Behavior of multi-steel reinforced concrete columns with encased steel H-sections

The multi-steel reinforced concrete (MSRC) columns in which multiple encased steel sections form an encased steel cage have been used in major projects at the bottom floors, due to the many advantages brought by the separate encased steel sections, including convenience of transportation and construction as well as lesser on-site welding workload. This paper investigated the behavior of MSRC column with four steel H-sections, one on each corner. Four specimens were tested under axial load, and six were tested under combined axial and cyclic lateral loads. Test results showed that the MSRC columns with sufficient encased steel ratios and proper deployment of battens were able to sustain axial and seismic loads in a rather ductile mode. Both the encased steel ratio and batten thickness had a minor effect on the dimensionless peak resistances, but greatly affected the behavior of the MSRC columns in the post-peak resistance range, resulting in varying ductilities (for both axial and cyclic lateral loads) and hysteretic behavior of the MSRC columns. Especially, if the battens were removed, the H-sections were prone to buckling globally due to the lack of tying from the battens. The stress–strain models for the steel cage-confined concrete in the MSRC columns were proposed, based on which the numerical approach for modelling MSRC columns in OpenSEES were developed, validated, and used for parametric studies. It is suggested that a proper steel ratio should be between 7% and 10%, and the battens have spacings equal to four times their heights, with the equivalent batten ratio which is defined as the ratio of volumes between the battens and the whole specimen within the range of 0.4% to 1.2%.