## 2 Experimental Investigation
### Specim Details  
The experimental investigations were performed during the reconstruction of the existing building of the company Napred in Belgrade. Dimensions of the columns, grade of steel and the quality of the concrete were determined based on the required design values of the forces in the columns during the top-down construction method. The column height corresponds to the height of the lowest floor plus the excavation depth required for the thickness of the drainage layer and the foundation slab. After construction of these CFT columns, these columns are reinforced and concreted to columns of final diameter with D = 40 cm for the latter stages. Figure 1: Schematic diagram of the paper flow. Figure 2: Steel frame with the hydraulic press for pile jacking. Four CFT columns 4.00 m long, connected to the previously jacked concrete piles of diameter \(\mathcal{O}\)450 mm and 7.50 m long, were tested. Table 1 shows the dimensions and material properties of specimens denoted as CS1, CS2, CS3 and CS4 with different outer diameter ratios and the wall thickness of the steel tube (D/t) and L/D ratio. For the outer steel tubes of the CFT column carbon steel tubes, S355 was used. The designed concrete mix for the core was C25/30, while the material properties of concrete were determined from three 150 mm \(\times\) 300 mm cylinder tests from the same concrete mixture: 27.2 MPa, 26.3 MPa and 26.6 MPa. The mean values of obtained concrete compressive strength and modulus of elasticity were f\({}_{\text{c}}\) = 26.70 MPa (the standard deviation 0.458 MPa) and E\({}_{\text{c}}\) = 31.95 GPa, respectively. Table 1 summarises all the specimen properties. In order to assign an axial load to the steel tube and the concrete core simultaneously, it was essential to prepare the end surfaces of the CFT column. The concrete core of the specimens was left 2 centimetres longer than the steel tube during concreting. After 28 days of specimen curing, the concrete out of the tube was cut with a diamond plate grinder, and the surface was finely leveled with the grinding wheel. The first step in the experiment was the installation of four jacked piles. The jacking of 7.5 m long piles went in 1.50 m long segments. The nanometer registered the force at each jacked segment's beginning, middle and end. These values at the end of the last jacked segment multiplied by the cross-sectional area of the cylinders showed an average total jacking force of 1130.40 kN. After pile jacking, the steel endplate \(\mathcal{O}\) = 450 mm, 30 mm thick, was welded to the pile top end. The CFT specimen was put in the testing position and welded to the steel plate top. Figure 3 shows the general test setup and the testing of the specimen CS1. Figure 4 shows the testing of the other three specimens, CS2, CS3 and CS4. All specimens were loaded to failure. Axial loading was applied centrically to the top endplate using a particular calotte device which simulated simple support. The load was applied simultaneously to the steel tube and the concrete core in 5-bar increments corresponding to a force of 31.42 kN. The total load can be considered short-time loading since the load application until failure took about 3 min for each sample. The expected results for the column buckling length were in the range of 1.0 L (which corresponds to a pinned connection at the bottom end of the column) to 0.707 L (that corresponds to the fixed connection at the bottom end of the column). Therefore, horizontal displacements at distances of 0.35 L, 0.40 L and 0.50 L from the top end of the column were continuously measured. Besides, applied loading at the top of the column and the column axial shortening was also recorded. The dose type C6A2 MN of 2000 kN capacity measured the axial loading at the top end of the column with \(\pm\)0.1 kN accuracies. The W100 displacement gauges measured the horizontal and vertical displacements with \(\pm\)0.01 mm accuracy.