# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 6 Conclusions  
The following conclusions were drawn based on the test parameter ranges:  
1. In this research, steel with a 600 \(\mathrm{N/mm^{2}}\) yield stress was used in combination with concrete with a cylinder compressive strength of 95 \(\mathrm{N/mm^{2}}\). During the experiments, both materials reached their maximum strength at approximately the same strain level; thus, the full potential of the high-strength steel and the high-strength concrete was utilized. 2. The load-bearing capacity of the hollow two-chord columns improved by around 1.74 times due to the concrete infill, while the presence of the concrete infill in the chords increased the slenderness of the column by 16%. 3. The filled specimens exhibited a stiffer behavior than and a similar deformational capacity to the hollow steel tube column models. The failure of the column models occurred as a result of the global buckling of the chords, the crushing of the filled concrete, and steel yielding. Figure 19: Comparison of deformations vs. load curves (**a**) and comparison between predicted and experimental failure modes (**b**). Figure 20: von Mises equivalent stresses in steel (**a**) and normal stresses in concrete (**b**). * A load distribution analysis indicated that composite action between all cross-section parts was achieved. It was observed that 36% of the load was carried by the concrete, while 64% of the load was carried by the steel tube. * A hoop stress effect was registered in the steel tube; i.e., a plane state of stress was formed, which led to the enhanced properties of the concrete due to the confining effect provided by the steel tube. * The behavior of the TCCFST column could be divided into two parts, mainly the behavior before and after reaching the yield stress of the steel tube. In the first part, the structural response of the column was linear, and, after reaching the yield stress, the buckling of the chords occurred. * All three analyzed structural codes gave relatively good forecasts of the axial compressive capacity of the TCCFST column. The Eurocode 4 and AISC 360 predictions were proven to be underestimated. The DBI/T13-51 design code gave the best predictions and, thus, could be recommended for the calculation of the design strength of TCCFST columns. * A numerical analysis can be successfully applied if the appropriate material models are correctly entered into the FEM model. The FEM model showed good predictions of strength, stresses, deformations, and buckling. The validity of the presented research is limited to TCCFST columns with a nominal slenderness of around 30, as well as to the high-strength steel and high-strength concrete with the full strain compatibility. Further research is required on the material and geometric properties of TCCFST so that the validity of the DBI/T13-51 predictions can be verified. Conceptualization, M.B. and J.K.-S.; methodology, M.B. ; software, A.L. ; validation, M.B., A.L. and A.C.; formal analysis, J.K.-S.; investigation, J.K.-S.; resources, J.K.-S.; data curation, J.K.-S. and A.L. ; writing--original draft preparation, A.L. ; writing--review and editing, A.L. and A.C.; visualization, A.L. ; supervision, M.B. ; project administration, J.K.-S.; funding acquisition, J.K.-S. and A.C. All authors have read and agreed to the published version of the manuscript. This research was funded by The Provincial Secretariat for Higher Education and Scientific Research, Autonomous Province of Vojvodina, Republic of Serbia (Project No. 142-451-2640/2021-01). Not applicable. The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available because they are part of the ongoing doctoral research of the first author. For support in the realization of the experimental research, the authors are thankful to their colleagues from the Laboratory for Material and Structure Testing at the Faculty of Civil Engineering Subotica, University of Novi Sad, Republic of Serbia. The authors declare no conflict of interest.