Wind Tunnel Tested

When engineers sat down to consider aerodynamic testing for the new Tacoma Narrows Bridge, they had one goal, says Bechtel’s David Climie, who manages superstructure and steel for the project. “We wanted to make absolutely certain it was nothing like Galloping Gertie.”

 
From the day it opened in July 1940, the first bridge across Tacoma Narrows was a disaster waiting to happen. Its roadway rolled and undulated noticeably in the wind, leading locals to nickname it Galloping Gertie. As word spread about the quirky bridge, people came from all over the Seattle area to cross it, watching cars in front of them vanish and reappear as the road heaved up and down.

Barely four months after opening, Gertie collapsed in a 65-kilometer-per-hour windstorm. The event was immortalized in a grainy black and white newsreel showing the bridge twisting, buckling, tearing apart, and finally sinking in Puget Sound. The film made Gertie famous around the world, but it also showed engineers how not to build a bridge. The lessons learned from the death of Gertie helped make bridges safer.

“The entire engineering community learned about issues related to poor aerodynamic performance,” says Manuel Rondón, Bechtel’s project manager for the Tacoma Narrows project. “Designing for wind loads is something we take for granted today.”

The big difference now is that before a suspension bridge is built, a scale model of it is subjected to tortuous wind- tunnel testing to make sure it can withstand any potential gale while several computer simulated conditions are modeled. In the Tacoma area, the prevailing winds blow south to north—perpendicular to the bridge. To test their design for the new Tacoma Narrows Bridge, engineers built a 9-meter-long model and sent it to one of the largest wind tunnels in the world, operated by RWDI in Ottawa, Canada.

Over the course of three months, the model was repeatedly tested at winds of more than 160 kilometers per hour—the highest that could be expected in the Tacoma Narrows for a thousand years. The model stood strong. “We had been prepared to make some changes if there was a problem, but the first design turned out to be good,” says Climie.

Wind wasn’t the only consideration. “This particular location required extensive analysis,” says Rondón. “We had to determine how two side-by-side bridges would interact in the presence of water currents as well as wind.”