The function of a portal in bridge trusses is to provide additional stability to the structure. A portal is a structural element that consists of two diagonal members connected at the top and bottom by horizontal members, forming a triangular shape. It is typically placed at the ends of a truss bridge.

The portal serves several important functions in bridge trusses:

1. Stability: The main function of a portal is to enhance the stability of the bridge truss. The triangular shape of the portal provides rigidity and helps resist lateral forces, such as wind loads and the dynamic loads caused by moving vehicles. By bracing the ends of the truss, the portal prevents the bridge from buckling or twisting under these external forces.

2. Load distribution: Portals help distribute the loads acting on the bridge truss. The diagonal members of the portal transfer the applied loads to the vertical supports or abutments at the ends of the bridge. This ensures that the loads are evenly distributed and do not concentrate at specific points, which could lead to structural failure.

3. Reducing deflection: The presence of portals helps reduce deflection or sagging of the bridge truss. When the bridge is subjected to vertical loads, such as the weight of the bridge deck and vehicles, the portals resist the bending action and help maintain the structural integrity of the truss. This is especially important for longer span bridges where deflection can become more significant.

4. Stiffness: Portals also contribute to the overall stiffness of the bridge truss system. The triangulated configuration of the portal makes it inherently stable and resistant to deformation. This stiffness helps in maintaining the shape and alignment of the bridge under different loading conditions.

Overall, the portal plays a crucial role in providing additional stability and load-bearing capacity to bridge trusses. It enhances the structural integrity, reduces deflection, and ensures that the loads acting on the bridge are distributed evenly.