# Bending Performance of a Prestressed Concrete Composite Girder Bridge with Steel Truss Webs
## 4 Test and Finite Element Result Analysis
### Strain Analysis  
Figure 9, in which the section numbers are the same as those in Figure 4a, shows the load-strain curves of the concrete bottom plate, top plate, and common steel bars for the specimen under a symmetrical load, obtained by the test and FE analysis. It can be seen in Figure 9a that when the structure loading was in the elastic stage, the tensile strain of the concrete bottom plate basically increased linearly. When the load value increased to 95 kN, the load-strain curve also increased linearly, although the mid-span section of the specimen was cracked. At a load of 120 kN, the test specimen entered the nonlinear growth stage, indicating the onset of significant plastic deformation in the bottom plate's reinforcement. As can be seen in Figure 9b, when the load was less than 120 kN, the load-strain curves of the concrete top plate basically presented a linear growth trend throughout the whole process, indicating that micro-cracks may occur inside the concrete top plate, but no obvious crushing phenomenon occurred. As can be seen in Figure 9c, when the load value reached 120 kN, the reinforcement of the concrete bottom plate in the mid-span section of the specimen yielded.  
The distribution of the concrete strain along the height direction in the mid-span section of the specimen during loading is shown in Figure 10. Figure 10 illustrates that during the elastic stage, the compressive and tensile strains of the top and bottom plates varied linearly with the section height. This indicates that the deformation of the top and bottom plates of the test specimen conforms to the 'plane-section assumption'.  
Figure 10: Mid-span strain distribution along the height: (**a**) Central axis; (**b**) The thickest part of the top plate.  
Figure 9: Load–strain curves: (**a**) Bottom plate; (**b**) Top plate; (**c**) Reinforcement in the bottom plate.  
The steel truss web on each side of the test specimen was composed of 38 inclined steel tubes (seen in Figure 2a), numbered 1 to 38 from left to right. Figure 11a shows the strain distribution of the steel truss web of the test specimen along the longitudinal bridge direction when the load is 60 kN. As can be seen in Figure 11a, the maximum strain value of the steel truss web appeared at the \(L/6\) position (#6 and #7 steel tubes). Figure 11b shows the load-strain curves of the steel truss web at the position of \(L/6\). It can be seen that the strain of the steel truss web increased linearly with the increase in the load but was less than its yield strain before the reinforcement in the concrete bottom plate yielding. According to Section 2.1 of this paper, the yield strain of the steel tube is 1.81 \(\times\) 10\({}^{-3}\), and when the load reaches 120 kN, the maximum strain value of the steel truss web is 6.18 \(\times\) 10\({}^{-4}\), which is 34.1% of the yield strain value. When the loading was stopped, the strain of the steel truss web was 7.68 \(\times\) 10\({}^{-4}\), which is 42.4% of the yield strain, indicating that the steel truss web was basically in the elastic stage in the whole process of test loading, and the steel truss web was suitable for the composite girder with a large cross-section height.  
The comparative analysis of the test and FE method results in Sections 3.1 and 3.2 shows that the deformation of composite girders obtained by two methods is similar, and the deformation increase rates of each section are basically the same, indicating that the flexural stiffness of the composite girders obtained by both are basically the same. The load-strain curves of the top and bottom plate concrete and the steel reinforcement in the bottom plate are basically consistent with each other. The load-strain curves of steel truss webs #6 and #7 basically coincide. The maximum error between the test and the FE results is less than 10%. Therefore, the FE model established in this paper can simulate the test loading process more accurately.