# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 2 Experimental Program  
### Column Models  
The experiments presented in this study were carried out on models of TCHST and TCCFST columns. The dimensions of the column models correlated with those of actual physical structures at a ratio of one by three; i.e., the models were three times smaller than the equivalent pillar in a typical structure. Considering that a typical residential building has a 3 m floor height, the experimental model was adopted to have a height of one meter. The global size of the experimental specimens was also selected while taking into consideration the laboratory testing equipment limitations, in which the largest test specimen could be around 1000 mm high. Additional properties of the experimental models, such as the dimensions of the column's cross-section, the width and wall thickness of the steel tubes, and the concrete aggregate sizes, were properly scaled. Five axially compressed two-chord built-up composite columns were tested. The specimens were separated into two groups. The first set of two columns were specimens made as hollow two-chord columns (labels S-21 and S-22), while the second set of three models were TCCFST columns filled with high-strength concrete (labels SC-21, SC-22, and SC-23). The hollow and the filled columns had cross-sections made from two parallel 1000 mm high main steel tubes placed at a 200 mm distance apart. Along the height of the specimens, the main tubes were interconnected via five horizontal steel members. The diameter of the main chord tubes was 60.3 mm, and they had a wall thickness of 4 mm, while the connector members were made from 33.7 mm diameter and 3 mm thick wall tubes. A horizontal batten tube was then welded to a vertical chord tube at a distance of 239 mm, starting at 22 mm from the top and bottom of the column. The dimensions of the column models are presented in Figure 1a. The inside surfaces of the chord tubes were cleaned to remove rust and oil deposits, and the chords were carefully filled with high-strength concrete. Fresh concrete was poured in layers and then compacted with a steel rod (Figure 1b). Only vertical tubes were filled with concrete. The test specimens were kept at a temperature of 20 \({}^{\circ}\)C and a 60% humidity to air-dry. A 280 mm \(\times\) 80 mm \(\times\) 15 mm steel plate was welded to each end of the test specimens. The bottom plate was welded before the casting of the fill concrete, while the top plate was welded four weeks after the concrete had cured. One of the parameters evaluated in the experimental analysis was the global nominal slenderness ratio (\(\lambda_{n}\)) of the hollow and the filled specimens. The slenderness ratio was calculated as follows:  
\[\lambda_{n}=\frac{kL}{\sqrt{I/A}} \tag{1}\]  
where \(L\) is the height of the specimen; \(k\) is the effective length factor that depends on the boundary condition so that \(k\) = 0.7 for a column that is fixed at both ends [24]; and \(I\) and \(A\) are the moments of inertia and the total area of the chord members in the cross-section, respectively. For the TCHST specimens, \(I=I_{s}\) and \(A=A_{s}\), while for the TCCFST specimens, \(I\) and \(A\) are characteristics of the idealized composite cross-section calculated as \(I=E_{c}I_{c}/E_{s}+I_{s}\) and \(A=E_{c}A_{c}/E_{s}+A_{s}\). In previous expressions, \(A_{s}\) and \(A_{c}\) are the total areas of the steel tubes and the concrete infill, and \(I_{s}\) and \(I_{c}\) are the moment of inertia of the steel tubes and the concrete infill, respectively. In the presented study, the cross-section had both material and non-material axes. However, only buckling around the material axis was investigated, as, due to the size and shape of the cross-section, buckling instability could occur only around the material axis. By material axis, we mean the axis that goes through both chords longitudinally over the cross-section (cuts through material), while the non-material axis goes across the battens.