Which twisting force, often caused by wind, can cause dangerous movement in long-span structures like suspension bridges?

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

Answer: (A)

Wind can cause a twisting effect on long-span bridges, and the twisting force involved is torsion. Torsion is a moment that causes rotation about a member’s longitudinal axis, producing shear stresses across the cross-section and potential angular movement of the deck or suspenders. In tall, flexible structures like suspension bridges, aerodynamic forces can couple with the shape to create torsional oscillations; this is why torsion is a major design consideration—historically seen in aeroelastic events where wind-induced twisting led to dangerous movements. Gravity, tension, and load describe vertical or axial forces, not twisting about the axis, so they don’t capture the hazardous twisting action caused by wind.