An Innovative and Simplistic Reinforcement Detail for Reinforced Concrete Coupling Beams

Abstract eng:
Diagonally reinforced coupling beams (DCBs) are commonly used as seismic-force resisting members for medium- to high-rise buildings in high seismic zones. The diagonal reinforcing bars in DCBs are most effective when the beam has a span-to-depth ratio less than 2. However, modern construction typically requires span-todepth ratios between 2.4 to 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 utilizing 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 has high stiffness and acts like a conventional coupling beam under small displacements. When large displacements occur, cracks begin developing at the beam’s mid-span and mid-height area where the narrow unreinforced concrete strip is located, gradually propagating towards the beam’s ends. The cracks eventually separate the coupling beam into two 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 towards 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-to-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, because the cracks always initiate at mid-span and mid-height, the damage location can be easily predicted, which makes repair work easier after moderate earthquakes.

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