# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 3 Results of the Experiments
### Comparative Analysis of the Test Results
The axial load (\(N_{i}\)) applied in load increments (\(i\)) to a built-up composite steel-concrete column is divided among its parts so that  
\[N_{i}=N_{c,i}+N_{s,i} \tag{3}\]  
where \(N_{c,i}\) and \(N_{s,i}\) are the forces on the concrete and steel in one load increment, respectively. The theoretic distribution of the load on parts of the column's cross-section could be calculated using the equivalence of the axial strain because, in the composite cross-section, all parts must have identical strains in the axial direction. The distribution of the applied force \(N_{i}\) can be calculated from ratios:  
\[\frac{N_{c,i}}{N_{i}}=\frac{A_{c}^{*}}{A^{*}},\quad\frac{N_{s,i}}{N_{i}}=\frac {A_{s}^{*}}{A^{*}} \tag{4}\]  
The modified area of the concrete (\(A_{c}^{*}\)) and steel (\(A_{s}^{*}\)) part and the idealized area of the composite cross-section (\(A^{*}\)) are determined as follows:  
\[A_{c}^{*}=\frac{E_{c}}{E^{*}}A_{c},\;\;A_{s}^{*}=\frac{E_{s}}{E^{*}}A_{s},\;\; A^{*}=A_{c}^{*}+A_{s}^{*} \tag{5}\]  
The elastic modulus of the steel was chosen as the comparative modulus of elasticity (\(E^{*}\)) for the cross-section of a composite member. The theoretical distribution of the force between two parts of the TCCFST chord was determined in the form of a load percentage. In the analyzed member, the concrete fill carried 42% of the applied load, whereas the steel tube carried 58% of the applied load. This distribution is presented in the first column in Figure 13.  
The confinement factor, or the relative strength ratio (\(\xi\)), is generally used to assess the compressive response of concrete-filled steel tubes, as noted in [32; 33; 34]. As one of the major factors in interactions between the steel tube and the concrete fill, it was defined as follows:  
Figure 10:0  
where \(A_{c}\) and \(A_{s}\) are the total areas of the concrete and steel tubes, respectively, while \(f_{y}\) is the steel's yield strength. The characteristic concrete compressive strength \(f_{ck}\) is determined from the cube strength (\(f_{cu}\)) as follows:  
Figure 10:1  
with the coefficients \(\alpha_{1}=0.82\) and \(\alpha_{1}=0.87\). Details on the reduction coefficients are given in Ref. [35]. The values of the concrete compressive strengths are given in Table 3, while  
Figure 10:2 Comparative relationships for theoretical and measured load distributions. the confinement factor value is shown in Figure 10:3 In concrete-filled steel tube columns with a high confinement factor, the steel tube is stronger and is less likely to be influenced by the local buckling of walls, and more confinement is provided to the concrete infill. The relationship between the cube's compressive strength and the confinement factor is presented in Figure 10:4 Additionally, the value of the confinement factor for the TCCFST test specimen is also given. It can be observed that the values of \(\xi\) decrease with the increase in the compressive strength of the concrete, which means that the TCCFST column filled with low-strength concrete is relatively more confined by the steel tube than the TCCFST column model filled with high-strength concrete. The lower confinement factor for the test specimen provided by the steel tube is a direct consequence of the high concrete strength used in this research. Furthermore, a composite column with a higher \(\xi\) factor will have considerable ductile behavior, which was shown in [36]. Generally, the TCCFST column's load-bearing capacity increases with a higher compressive strength of the infill concrete. It was shown that, when the confinement factor increases, the influence of the concrete strength on the coefficient of the enhanced strength, \(ks\), decreases [1]. After the concluded experiments, the steel tube was partially removed from one chord of the TCCFST column to investigate the mode of failure and to note the cracking and/or crushing of the concrete fill. Figure 15a shows the TCCFST specimen with exposed concrete. Cracks in the concrete were only observed in the middle of the chord (Figure 15b).