What is the function of cross-frames and vertical stiffeners in steel girder bridges?

• A. Cross-frames and vertical stiffeners resist distortional buckling, improve bending stiffness, and distribute loads between girders.
• B. They primarily reduce dead load without affecting buckling.
• C. They only provide cosmetic reinforcement.
• D. They are used only in arch bridges.

Answer: (A)

Cross-frames and vertical stiffeners act as the lateral bracing and load-sharing network that holds steel girders together and restrains their slender members from buckling. The cross-frames connect adjacent girders in a lattice, creating a stiff, truss-like layout that resists lateral movement and distributes vertical shear and moments across the bays. This tying effect helps prevent distortional buckling of the flanges and web, while also increasing the overall bending stiffness of the girder group and improving how loads are shared among the girders.

Vertical stiffeners run along the web between bays, reducing the unbraced length of the web and preventing web buckling and distortional modes under bending and shear. They tighten the connection between girders and help transfer forces more evenly, further boosting bending stiffness and stability of the system.

So, these elements primarily resist distortional buckling, enhance bending stiffness, and distribute loads between girders, which is why this is the best description. They do not simply reduce dead load, are not purely cosmetic, and are not exclusive to arch bridges.