The Tacoma Narrows Bridge Collapse

Slated to be the third largest bridge in the world, the Tacoma Narrows Bridge was plagued with issues before ground was even broken. Scale models showed it was unstable even in moderate winds. Nevertheless, construction started and the bridge was completed as planned. The bridge deck would shift up and down a staggering 10 feet with only a slight breeze. Construction workers needed to chew on lemon slices to attempt to avoid motion sickness while working.

The bridge was designed by engineer Clark Eldridge, and later revised by Leon Moisseiff, who worked on the design of the somewhat similar Golden Gate Bridge. The bridge used unconventional design ideas in attempt to save money but preserve stength. The Tacoma Narrows Bridge was designed using I-beams rather than the standard lattice-style supports of the day with this in mind. Ultimately, this would contribute to its literal down-fall.

Bridge construction began on September 27th, 1938 and construction took only 19 months, at a cost of $6.4 million.

Models built of the bridge showed no reason for the abnormal movement, and engineers concluded that although uncomfortable, the movement did not affect the structural integrity of the bridge. Even with the issues, the bridge opened July 1st, 1940 and the scale of the issue became clear. Driving on the bridge proved to be a white knuckle affair. Thrill seekers would pay the 75 cent toll to simply to experience the swaying of the bridge. Some drivers avoided the bridge altogether, disturbed by the cars ahead of them bobbing in and out of view as the bridge moved. The bridge came to be known as "Galloping Gertie."

A group of professors was assembled to come up with a solution to minimize the sway. The ideas proposed ranged from cable tie-downs to hydraulic dampers to wind deflectors. None of the ideas could overcome the inherent design issues of the bridge. Wind deflectors most likely would have solved the issues, but the bridge collapsed just 5 days after they were recommended.

The collapse occurred on November 7th, 1940, at 11:00 am, just 4 months after it was completed. Winds were blowing at a steady 40 miles per hour. Windy, but far from what should have been the limit of the bridge. It was later concluded that aeroelastic fluttering was responsible for the collapse, not the commonly referenced theory of resonance caused by the wind.

Aeroelastic fluttering occurs when air is passing over an object at a steady speed and the object shifts slightly. The force of the wind transfers into the object, which then pushes back in the opposite direction slightly harder than the wind naturally would. Because of this, the object catches more wind, and is forced even farther, building up even more energy. This repeats until the object, in this case, the road bead of the bridge, cannot sustain the forces anymore.

Prior bridges didn't suffer from this phenomenon because the lattice-work like supports allowed wind to simply flow through the bridge. With the Tacoma Narrows Bridge the new I-beams forced the wind to go either over or under the roadbed.

Amazingly, no one died in the collapse, save for driver Leonard Coatsworth's dog Tubby. Coatsworth give this description of the collapse:

"Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car...I jammed on the brakes and got out, only to be thrown onto my face against the curb...Around me I could hear concrete cracking...The car itself began to slide from side to side of the roadway.

On hands and knees most of the time, I crawled 500 yards [450 m] or more to the towers...My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb...Toward the last, I risked rising to my feet and running a few yards at a time...Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows."

A nearby camera shop owner, Barney Elliott, got to the scene just in time to capture Coatsworth escaping. Coatsworth's car, and dog, were never found. The state put together a team to salvage the valuable steel for war time efforts. In the end, the cost of salvage was $350,000 more than what they were able to sell the steel for. 10 years later a new bridge was constructed, which still stands today.

Video from the bridge's opening day, along with it's final moments, below:

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