# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 2 Experimental Program
### Test Setup  
Two types of deformations were examined in the presented research. The first type was local deformations, e.g., the longitudinal and lateral strains, and the second type was global deformations of the columns, such as shortening and out-of-plane deflections. The specific strains were measured using eight strain gauges placed on a section positioned between the second and third batten. The strain gauges were positioned symmetrically along the longitudinal axis of the column, as presented in Figure 4a. The strain gauges used to measure axial strains were labeled with V (e.g., the vertical direction), while the strain gauges used to measure strains in the lateral direction were labeled with H (e.g., the horizontal direction). The out-of-plane deflections at the mid-height of the chord were measured using two electrical displacement transducers and two mechanical gauge dials, as shown in Figure 4b. An additional two displacement transducers (DTs) were used to measure the deformations in the axial direction. The shortening of the specimen was measured by DT1, while DT2 was used to track down any support movement. The readings from the electronic displacement gauges and strain gauges were automatically recorded throughout the loading process by two HBM SPIDER 8 acquisition devices. The analysis of the tested specimens included the tracking of the stresses and deformations of all parts of the built-up column, the load-bearing capacity, the mode of deformation at failure, and the force distribution over the composite cross-section parts. This study investigated only short-term loading, and the effects of the shrinkage and/or creep of the concrete were not considered.  
Figure 3: Representative tensile stress—strain curve for steel (a); test specimens (b).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Class** & \(D\) **(mm)** & \(t\) **(mm)** & \(f_{y}\) **(N/mm2)** & \(f_{u}\) **(N/mm2)** & \(f_{u}\)/\(f_{y}\) & \(E_{s}\) **(GPa)** & \(\mu_{s}\) **(–)** & \(\delta\) **(\%)** \\ \hline S600 & 60.3 & 4.0 & 600 & 640 & 1.07 & 190 & 0.30 & 11.1 \\ \hline \end{tabular}
\end{table}
Table 3: Properties of steel.  
The testing of the column models was accomplished using a hydraulic press with a 2 MN capacity. A column was carefully positioned in the center of the bottom rigid steel plate of the press to eliminate any potential secondary bending moments. Concentric force was applied to the column model via the top and bottom thick steel load-bearing plates. The force was applied in an upward direction, pushing it against the top support plate. Figure 4a presents a schematic representation of the test setup and the positions of the used instruments. All column models were continuously loaded until failure occurred. The hydraulic press was operated using a force control technique in which a load step of 1/10 of the expected ultimate capacity of the tested model was used. The load-bearing capacities of the TCHST and TCCFST models were estimated using a finite element analysis. Every load interval was maintained for about 2 min. After the applied load reached 50% of the estimated ultimate load, or if a change in the slope was recorded in the force-displacement curve, the value of the load step was changed to 1/20 of the estimated load-bearing capacity. The progression of global deformations, the mode of failure, and the ultimate capacity of the column models were all noted. The test ended when the load on the column fell under 80% of the highest recorded value or when the lateral displacement in the middle of the specimen reached \(L/25\). The TCCFST specimen just before the start of the test is presented in Figure 4b.