. Consequently, the current ACI code (2014) requires that lateral confinement of coupling beams should be used instead of diagonal reinforcements, as shown in Fig. 3b, which were recommended in the ACI Committee 318 2011 edition of the ACI code (ACI 2011). These construction details, however, still pose difficulties in construction due to the excessive use of lateral confinement reinforcements and diagonal reinforcements. In the current standards (ACI 2014; Eurocode 8 2004; KCI 2012), all the shear forces and moments that act on coupling beams are assumed to be borne only by the diagonal reinforcements. Therefore, transverse and longitudinal reinforcements that confine the diagonal reinforcements are not reflected in the design at all. Consequently, all the reinforcements that are placed in the longitudinal direction of the beam are cut off at the interface, without being affixed to the walls. In other words, plastic behavior of the diagonal reinforcements is encouraged in order to dominate the behavior of the coupling beams at the ends of such beams where the moment and shear forces are greatest.