Wind Shear and Resonance

• When wind flows horizontally at right angles to a bridge's length, it splits when it encounters the bridge, flowing over and under it. Wind shear is a change of wind speed with altitude. It produces twisting forces along the length of the bridge. An improperly-designed bridge will resonate, accumulating vibrational energy until it fails.
  
  

Vibration and Tension

• The more tension a guitar player puts on a string, the faster it vibrates. This effect also applies to bridges. The faster the vibration, the more energy it takes to make it vibrate. A bridge under a large amount of tension will not vibrate in any reasonable wind; its tension is too high. The Tacoma Narrows bridge had a very low weight, 5,700 pounds per linear foot, compared to 30,000 for comparable bridges. Its low weight gave it low tension, which made its vibration frequency low, and a mild breeze would set it into motion.
  
  

Fatigue and Collapse

• The Tacoma Narrows bridge did not collapse right away. Engineers observed its vibrating behavior and attempted to correct it, but were not successful. The stresses produced in the swaying bridge fatigued its metal and concrete supports for several months, finally leading to its collapse.
  
  

Rebuilding

• The bridge was rebuilt ten years later after extensive redesign work and testing with scale models. The new bridge had a heavier and wider structure and hydraulic dampeners to absorb vibrations. It has been in service since 1950 and has had no problems with wind vibrations.