# Mechanical Properties of Disconnectable Coupling Joints for Steel Bracing under Eccentric Load
## 1 Introduction  
At present, the \"row pile plus steel support\" internal support system is widely used in foundation pit engineering [1, 2, 3]. This support system improves the overall bearing capacity of a foundation pit through prestressing [4, 5], so that the deformation of the foundation pit is effectively controlled. The DC joint is the device to apply and maintain prestressing [6]. The \"row pile plus steel support\" internal supporting system of the foundation pit is shown in Figure 1. The DC joint is a crucial connection between steel support and row piles, composed of steel wedges, a fixed part and an active part, as revealed in Figure 2. The role of the end plate is to connect the steel support with the purlin or top beam. The jack bracket is used to place the jack. The DC joint and the steel support are associated by flange and bolt. An essential advantage of DC joints is convenient installation. The active part is separated gradually from the fixed part by the jack, and prestress is applied. The steel wedges are wedged between the fixed part and active part when the design value of prestressing is achieved; then, the jack is removed. Therefore, the critical role of the DC joints is to apply prestress to the whole support system. The DC joint sits at the junction of the vertical member and the horizontal member, which is a crucial joint connecting the row pile and the steel support. The importance of the joints generally exceeds the components which connect to them [7], since the failure of joints can arouse the progressive collapse of row piles. Further, the transfer path of force at the joint is more complex and fuzzier than thatof the components which it connects to. Therefore, the influence of the DC joints should be considered when evaluating the mechanical properties of the steel support system. Current research on joints mainly focuses on the joints of frame structures [8; 9; 10] and beam-column structures [11; 12; 13], such as K-joints [14; 15], T-joints [16; 17; 18], and X-joints [19]. The research on DC joints is relatively minor. Most studies focus on bearing capacity under axial compression. For example, The literature [20] studied the mechanical properties of DC joints through experimental methods, and corresponding improvements were proposed. The literature [21] improved the DC joint. The load holding performance of the improved DC joints increased about 30%, verified by field test. The literature [22] systematically analyzed the strain evolution process, load-displacement curves and failure modes of DC joints under axial load, and the fitting formulas of bearing capacity indexes were obtained. DC joints inevitably bear eccentric load [23; 24], due to the incomplete vertical of the embedded steel plate at the connection, in addition to other quality problems. Local buckling, or even overall instability of the support system, is easily caused by eccentric load [25]. Eccentric compression reduces the bearing capacity of the support system [26]. Figure 1: Steel bracing system [22]. Figure 2: Composition of a DC joint. Therefore, the flexural performance of DC joints affects the bearing capacity of the whole support system. Currently, the research on the flexural performance of DC joints focuses chiefly on improved DC joints. The literature [27] studied the influence of eccentricity on the bearing capacity of disconnectable coupling with a rectangular plate through numerical simulation and theoretical calculation. The results showed that the ultimate bearing capacity of disconnectable coupling with a rectangular plate decreases by 20% when the eccentricity increases from 0 to 30 cm. The literature [28] experimentally studied the mechanical properties of disconnectable coupling with a bolt fasten wedge under eccentric load. The results showed that the compression stiffness of disconnectable coupling with a bolt fasten wedge was clearly outstanding; the fracture of the bolts was irregular. The moment-rotation model is usually used to describe the flexural performance of structures.