# Study on Mechanical Properties of Multi-Cavity Steel-Concrete Composite Floor
## 5 Finite Element Modeling Investigation
### Finite Element Model Result  
The size of the composite floor for the finite element analysis was exactly the same as that in the test. In the test, when 96 kN of load was applied to the floor, the deflection limit of the normal use stage was basically reached. In order to simulate the deformation of the composite floor in the elastic stage, the load applied to the model was set as 96 kN. The shell steel plate was in the tensile state when the floor was under load, and the tensile stress along the X tangent direction of the shell steel plate at the bottom midspan position was the maximum. The vertical deflection nephogram of the composite floor is shown in Figure 14.  
It can be seen from Figure 14 that pure bending deformation occurred in the composite floor specimen under load, and the cavity steel plate did not show obvious buckling deformation and stress concentration. The deformation of the midspan position was the largest at the composite floor, and the deflection value reached 9.61 mm. The deformation decreased along the longitudinal direction to both sides. The minimum deflection was at both ends of the composite floor, which was only 3.7 mm. The deformation characteristics were similar to those of ordinary cast-in-place reinforced concrete floor. It can be seen from Figure 15 that for both simulation and test results, the load-deflection curves are close to straight lines, and the composite floor was basically in elastic deformation stage under the load <96 kN. When the load value was greater than 96 kN, plastic deformation occurred and the deflection increased much more rapidly, which was caused by the large deformation of the cavity steel plate on the side of composite floor. As a result, the restrains on concrete were weakened and the stiffness of composite floor was reduced. Under the same load, the deflections of the floor in the simulated case were smaller than that in the test. For example, under the lo  
Figure 14: Displacement nephogram of composite floor in vertical direction. Figure 13: Finite element model of composite floor. deflection in the test and simulation condition was 10.9 and 9.6 mm, respectively. The reasons for this deviation are as follows:  
1. In the simulation, the constraint condition and load application were ideal. However, in the test, the x-direction displacement at the two simply-supported points was unlikely to be zero under load. Therefore, during the loading process, the two sides of the composite floor moved to the middle, which led to the increase in the mid span deflection. 2. Insufficient displacement transducers were set up, which made the test data slightly deviate. The displacement transducer might be disturbed if it is fixed at the center of composite floor when the composite floor is damaged. Therefore, only one displacement transducer was set up at the bottom surface of midspan. The upper part of the actuator which applies load on the span is a spherical hinge. Therefore, during the loading process, the composite floor may incline slightly to the side with the displacement transducer. For these reasons, the simulation results are relatively accurate. The stress nephogram of composite floor under load is shown in Figure 16.  
Figure 16: Stress nephogram of composite floor: (**a**) concrete stress nephogram of composite floor and (**b**) steel plate stress nephogram of composite floor along X direction. Figure 15: Load–deflection curve of composite floor in midspan. It can be seen from Figure 16: (1) when the load was 96 kN, the maximum stress of the cavity steel composite floor was 182.7 N/mm\({}^{2}\), which did not reach the steel yield strength of 235.7 MPa. Moreover, the compressive stress on the upper edge and the tensile stress on the lower edge of the cavity steel plate were basically symmetrical, and the stress distribution was relatively uniform. The neutral axis of the composite floor is located in the middle position where the stress is basically zero, which indicates that the composite floor has good mechanical performance.