## 1 Introduction  
In recent decades, steel-concrete composite structures, such as concrete-filled steel tubes (CFST) [1; 2; 3], steel-plate concrete beams [4; 5; 6], partially encased composite (PEC) columns [7; 8; 9; 10], and steel-concrete composite frames [11; 12], have received much attention [13; 14; 15]. Among them, the steel-concrete composite frame is currently a widespread structural system in multistory and high-rise buildings since it combines the advantages of both steel and concrete frames, thus leading to, in many cases, a reduction of costs and optimization of the structural performance [16; 17]. The mechanical behavior of composite frame systems including experimental and theoretical research, has been reported in detail over the last few decades [18; 19; 20; 21], and previous research, has demonstrated that the presence of reinforced concrete (RC) slabs is beneficial to the flexural capacity, stiffness and lateral stability of steel beams, which also improves the seismic performance of the frame [22; 23]. However, under vertical and seismic loads, tensile stresses and cracks may occur in the concrete slab at beam ends where a hogging moment exists, which has always been a critical issue hindering the widespread application of steel-concrete composite structures. Concrete cracking significantly influences the stiffness and durability of structures [24; 25; 26]. To reduce the tensile stress and control the cracks in the concrete slab, the prestressed technique has been widely used in the last few decades. However, most of the prestress applied to the composite beam is undertaken by the steel beam due to the strong composite effects between steel and concrete, which means that the prestressing efficiency is very low in composite structures. Moreover, construction the using prestressed technique is very complex and time-consuming. The other approach employed to reduce tensile stress and control cracking in concrete is releasing the composite effects by adopting a new type of connector named the uplift-restricted and slip-permitted (URSP) connector instead of the traditional shear connectors [27; 28; 29; 30; 31], as illustrated in Figure 1. The traditional shear connectors are mainly the headed stud connector and the Perfobond strip [32; 33]. To provide an alternative to replace the stud connector in special situation, an innovative connector named the truss-type shear connector has been proposed and received much attention recently, and has been proven to reduce the production cost and improve the resistance value [34]. However, to reduce the tensile stress and control the cracking in slabs in negative moments, a new type of connector in this paper is designed to partially release the slip constraints along the longitudinal direction while retaining the uplift resistance of the steel-concrete interface. To achieve the structural functions of connectors, foamed plastics are wrapped around the screw and nut. In this way, the slip between steel and concrete is partially allowed. Several theoretical and experimental studies on the performance of URSP connectors have been conducted. The slip and uplift performance has been investigated and a shear force-slip hysteresis model has been proposed based on the push-out tests [27]. In addition, an optimization analysis of URSP connectors in composite bridges and some practical design guidelines, as well as construction methods, have been provided by [PERSON] et al. [31]. However, theoretical and experimental studies on the practical application of this new type of connector in a composite frame system for building structures are still insufficient, and the influence of the URSP connector on the strength, stiffness and cracking performance of composite frame structures remains a research blank in this field. Therefore, this paper focuses on the application of the URSP connector in steel-concrete composite frame buildings. Firstly, the mechanism of the frame beam with URSP connectors is described.