Which force is a twisting moment that can threaten the stability of long-span bridges?

• A. Torsion
• B. Gravity
• C. Load
• D. Compression

Answer: (A)

Torsion is the twisting moment that acts about the longitudinal axis of a member, causing cross-sections to rotate relative to one another and producing shear stresses. In long-span bridges, torsional effects are especially risky because wind, uneven loading, and geometric irregularities can create unbalanced moments that twist the deck or girders. If the torsional stiffness is not adequate or if aerodynamic forces amplify the twist, the structure can become unstable through torsional buckling or flutter.

Gravity is a vertical force that mainly causes bending and axial weight effects, not twisting. A general load can include many components, but by itself it does not specify a twisting moment. Compression is an axial load that shortens the member along its length, again not twisting. The distinct twisting action required to threaten stability is torsion.