One way to increase the rigidity of a bridge girder is to increase what geometric dimension?

• A. Increasing the deck width
• B. Increasing the girder's height
• C. Increasing the span length
• D. Adding more piers

Answer: (B)

Rigidity in bending comes from the bending stiffness EI, which depends on the cross-sectional shape. For a rectangular girder, the second moment of area I scales with height cubed (I = b h^3 / 12). That means increasing the vertical dimension (height) dramatically boosts I, and thus EI, making the girder much stiffer under loads. Doubling the height increases I by a factor of eight, while doubling the deck width only doubles I. Extending the span or adding more piers changes how the load is carried and how much deflection the system experiences, but it doesn’t strengthen the girder’s own cross-section. So, the most effective way to increase the girder’s rigidity is to increase its height.