Reduction of reinforcement congestion in slender coupling beam using bundled diagonal bars

The current design method in the ACI building code 318M-14 for reinforced-concrete coupling beams requires complex details, which leads to reinforcement congestion and difficulty in construction. As an original solution for this issue, the use of steel fibres to reduce the amount of transverse and bundled diagonal reinforcement in coupling beams is investigated in this study. In order to estimate the effect of the expected design method, four coupling beam specimens with 2/3 scale subjected to cyclic lateral loads were tested. All specimens were made of steel-fibre-reinforced concrete. One specimen complied with ACI code-specified reinforcement details (for all diagonal, transverse and horizontal bars) and three specimens used bundled diagonal reinforcement with variation in the spacing of stirrups. The global behaviours of the tested coupling beams are discussed, especially focusing on the strength, ductility and energy dissipation capacity as displacement demand increases. The test results illustrate that the lateral force-resisting performance of coupling beams was effectively improved by using steel fibres and bundled diagonal reinforcement, even with a reduced amount of stirrups, compared to that designed following ACI code-specified reinforcement details.