# Fatigue Performance of Different Rib-To-Deck Connections Using Traction Structural Stress Method
## 3 Fatigue Performance of Rib-To-Deck Connection Types in Orthotropic Steel Bridges
### Comparative Results of Three Typical Connections and Discussions  
The Mises stress diagram of the connection of the bridge deck and U-rib under fatigue load was obtained with the finite element simulation method. The equivalent structural stress contained membrane stress and bending stress. The equivalent structural stress and the stress components of the bottom side of the bridge deck were obtained based on the traction structural stress method. The fatigue cyclic life was calculated using the master _S-N_ curve and Equation (4). Furthermore, in the figure, the vertical axis represents the equivalent structural stress and the horizonal axis represents distance from the side of the weld toe or weld root. The NSS represents the normal structural stress and TSS represents the in-plane shear structural stress. \"Membrane\" stands for the membrane structural stress and \"bending\" represents the bending stress. \"Total\" means the sum of the membrane stress and bending stress. The equivalent structural stress of the bottom and the top side of the bridge deck were obtained similarly. As shown in Figure 5, the left weld seam (Trad-Weldline1) is a traditional single-sided weld seam of the weld toe and bridge deck. The right weld seam (Trad-Weldline2) is a traditional single-sided weld seam of the weld root and bridge deck. Figure 9 shows the Mises stress and fatigue cyclic life of the bottom side of the bridge deck of a traditional weld seam. It can be seen clearly that the maximum stress of the conventional single-sided weld seam occurred in the middle of the conventional weld seam between the bottom of the bridge deck and the groove weld seam. The equivalent structural stress and the stress components of the Trad-Weldline1 and Trad-Weldline2 are plotted in Figure 10. The comparative results indicate that the structural stress changed sharply at the location of the rib-to-deck connection and the crossbeam. The bottom of the two weld seams were subjected to the normal tensile stress and the top of weld seams were subjected to the normal compressive stress. The major component of the equivalent structural stress was bending stress with little in-plane shear stress. The maximum tensile stress of Trad-Weldline1 was 39 MPa and that of Trad-Weldline2 was 62 MPa. The equivalent structural stress increased slowly from the side of the weld seam to the middle of the seam. The total structural stress in the middle of the bottom surface of the bridge deck was the most significant with the lowest fatigue cyclic life. As can be seen from the figure, the location of the minimum fatigue life corresponded to the middle of the connection  
Figure 8: Hypothetical fatigue crack initiation and propagation on the bridge deck. between the weld seam and the bridge deck. The location was regarded as the location of initiation of the fatigue crack and the crack propagated along the weld seam. Figure 11 plots the Mises stress and fatigue cyclic life of the bottom side of the bridge deck of the weld seam of a large root. As shown in the figure, the left weld seam (Large-Weldline1) is the weld seam of the large root of the weld toe and bridge deck and the right weld seam (Large-Weldline2) is the weld seam of the large root of the weld root and bridge deck. Compared to the distribution contours of the traditional weld seam (Figure 5), the distribution trend of the Mises stress contours and fatigue cyclic life were similar. Moreover, the location of the maximum stress was the same as in the traditional weld seam. Figure 12 shows the equivalent structural stress of the weld toe for Large-Weldline1 and Large-Weldline2. The equivalent structural stress of Large-Weldline1 and Large-Weldline2 indicate that the top side of the bridge deck was mainly subjected to the normal compressive stress and the bottom of the bridge deck was subjected to the normal tensile stress. The bending structural stress was more significant than the in-plane shear structural stress.