# 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  
### Fatigue Behavior of Three Typical Connections of the Rib-To-Deck  
OSBD involves groove welding joints among bridge deck, longitudinal ribs and crossbeams. Due to the differences in the stiffness of the longitudinal rib and the crossbeam, fatigue cracks can occur at various locations and can have different propagation directions. Literature [22] showed that fatigue cracks occurred mainly at the location of the connection of rib-to-deck (30.2%), rib-to-crossbeam (62.4%), and the arc-shaped zone of the crossbeam and other locations (7.6%). Single-sided weld seam is the traditional connection of the rib-to-deck and this type of connection was applied widely in OSBD. Some scholars [12; 13] proposed new types of connections of the rib-to-deck, namely large-filtel weld seam and double-sided weld seam. Traction structural stress method can be effectively used to study the differences in the fatigue behaviors of the traditional and new types of the rib-to-deck connections. The equivalent structural stress range under the fatigue load is considered as a control target in this study. Besides, the effect of the crossbeam on the fatigue performance of the rib-to-deck is considered for more accurate results. Figure 6 shows a local model of an OSBD constructed with a bridge deck, a U-rib and a crossbeam. The overall size of the local model was 600 mm (length) \(\times\) 400 mm (height) \(\times\) 200 mm (width) based on the size of an actual bridge. The sizes of the bridge deck, crossbeam and U-rib were 18 mm, 14 mm and 8 mm, respectively. The model of the OSBD and three typical weld seams of the connections between the bridge deck and U-rib are plotted in Figure 6.  
Furthermore, the size of the weld seams, including the size of the weld root and weld throat, are shown in the figure. The equivalent structural stress of the four typical weld seams, including the weld toe of the outer seam, weld root of the outer seam, weld root of the inner seam and weld toe of the inner seam, were identified as the evaluation target. Based on the traction structural stress method, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Loading** & **Fatigue Test** & \begin{tabular}{c} **Hot Spot** \\ **Stress** \\ **(HSS/MPa)** \\ \end{tabular} & \begin{tabular}{c} **Structural** \\ **Stress** \\ **(ISS/MPa)** \\ \end{tabular} & \begin{tabular}{c} **Loading** \\ **Mode** \\ **(SS/MPa)** \\ \end{tabular} & \begin{tabular}{c} **Fatigue Test** \\ **Cyclic Life** \\ \end{tabular} & \
\begin{tabular}{c} **Hot Spot** \\ **Stress** \\ **(HSS/MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Structural** \\ **Stress** \\ **(HSS/MPa)** \\ \end{tabular} \\ \hline I & 782,325 & 212.1 & 252.02 & II & 643,540 & 236.2 & 270.50 \\ I & 354,321 & 255.6 & 314.42 & II & 614,865 & 225.5 & 275.98 \\ I & 797,970 & 205.6 & 251.90 & II & 187,758 & 315.1 & 406.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fatigue test results and numerical simulation results. Figure 5: Master S-N curve based traction structural stress method. the fatigue performance of the region with higher equivalent structural stress was more significant than other zones with lower stress. The finite element model of the local model was made of Q370 qE steel, and the SOLID 185 element was used for higher accuracy, as shown in Figure 7. The stress ratio of the fatigue load was \(-1\). The size of the hexahedron element was 6 mm and the groove seam of the connection between the U-rib and bridge deck was 2 mm. The element grid layer of the rib-to-deck along the direction of thickness was six and that of the rib-to-crossbeam along the thickness direction was three, which satisfy the recommendations of the traction structural stress method [17].