# Experimental Study on Mechanical Behavior of Shear Connectors of Square Concrete Filled Steel Tube
## 1 Introduction  
In common structural designs, concrete filled steel tube (CFST) members have been widely used as piles, building columns and bridge piers. This is due to the advantages of the composite members, such as high strength, good ductility, high energy absorption capacity, and ease of construction. The CFST members' behavior has been investigated extensively since the 1960s. [PERSON] and [PERSON] [1] performed an experimental study on the behavior of square CFST stub columns with different diameter to thickness ratios (\(D/t\)). The test results showed that the composite columns had high ductility as well as high strength due to the mutual confinement between the steel plates. [PERSON] [2] studied the effect of the constraining factor and width to thickness ratio of a steel tube on the compressive load capacity of concrete filled rectangular hollow section (CFRHS) and the ductility of the steel tube. [PERSON] et al. [3] studied the behavior of the composite CFST columns with various concrete strengths and wall thicknesses. The results showed that for high-strength CFST columns, the peak load was achieved for small shortening (3.0 mm), whereas for normal concrete the ultimate load was gained with large displacement. By performing tests on the circular CFSTs, [PERSON] [4] investigated the effect of compaction method of concrete on the strength of the circular CFST. The results showed that using denser compacted concrete, the ultimate strength of the CFST was higher. In the design of a CFST structure, the steel beam-CFST column is a common joint type. The vertical load of steel beams is transmitted through the beam-column joints to CFST columns. Usually, steel beams are connected directly with the outer steel tube of the CFST columns, and vertical loads are transmitted from the steel tube to the core concrete through the bonding between the steel tube and concrete. In the past few decades, numerous studies have been conducted to investigate the bond strength between the steel tube and concrete in CFST columns. [PERSON] et al. [PERSON] et al. (2016) conducted push-out tests on circular CFST columns, where the in-filled concrete was of moderate shrinkage or little shrinkage. The test results showed that the concrete shrinkage had a negative effect on the bonding in the CFST columns. [PERSON] et al. [PERSON] et al. (2016) tested the bond behavior in CFST columns with expansive concrete. The results showed that expansive concrete was effective in improving the short term bond strength of the CFST columns. [PERSON] et al. [PERSON] et al. (2017) investigated the effect of concrete age on the push-out resistance of CFST columns. The results showed that the age of the normal strength concrete caused a small reduction in the push-out resistance. A statistical analysis conducted by [PERSON] et al. [PERSON] et al. (2017) indicated that the bond strength decreases with increasing height to diameter ratio (\(L/D\)) up to about three times. On the other hand, for circular columns with \(L/D>3\) and for rectangular columns, the effect of \(L/D\) ratio on the bond strength was not clear. Further, in a CFST structure with braces or an ultra high-rise CFST structure, as shown in Figure 1, it is not reliable to transmit the vertical load of the beam-column joints by the bonding between the steel tube and core concrete. The steel tube and core concrete cannot work together. When the bonding between the steel tube and concrete is not reliable, the column deformations shown in Figure 2 may occur, which is not acceptable in the design of a CFST structure. Figure 1: Tianjin 117 super high rise building. (**a**) Tianjin 117 under construction; (**b**) Brace in Tianjin 117 building. Figure 2: Deformation and slippage of concrete filled steel tube (CFST) column when bonding is not reliable. (**a**) Axial deformation; (**b**) Bending deformation; (**c**) Shear deformation.