# Influence of Lower Lateral Bracing on the Seismic Pounding Damage to Slab-On-Girder Steel Bridges
## 3 Numerical Results
### Failure of the Lower Lateral Bracing  
The different design details of the lower lateral bracing have a significant impact on its lateral seismic response, and the key is the connection between the lower lateral bracing and the steel beam. As shown in Figure 17, the lower lateral bracing with the two connection designs exhibits two failure modes, vertical failure and connection failure, respectively, under lateral bounding. In Figure 17a, the stiffener of the lower lateral bracing is not interrupted and is tightly connected to the gusset plate on the steel beam. In Figure 17b, the lower lateral bracing connecting part of the gusset plate and the stiffener is too short and interrupted. Under the transverse load, the lower lateral bracing with continuous vertical stiffness will have bending deformation, and the failure mode is a vertical failure. The lower lateral bracing with discontinuous vertical stiffness will bend and deform in the area of sudden stiffness change, and the failure mode is a connection failure. It should be emphasized that under the action of earthquakes, it is difficult and unnecessary to ensure that the lower lateral bracing never fails. Under strong seismic action, the bounding force of the beam may be large, and significant deformation will occur at the bottom. In this case, repeated plastic bending deformations and energy dissipation in the lower lateral bracing and its stiffener are necessary. However, once the connection fails, lower lateral bracing cannot transmit and share the lateral bounding force of the steel beam, and there is a risk of breaking at the joint. Therefore, the lower lateral bracing is allowed to fail in an earthquake, but the reliability of the joint must be guaranteed. Figure 16: Vertical displacement and stress time history of the lower lateral bracing. Figure 15: Vertical stress distribution of steel webs. (**a**) Stress under self-weight; (**b**) peak stress on bounding; (**c**) residual stress after the earthquake. Figure 18 shows the stress cloud of two lower lateral bracings. It can be seen that the lower lateral bracing with uneven vertical stiffness will cause stress concentration at the connection, and obvious bending deformation will occur at the gusset plate. The lower lateral bracing with uniform vertical stiffness only has a large area of stress at the stiffener and no significant deformation concentration. In order to explore the response of lower lateral bracing with a truncated stiffener after connection failure, the vertical displacement and stress time history are further plotted as shown in Figure 19. The reference point of vertical displacement is the bending vertex of the lower lateral bracing, and the reference point of the stress is the connection point of the lower lateral bracing and the steel beam. It can be seen that when the bounding occurs,  
Figure 17: Failure mode of lower lateral bracing. (**a**) Vertical failure; (**b**) connection failure. Figure 18: Stress cloud plot of lower lateral bracing (unit: MPa). (**a**) Lower lateral bracing with truncated stiffener; (**b**) lower lateral bracing with continuous stiffener. the stress peak occurs at the connection between the bracing and the steel beam, and the bracing has an upward vertical displacement. Comparison with Figure 16 shows that the vertical displacement of lower lateral bracing with a truncated stiffener (76 mm) is much higher than the vertical displacement of lower lateral bracing with a continuous stiffener (2 mm). Such a large vertical displacement of the lower lateral bracing with a truncated stiffener indicates that it has broken and failed the node, and the excessive deformation is concentrated in a small area. The lateral bounding causes a bend in the joint, which leads to the failure of the lower lateral bracing.