Seventy-four years ago last Friday, there was nothing extraordinary about the weather in Tacoma, Washington. A near gale was blowing across Puget Sound, causing trees to sway and inconveniencing walking, but the notoriously stormy region had seen worse. However, from the way the deck of the Tacoma Narrows suspension bridge jumped, twisted, and swung in the air, you would have guessed that a hurricane of biblical proportions was battering the northwest coast. At about 11:00 AM, the contortion became too much for the bridge’s fatigued structure, and the main deck collapsed. All that was left of the bridge was its towers and an ignominious legacy as the epitome of bad engineering.
The Tacoma Narrows Bridge was completed in July 1940, after being built at a cost of $6.4 million (around $90 million today). At the time, it was the world’s third longest suspension bridge. The main span was 2800 feet long. However, the deck had a width of 39 feet and a height of only eight feet. This daringly delicate design was the brainchild of Leon Moiseff, a famed engineer who had assisted in the designs of the Golden Gate Bridge and the Manhattan Bridge. The bridge builders selected his shallow deck design for its elegance and cost efficiency. They rejected a more expensive design by Clark Eldridge that had a 25-foot-deep stiffening truss. This would prove to be a fateful decision.
The twisting bridge deck
When the bridge opened, it immediately began showing the flaws that would lead to its collapse. Even in light winds, the deck oscillated like a wave at various points. It earned the cognomen “Galloping Gertie” and became something of a tourist attraction. Thrill seekers would drive back and forth across the undulating structure. It was clear that the “galloping”- which rendered drivers unable to see cars only a short distance in front of them- was a problem that needed fixing, engineers didn’t even have time to put a solution on the drawing board. On November 7, 1940, the bridge collapsed after undergoing severe torsion and vertical oscillations with a height of 35 feet. Fortunately, there were no human fatalities.
What went wrong? The underlying problem behind the disaster was that the engineers of the time had a limited understanding of fluid dynamics, including aerodynamics. In the eyes of chief engineer Leon Moiseff, the bridge deck’s extremely low height-to-width ratio was ideal aerodynamically. He probably thought that lateral wind would pass around the deck with ease, like air moving around an airplane’s wing. In fact, the deck did act like an airplane wing in ways the Moiseff did not foresee. Its lack of rigidity (due to its shallow height) and its thinness relative to its length led it to undergo aeroelastic flutter. An object obstructing flowing fluid, like air, creates a street of vortices behind it as the air moves in to fill the gap of fluid created in the wake of the object.
The Karman Vortex, the beautiful mathematical pattern that was fatal for the bridge
In skyscrapers and other tall structures, these vortices exert lateral forces perpendicular to wind direction on the structure, causing lateral oscillation. In the Tacoma narrows bridge, this phenomenon led to the up-and-down motion in the bridge deck even in low winds. In the high winds of November 7, 1940, the bridge not only underwent even larger vertical oscillation but also twisting due to the wind’s direction not being horizontal. The twisting and vortices combined constructively, and the intensity of the torsion increased until the final, calamitous structural failure.
The collapse sent waves across the fields of civil and structural engineering. After inquiry and research, the causes of the collapse became clear. At the time of the bridge’s design, the trend was creating thin, elegant, simplified bridges- much like the plain and elegant international skyscrapers of the era. Such a trend did not survive the general anxiety after the disaster. The bridge was rebuilt in 1950, with a deck stiffened with wider roadways and a deep supportive truss. This truss was also open, to allow air to pass through. Bridges today use a variety of methods to avoid Galloping Gertie’s fate, including trusses designed to avoid torsion and tuned mass dampers. Modern engineers also have the aid of computer algorithms and programs that can simulate the behaviors of designs at various wind speeds.
The rebuilt Tacoma Narrows Bridge. There is now a twin suspension bridge next to it.
Leon Moiseff’s career collapsed along with the bridge, and he did not collaborate in any more projects before his death, three years later. It has often been debated whether or not Moiseff was at fault for the failure- after all, he had no more knowledge of fluid dynamics and aeroelastic flutter than any of his contemporary engineers. However, his thin deck design was risky in other ways, such as load-bearing capacity. Safety was unwisely sacrificed by Moiseff in the name of cost and elegance. Today, Galloping Gertie’s collapse teaches a valuable lesson to engineers in prudence. A famed video of the collapse is a staple of engineering and physics courses, not so much because of its depiction of aerodynamics but because it perfectly captures one of the most interesting and darkly humorous episodes of the history of either field.
Total Views: 1735 ,