## 2 Mechanical-Performance Analysis of Built-In Double-Fixed Steel Anchor
### Force-Transmission Mechanism  
In order to clarify the force-transmission mechanism of the built-in double-fixed steel anchor cable-girder box, local finite element analysis of the steel anchor box was carried out. Four welds connected to the main beam were selected and numbered, among which the inner-web and outer-web welds were numbered N1-F1, N2-F1, N1-F2, and N2-F2 (see Figure 4 for the numbering). In the local calculation model of the steel anchor box, the shear-stress distribution of the four main welds, N1-F1, N2-F1, N1-F2, and N2-F2 along the direction of the diagonal cable was plotted, as shown in Figure 5, with the apex point \"O\" of support plates N1 and N2 as the origin and the direction of the diagonal cable, pointing to the bridge tower as the positive direction. Figure 4: (**a**) Inner-web-weld-numbering diagram; (**b**) outer-web-weld-numbering diagram. Figure 3: The finite element model of the half-section of the beam section where the M16 cable is located. Analysis of Figure 5 shows that the two connection welds, N1-F1 and N2-F1, in the inner web of the main beam had essentially the same shear stress values and the same stress variations along the diagonal cables, and that the same pattern occurred for the two connection welds in the outer web of the main beam. The shear stresses in the weld between the inner web and the support plate were slightly higher than those in the weld between the outer web and the support plate. Further analysis showed that the four welds, N1-F1, N2-F1, N1-F2, and N2-F2, had a large variation in shear stress along the diagonal cable direction, and that when the distance was between 0 m and 0.31 m, the shear-stress value was small. At 0.31 m, the shear stress increased steeply and reached its maximum value in the range of 0.5 m to 0.6 m. This was due to the fact that the cable force was transferred from the pressure plate N7 to the support plates N1 and N2 in the form of diffusion. When the distance was >0.6 m, the shear stress gradually decreased, but the shear stress still occurred at the end of the weld of the N1 and N2 plates, indicating that the rest of the shear force was transferred to the lower pressure plate N5 and the stiffening rib N6 through the steel anchor-box-support plates N1 and N2. In order to further investigate the force-transmission mechanism of the built-in double-fixed steel anchor cable-girder box and determine the proportion of the load carried by the four main transmission welds, F1-N1, F1-N2, F2-N1, and F2-N2, the shear stresses along the diagonal cable direction at the welds were extracted through finite-element-analysis calculations and integrated to obtain the cable force and the proportion of the load carried by the four welds. The calculation results are shown in Table 1  
As can be seen in Table 1, the main welds of the built-in double-fixed steel anchor box transferred a total of 93.08% of the cable force, in which the weld between the support plate N2 and the inner web transferred the largest cable force, with a value of 1092.27 kN, accounting for 23.74% of the total. Due to the symmetry of the steel anchor-box structure, the four welds transferred almost the same proportion, which was generally around 23%. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Welding-Seam Number**} & \multirow{2}{*}{**Weld Length**} & **Transmitted** & **Designed Cable-Force** & **Proportion of Shared** \\  & & **Cable-Force** & **Value** & **Cable Force** \\ \cline{2-4}  & **(m)** & **(kN)** & **(kN)** & **(\%)** \\ \hline N1–F1 & 1.71 & 1089.59 & & 23.68 \\ N2–F1 & 1.71 & 1092.27 & & 23.74 \\ N1–F2 & 1.71 & 1040.64 & & 22.62 \\ N2–F2 & 1.71 & 1059.12 & & 23.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main weld-cable-force transmission and proportion of the built-in double-fixed steel anchor cable-girder box.