In recent years, since steel plate reinforced concrete structure (hereinafter abbreviated as SC Structure) is expected to contribute to shortened work periods and less impact on environment compared to conventional reinforced concrete structure, it is applied in shear walls in nuclear power related facilities and turbine pedestals in addition to ordinary buildings. Because the thickness of the floor can be several meters when applying SC Structure to horizontal member such as floor slabs in nuclear power related facilities, gaps appear in the back of upper steel plates due to concrete contraction or settlement after casting (as in Fig. 2). Therefore, it may result in degrading the slip preventing performance of headed stud. However, the effect exerted by the presence or absence of gaps to mechanical properties of SC structure has yet to be elucidated. Authors conducted a push-out shear test of stud with gaps as a first step in order to grasp the effect exerted by gap caused in the back surfaces of steel plates to mechanical properties of SC structure after casting. The test results have proven the tendency that the maximum bearing force decreased with increasing the thickness of gaps and the displacement at the maximum bearing force increased. In addition, the evaluation formula of the stud sheer strength considering gaps was proposed. In this paper, loading test of SC structure beam member simulating gaps caused in the back surfaces of steel plates was conducted and impact on the slip preventing performance of studs by gaps was assessed. The following knowledge was obtained from the results.
 According to the measurement results of relative displacement (referred to as slip displacement) in interface between steel plates and concrete, vertical slip displacement is minute compared to horizontal slip displacement and horizontal component is dominant. Furthermore, the relationship between the horizontal slip displacement and the axial strain of steel plates was confirmed by the experiments. In regard to the relationship between the stud sheer forces considering gaps and the slip displacement (Q-S relationship), the calculating method of the slip displacement (S_y) at the yield point was proposed. Q-S relationship model of sheer strength and S_y as a parameter was also constructed. From the comparison of past test results and analysis results by model, the applicability was determined and indicated S_y is substantially constant regardless of thickness of gaps. Based on the equilibrium of force in interface between steel plates and concrete, the equation related to horizontal slip displacement was derived. The steel plate strain and the stud slip displacement calculated by this equation mostly expressed test results. From the test and analysis results, it is confirmed that the load-displacement relation of the beam member with gaps is almost the same as the relationship of the member without gaps if the stud slip displacement is S_y or less. Although the test case is limited, it is found that the stud slip displacement under S_y is important in order to maintain the structural performance on part concerned in generation of gaps.