Suspension Bridge Wind Effects

Properly constructed bridges will not suffer catastrophic failures when exposed to wind loads.

Suspension bridges, flexible spans hovering over waterways supported by a system of cables, are constantly buffeted by winds strong enough to rock and contort them until they collapse. These engineering marvels have undergone numerous design changes over the years, however, to prevent such disasters.

Wind Load Indentification

As heights above waterways increase, lateral wind loads pummeling the suspension bridge also increase. Engineers, therefore, must verify wind speed, turbulence and strength at the bridge site. They apply these wind load calculations, and note the bridge building materials and height of the bridge over the water when designing bridges or planning design revisions to existing bridges.

Effects of Wind Load

Strong wind loads and the weight of cars tax the cable strength and span elasticity of suspension bridges. Lateral winds striking suspension bridges affect the span's sides and the top and bottom surfaces, causing them to flex, or oscillate. Each oscillation increases the winds' contact with the span's surfaces, which further intensifies the twisting motion, or torsion effect.

Significance

Dynamic wind forces cause suspension bridge failures when bridge spans are too light, narrow and thin, and when excessive flexibility is designed into them. The materials (concrete slab and steel girders) and the design produce unbalanced lift and drag, leading to increasing torsion.

References

• Wind & Earthquake Response in Very Long Span Cable-Stayed and Suspension Bridges. Henry J. Farran.
• Brookscafe.com: Galloping Gertie: The Tacoma Narrows Bridge Collapse. Robert G. Fuller and Dean A. Zollman .
• Washington State Department of Transportation: Tacoma Narrows Bridge.

Resources

• Bridge Aerodynamics Laboratory: History
• NOVA: Suspension Bridges
• Oscillation of Bridges
• Basic Wind Speed Map