# Seismic Performance of Ductile Column with Rectangular Hollow Cross-Section in RC Building
## 5 Conclusions  
In this study, detailed investigations on RC columns with rectangular hollow cross-sections, in building structures, were carried out to understand their structural characteristics and seismic performance. The following are the major findings obtained from this study. 1. Although the moment of inertia and cross-sectional area decreased in the hollow-sectioned column, by using higher strength concrete with a higher Young's modulus, the initial lateral stiffness of the hollow-sectioned column could be reproduced as that of the solid-sectioned column. 2. In the hollow-sectioned columns, the experimental maximum strength was 1.00-1.06 times the ultimate flexural strength estimated by the cross-sectional analysis using plane section assumption, and the calculations showed good agreement with the experiments. Accordingly, the maximum strength evaluation by the plane section assumption is also effective for the RC columns with rectangular hollow cross-sections. 3. Under an axial force ratio of 0.16, the structural performances between hollow- and solid-sectioned columns were found to be almost equivalent, because their hysteresis curves, failure patterns, strain distributions of reinforcement, flexural and shear deformations, and energy absorption capacity were almost the same. 4. In the hollow-sectioned specimen CM12 with an axial force ratio of 0.3, the structural performance was almost the same until the 2.0% drift angle; however, sudden deterioration of the load-bearing capacity occurred at the second loading cycle, with concrete compressive failure occurring at the plastic hinge region near the column end. Such brittle failure was induced by the location of the neutral axis at the ultimate flexural state, which was estimated to exist beyond the flange of the hollow cross-section. Figure 18: Minimum principal stress distribution of concrete (\(R=1.0\%\)): (a) CM12; (b) CM13. In the hollow-sectioned specimen CM13 with an axial force ratio of 0.3, the position of hollow sections was, therefore, planned at 0.75\(D\) from the column end, beyond the plastic hinge regions. Then, the structural performance between the hollow- and solid-sectioned columns was found to be almost the same. 6. The relationship between the magnitude of the compressive axial force and the neutral axis location at the ultimate flexural state was found to be one of the crucial factors for the deformation capacity and the application scope of RC columns with rectangular hollow cross-sections. If the neutral axis is located within the flange of the cross-section, the concrete compressive stress does not work on the hollow section; therefore, the compressive stress distribution of concrete, at the plastic hinge region, would be similar to that of a solid cross-section. On the other hand, if the neutral axis exists on the web of plastic hinge regions, the concrete compressive stress on the hollow cross-section is likely to increase abruptly, due to the concrete crushing, at around the ultimate flexural state. 7. The nonlinear FEM analysis was carried out to further grasp the structural behaviors of the rectangular hollow sectioned columns. The drift angles of reinforcement yielding, the maximum strength, and the hysteresis curves by the analysis reasonably corresponded with the experiments. The compressive stress concentration of concrete at the hollow-cross section near the column end was also observed from the minimum principal stress distribution by the analysis, which is considered to result in abrupt brittle failure. Therefore, as explained in this study, for the use of RC columns with rectangular hollow cross-sections, it is recommended that the neutral axis position against the design axial force, at the ultimate flexural state, be calculated and checked in advance. If the neutral axis is located on the web of the hollow cross-section, the failure mode and ductility can be ensured by properly separating the hollow section from the plastic hinge region.