# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 3 Results of the Experiments
### TCCFST Specimens  
The experimental testing of the TCCFST models was conducted in a similar manner to that of the specimens of the hollow group to ensure that the results of both groups could be effectively studied. Thus, a direct assessment of the effect of the compressive strength of the concrete on the ultimate capacity of the built-up two-chord column was possible. Figure 7 shows a diagram of the compressive force versus axial and lateral strains. The yield strains of steel (\(\epsilon_{y}\)) are indicated by the dashed lines. In the initial loading stage, or for forces up to 50% of the maximal load, the TCCFST specimens were in the elastic state, and the response of the specimens was linear. Afterwards, the models demonstrated an indistinct nonlinear increase in deformations, and a noticeable increase in the deformations was observed for the forces around 70 to 80% of the peak load or when the yield strain in the axial direction was reached. At the maximal load on the specimen, the calculated values of normal stresses in the steel tube and the concrete fill were 515 N/mm\({}^{2}\) and 121 N/mm\({}^{2}\), respectively. It was observed in the diagrams that the axial strains of the steel tubes were fairly similar, which led to the conclusion that the specimens behaved as axially loaded members and that the initial imperfections or material variations were negligible.  
Figure 6: Failure modes of hollow built-up column S-21 (**a**) and column S-22 (**b**).  
Figure 7: Measured strains vs. axial load for the filled specimens.  
A diagram of the axial load leading to longitudinal shortening and mid-height lateral deformations of the specimens is presented in Figure 8. The specimens exhibited uniform axial shortening almost to the bearing capacity. As for the previous group, for forces over 90% of the ultimate load, the column started to buckle, which was followed by a sudden increase in side displacements \(U_{m}\) at the mid-height of the specimen, as shown in Figure 8.  
Similar behavior was observed in all TCCFST specimens. Mainly, the axial strains of the filled specimen attained the yield strain before the specimen reached its bearing capacity, which can indicate full utilization of the steel's strength. The effective confining of the concrete infill was uniformly achieved over the entire height of the column. An increase in the load over the elastic limit initiated the stretching of the tube's wall, which influenced the bond between the steel tube and the concrete fill. Regardless, the steel tube was still able to deliver partial confinement of the fill concrete, which resulted in a continued load-bearing response of the specimen. At that time, the response mainly depended on the compressive strength of the concrete. Failure of the specimens occurred when the steel tube could not provide confinement to the concrete, and a further increase in the force led to the crushing of the concrete and the buckling of the chords. The top of the SC-22 specimen is presented in Figure 9a,b. As for the previous group, neither cracks in the welds nor local buckling was detected on the wall of the steel tube. Also, no buckling or deformations were detected in the batten  
Figure 8: Measured axial load versus deformations of concrete-filled steel tube specimens.  
Figure 9: Top of the column model SC-22 after the experiment front view (**a**) and side view (**b**).  
The two-chord column models filled with concrete after the experiments are shown in Figure 10. The breakdown of the columns with the composite cross-section was attributed to a combination of out-of-plane buckling, concrete cracking, and yielding of the steel. The load-bearing forces of the TCSFST columns recorded in the experiments are listed in Table 1. The statistically analyzed results, which include the mean value, standard deviation, and coefficient of variation, are presented in Table 4. The variation between the results was under 1%.