# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 3 Results of the Experiments
### Comparative Analysis of the Test Results  
The comparative relationships between the axial load and the lateral and axial strains in the steel tubes for all tested models are presented in Figure 11, where the negative and positive values indicate the compressive and tensile strains, respectively. The compressive and tensile yield strains of the steel are marked with vertical dashed lines. It was observed that specimens of both groups behaved in a similar manner and that the chords and the battens were engaged in load bearing. The models of both groups showed linear behavior until they reached the steel yield stresses in the axial direction. The behavior of the built-up columns outside the elastic range largely influenced whether the chord was filled or not. The TCHST specimens exhibited a  
Figure 11: Comparative relations of load vs. strains of all specimens. Figure 10: TCCFST column models after the experiment: SC-21 (**a**); SC-22 (**b**); and SC-23 (**c**). nonlinear increase in deformations, while the TCCFST specimens exhibited almost a linear response until the peak load. After the load reached approximately 90% of the bearing capacity, the strains in the lateral direction reached the yield strain of the steel, and the out-of-plane deformations increased considerably. Because of the second-order effect and the loss of stability, the columns entered the plastic stress range. After achieving the peak load, excessive lateral deformations led to a sudden loss of the load-bearing capability. All columns failed after the loss of global stability as out-of-plane deflections of the chords developed. The chords of both groups performed as flexible members without observable local buckling of the steel tube walls throughout the experiment. Local failure along the battens or weld fracture, at their connections with the chords, where the maximal internal forces in the battens are reached, was not detected. A minor variance among the ultimate strains of the columns of both groups yielded the conclusion that the high-strength concrete infill had a comparatively low influence on the column's ductility. The relationships of axial force vs. axial shortening for typical specimens are shown in Figure 12. Similarly to the previous figure, it can be seen that the concrete in the chords significantly enhanced the load-bearing capacity and overall stiffness of the column and that the specimens with filled chords had a linear increase in deformations almost to the maximum capacity. The battened two-chord specimens for in-plane conditions behaved like frame structures, where the chords were subjected to axial load and bending moments, while the bending and shear deformations affected the batten's behavior. The compressive strength of a hollow steel tube member could significantly be improved with concrete infill, as shown in Figures 11 and 12. The ultimate capacity and the initial stiffness of the specimens with concrete-filled chords were substantially greater than those of the built-up specimens with hollow chords. The mean value of load-bearing forces, the standard deviation (\(S_{n}\)) of the forces, and the coefficient of variation (COV) for the column models of both groups are shown in Table 4. Additionally, the coefficient of enhanced strength of the two-chord column with the concrete-filled tubes in comparison with the hollow tube two-chord column (\(ks\)) is given. The coefficient is calculated as a ratio:  
\[ks=\frac{N_{u,CF}}{N_{u,H}} \tag{2}\]  
where \(N_{u,CF}\) and \(N_{u,H}\) are the mean values of the ultimate forces for the filled and hollow two-chord columns, respectively. Coefficient \(ks\) shows that, in this research, the two-chord  
Figure 12: Deformation vs. axial load—comparative relationships. built-up columns with high-strength concrete-filled steel tubes will have a 1.74 times higher ultimate load.