Title: Seismic Performance of Innovative Reinforced Concrete Coupling Beam—Double-Beam Coupling Beam
Author(s): Youngjae Choi, Poorya Hajyalikhani, and Shih-Ho Chao
Publication: Structural Journal
Appears on pages(s): 113-125
Diagonally reinforced coupling beams (DCBs) are commonly used as seismic force-resisting members for medium- to high-rise buildings. The diagonal reinforcing bars in DCBs are most effective when the beam has a span-depth ratio of less than 2. However, modern construction typically requires span-depth ratios between 2.4 and 4, which leads to a very shallow angle of inclination for the diagonal reinforcement. The lower angles of inclination, when combined with the detailing requirements specified in ACI 318, result in reinforcement congestion and construction difficulties. These issues can be considerably minimized by using an innovative and simplistic reinforcing scheme consisting of two separate cages similar to those used for typical beams in reinforced concrete special moment frames. The proposed coupling beam acts like a conventional coupling beam under small displacements. Upon the occurrence of large displacements, cracks begin developing at the beam’s midspan and midheight area where the narrow unreinforced concrete strip is located, gradually propagating toward the beam’s ends. The cracks eventually separate the coupling beam into two relatively slender beams where each has nearly twice the aspect ratio of the original coupling beam. This split essentially transforms the shear-dominated deep beam behavior into a flexure-dominated slender beam behavior. Because damage initiates from the center of the beam and then spreads toward the ends, the beam’s ends maintain their integrity even under very large displacements, thereby eliminating the sliding shear failure at the beam-to-wall interface. Testing results on half-scale specimens with span-depth ratios of 2.4 and 3.3 showed that the proposed coupling beam not only has high ductility and shear strength, but can significantly reduce construction issues in conventional DCBs. In addition, the proposed coupling beam arrangement has great architectural flexibility, allowing utility ducts to be placed inside the coupling beams where the gap between the two steel cages is located.