# Experimental Study on Mechanical Behavior of Shear Connectors of Square Concrete Filled Steel Tube
## 1 Introduction
In order to guarantee the reliable transmission of the vertical load of the beam-column joints, the shear connectors are usually applied to the inner wall of the steel tube to ensure the co-working performance of the steel tube and concrete. Among the various forms of shear connectors, the shear studs are widely used as the shear connectors, as shown in Figure 3a. There are many tests to investigate the bonding performance in CFST columns with studs in the inner steel tube wall. [PERSON] [9] carried out a series of push-out tests with different types of shear connector, and the results indicated that the ultimate push-out load was a function of the interface type, shape and size of the cross-section, led to a better understanding of the load transfer mechanism in beam-column connections in the composite construction. [PERSON] et al. [10] presented push-out tests of studs in normal and high strength concretes. The results showed that the concrete compressive strength significantly affected the strength of the stud connections. [PERSON] and [PERSON] [11; 12] investigated the static and fatigue behaviors of large shear studs up to 30 mm diameter, which were beyond the limits of the current design codes. [PERSON] [13] conducted push-out tests on the shear studs in order to investigate the factors that affect stud behavior. The shear stud mechanism was studied, a new expression of stud load-slip relationship was proposed, and a model for calculating the shear stud bearing capacity was developed. 'Specification for design and construction of concrete filled steel tubular structures' [14] recommends the use of steel rings, as shown in Figure 3b, or steel diaphragms as shear connectors. To ensure the quality of the core concrete, the width is limited to less than \(D_{0}/15\) and 50 mm, whichever is smaller. 'Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures' [15] recommends the steel rings to be used as shear connectors. Steel rings have been proved to be effective in enhancing the bond behavior according to the earlier studies. [PERSON] et al. [16] conducted push-out tests on the circular and square CFST specimens. The experimental results showed that welding internal ring(s) onto the inner surface was more effective than welding shear studs in enhancing the bond behavior. [PERSON] [17] tested the CFST columns of various dimensions, which were installed with rings. The results showed that the rings caused the axial load-carrying capacity and stiffness to increase, and the strength degradation rate to decrease. Up to present, the studies on the steel rings have been mainly focused on the circular CFST columns. However, for the square CFST column, it is difficult to weld the whole steel ring to the square tube because of the R corner, which is caused by the production process (for instance, cold-formed or hot-rolling), as shown in Figure 3b. In order to simplify the construction work, instead of the steel ring, the steel plates can be used as the shear connectors, as shown in Figure 3c. Therefore, in this study, the push-out tests on the square CFST with steel plates have been carried out. The test variables are the width to thickness (\(B/t\)) ratio of steel tube, thickness of steel plate, length of steel plate, strength of concrete, welding condition of steel plates and number of shear plates layer and their  
Figure 3: Types of shear connectors. (**a**) Shear studs; (**b**) Steel rings; (**c**) Steel plate. spacing. The effects of these test variables on the shear strength have been quantitatively analyzed, and a formula for calculating the shear-bearing capacity has been developed.