# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 3 Results of the Experiments  
### TCHST Specimens  
Two axially loaded, hollow steel, built-up specimens were tested for comparison purposes with the results of the concrete-filled specimens. Both columns in this group exhibited similar behavior. The measured axial load (\(N\)) vs. strains (\(\epsilon\)) and load vs. displacement relationships are presented in diagrams in Figure 5a,b, respectively. A linear response of the specimens could be observed from the beginning of the load process to the steel yield strain in the axial direction (Figure 5a). The yield strain limits are marked with vertical dashed lines (\(\epsilon_{y}\)). After reaching the yield strain, the models demonstrated a nonlinear increase in deformations. The specimens reached failure soon after the strains in the lateral direction attained the yield level. This is typical behavior for axially loaded steel columns made from hollow circular steel tubes, for which a biaxial stress state develops. The longitudinal strains in the steel at failure were around 6 to 8%, while the normal stress in the steel tube calculated from the maximal load on the specimen was 513 N/mm\({}^{2}\). Both chords  
Figure 4: The instrument arrangement (**a**); specimen SC-21 before testing (**b**).  
of the same specimen had almost identical axial strains, which meant that minimal or no initial imperfections, such as an eccentric placement of the specimen on the testing machine, were introduced to the specimen. Figure 5b shows the load (\(N\)) vs. axial shortening (\(\Delta\ell\)), and load vs. mid-height lateral deflections (\(U_{m}\)). In the presented diagrams, it could be observed that the specimen exhibited uniform axial shortening up to approximately 70% of the bearing capacity. Afterwards, an increase in vertical deformations occurred at 90% of the ultimate load when the specimen started to buckle, which was noted via an increase in lateral deformations. Labels U\({}_{\text{m,L}}\) and U\({}_{\text{m,R}}\) represent the recordings from two opposed displacement transducers placed on the left (L) and the right (R) sides of a single chord.  
The members of the built-up hollow tube specimens showed a good interaction, and the cracking of the welding on the chord-batten contacts was unnoticeable. In the first part of the load process, the TCHST specimens exhibited elastic behavior, and only small lateral deformations could be detected. However, when the yield strain (\(\varepsilon_{y}\)) limits were reached, noticeable global deformations of the columns were observed. The specimens were in the plastic range until the peak load was achieved. When the load on the specimen reached around 90% of the capacity, an increase in out-of-plane deflections was observed, attributable to the loss of stability and second-order effects, as shown in Figure 6a,b. When the peak load was attained, outward buckling at the mid-height of the chords was observed, and the load that the specimen could take began to decrease. Behavior in the post-peak range was represented by a gradual load drop. Local buckling of the steel tube wall was not detected, and no buckling or deformations were observed in the battens. The load-bearing capacities of the specimens measured in the experiments are given in Table 1, whereas the average values are presented in Table 4. The variation in the results between the specimens was around 1%.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **Hollow Group** & **Filled Group** \\  & **S21 and S22** & **SC21, SC22, and SC23** \\ \hline \(N_{u,exp}\) (KN) & 726 & 1263 \\ \(S_{n}\) (KN) & 8.49 & 12.22 \\ COV (\%) & 1.17 & 0.97 \\ \(ks\) (\(\cdot\)) & - & 1.74 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Column models’ ultimate load forces.  
Figure 5: Strains vs. axial load (**a**); deformations vs. axial force (**b**) for hollow steel tube specimens.