Confinement Reinforcement of High-Strength Reinforced Concrete Tied Columns under High Axial Load

The use of high-strength concrete (HSC) in high-rise buildings can reduce the size of structural members and increase the resi-dential space. However, the behavior of HSC at ultimate loads is more brittle than that of normal-strength concrete. Hence, the ACI 318-19 building code requires that all the column longitudinal bars be confined by seismic hooks when using HSC (f(c)' > 70 MPa [10.15 ksi]) or under high axial loads (P-u > 0.3A(g)f(c)'). The require-ment for crossties with seismic hooks at both ends might cause a huge impact on the assemblage of the column reinforcement cage during field installation. This research proposes an experimental study of full-scale column specimens with a 600 x 600 mm (23.62 x 23.62 in.) cross-sectional dimension subjected to quasi-static cyclic loading under constant high axial load. These column spec-imens are composed of both HSC and high-strength reinforcement. Test results show that if crossties with a 90-degree hook at one end are alternated, the column specimen maintains satisfactory seismic performance. This change in detailing can facilitate the field instal-lation of the reinforcement cage for columns using HSC or under high axial loads.