**1. Introduction**

Reinforced concrete (RC) columns subject to both central axial load and flexural load undergo a rapid deterioration of strength due to lateral expansion of inner concrete after delamination of the concrete cover. At this point, transverse reinforcement of RC columns confines the lateral expansion of core concrete, thus increasing the compressive strength and ductility under lateral load. The lateral confinement performance of RC columns is influenced by the strength ratio of transverse reinforcement and concrete, the amount and shape of transverse reinforcement, and the shear span to depth ratio [1–10]. Many models have been developed based on experiments to predict the strength and behavior of RC columns and concrete cylinders with shear reinforcement [11–18]. Extensive research has also been conducted not only on conventional crossties, but also on interlocking spirals to suppress lateral expansion of core concrete and buckling of longitudinal reinforcement, as well as shear reinforcement of RC columns using Fiber-Reinforced Polymer (FRP) [19–27].

Transverse reinforcement of RC columns can be largely classified into crossties and spiral reinforcement. Compared to crossties, spiral reinforcement can effectively confine the inner concrete of RC columns, and is thus more advantageous in enhancing ductility. However, due to difficulties in bar arrangemen<sup>t</sup> during construction, there is a higher demand for columns with rectangular cross-sections than for those with circular cross-sections. As such, most columns have shear reinforcement in the form of crossties, which are also more common than spiral reinforcement even in rarer columns with circular cross-sections. In terms of ductility enhancement, introducing sub-ties is more efficient than decreasing the spacing of transverse reinforcement. Sub-ties are e ffective at suppressing the lateral expansion of core concrete, and also prevent the buckling of longitudinal reinforcement. The use of conventional sub-ties with 90-degree and 135-degree standard hooks can result in problems such as mutual interference with longitudinal reinforcement, overcrowding of reinforcement, poor filling of concrete, and deterioration of constructability. When subject to repeated lateral forces such as seismic loads, the loosening of 90-degree hooks causes a decrease in e ffective lateral confinement, which may contribute to buckling and weakening of longitudinal reinforcement.

This study proposed a new type of transverse reinforcement with spirally arranged crossties to improve the constructability of RC columns and to resolve structural issues associated with conventional crossties. Cyclic loading tests were performed on RC columns with the new spiral-type transverse reinforcement and subject to a central axial load. The e ffects of transverse reinforcement shape and yield strength on crack formation, ductility capacity, energy dissipation capacity, and e ffective sti ffness in relation to drift angle were assessed. In addition, the constructability of RC column members was evaluated by measuring time consumed in arranging the proposed transverse reinforcement.

#### **2. Experimental Program**
