In box girder bridges, how does depth vary in longer spans?

• A. Depth is variable to provide curved underside
• B. Depth remains uniform
• C. Depth increases toward ends
• D. Depth decreases toward midspan

Answer: (A)

Long-span box girders are shaped to match how the bending and torsional stresses vary along the span. Instead of keeping a constant height, engineers vary the depth along the length, which creates a curved underside. This variable-depth form lets the section be deeper where the moments are larger and shallower where they’re smaller, improving strength and stiffness while saving material. The resulting underside curve is a natural consequence of tailoring the cross-section to the load path, rather than forcing a uniform, flat bottom.