## 1 Introduction  
The concrete-filled steel tube (CFT) columns are widely used structural members due to many benefits over simple reinforced concrete or steel members: higher strength and stiffness, increased ductility, buckling delay or prevention and better fire performance [1, 2]. In the top-down construction method, these columns are used as the basement columns [3]. Contrary to the classical bottom-up approach, where construction progresses from the lowest floor to the top of the structure, in the top-down construction method, the construction of the underground floors and foundation progress simultaneously with the excavation from top to bottom. When used in the top-down construction method, CFT columns are supported on the previously jacked piles, and they must have sufficient capacity to sustain the construction load and act as a part of the bracing system. Piles installation starts from the ground level or level-1, and CFT columns are installed on the piles. They serve as temporary supports for the ceilings. Currently, the actual behaviour of the CFT columns in connection with the piles is not well known due to the lack of experimental results. The critical point during the top-down construction is when the final excavation level is reached. The columns are loaded with significant loading at this stage while having the longest effective length. Considering high load intensity onto a relatively small cross-section of one CFT column at that critical moment, important issues to investigate are axial load capacity and loss of column stability considering the stiffness of the pile column connection. Many studies investigated the axial load capacity of composite CFT columns, both numerically and experimentally [4, 5, 6]. For example, [PERSON] et al. [7] presented an experimental study of the behaviour of a steel-composite hollow reinforced concrete (RC) column under concentric loading. The effects of important variables, such as concrete strength, inner tube thickness, hollow ratio, column diameter and transverse reinforcement space, are presented in the study. [PERSON] and [PERSON] [8] investigated the axial capacity of composite columns made of square RC columns strengthened with steel tubes and filled with concrete. The use of welded composite columns for retrofitting frames proved to be very successful since it increased the ultimate capacity, effective stiffness and energy dissipation [8]. Several studies investigated the behaviour of CFT composite columns used in bridges. In their study, [PERSON] et al. [9] studied the compressive behaviour of steel tubes at low temperatures related to the cold region environment. Moreover, [PERSON] et al. [10] analysed the axial compressive behaviours of both circular CFT columns and square CFT columns at low temperatures through a 23-test program with different grades of regular weight concrete, different D/t ratios and different circular and square shapes of the cross-section. [PERSON] et al. [11] studied the concept of the innovative tall-pier system derived from the principle of earthquake-resilient structures and improved the seismic performances of the tall-pier bridges under strong near-fault ground motions. The new system for CFT columns used in the top-down construction method was proposed by [PERSON] et al. [12]. This system consists of a pre-founded circular CFT column and a new shear system for slab-column connection. When CFT columns are used in the top-down construction method, it is essential to consider the stability of these columns [12; 13; 14]. However, very few papers address this issue, considering the pile-column connection properties. [PERSON] and [PERSON] [15] analysed the structure and partially embedded pile interaction using the differential method and the LIPLE program and the finite element model in ABAQUS and SAP2000. The stability of eccentrically loaded reinforced concrete piles partially embedded in the sand was investigated by [PERSON] et al. [16].