The Tacoma Narrows Bridge in Washington state collapsed in 1940 due to a combination of design flaws, aerodynamics, and self-excitation. The collapse revolutionized bridge-building, and a new, safer bridge was built in 1950.
On November 7, 1940, the big news of the day was the total failure and collapse of the Tacoma Narrows Bridge in Washington state. The bridge simply ripped open and fell into the waters of Puget Sound. How and why this happened are questions that have only recently been adequately answered.
The Tacoma Straits mark the closest point to Puget Sound between the Washington coast and the Olympic Peninsula. A bridge across the narrows meant the state government could open up another section of the state for expansion, so it wanted to build one.
In 1930s America, the trend, especially for long structures, was the graceful suspension bridge. Architects and engineers sought the most graceful, lightest and thinnest designs. As a result, the Tacoma Narrows Bridge, when finished, was 5,939 feet (1,810.2 meters) long and 39 feet (11.9 meters) wide, stick-like. It was also light in weight, due to the shallow plate girders used in the construction. Pretty? No doubt. It opened to traffic on July 1, 1940.
Little did the public know that the bridge had exhibited unusual behavior during the construction phase, but soon had an insight that all was not well with their tall, slender bridge. Experts had noticed an unusual linear motion along the roadway when the wind picked up, as often happens across the Narrows. Engineers attempted to compensate for some of this “swing” by installing hydraulic buffers and steel “anchor” cables to stabilize it. However, the center span had a habit of undulating up and down, much like a roller coaster.
Drivers soon discovered that the cars ahead of them seemed to disappear into “valleys,” reappearing at the crest of the hill. People came from all over to ride on the roller coaster deck. Locals quickly dubbed the facility “Galloping Gertie” and stuck around.
On November 7, 1940, the State Police and Highway Department closed the bridge to traffic at about 10 a.m. after ripples 3 to 5 feet (1 to 1.5 meters) high were reported, with winds exceeding 22 miles /h. Fortunately, only one person was still on board, and he managed to get out before the central span collapsed. As the crowd gathered watched in fascinated horror, the center span began twisting from side to side, as well as moving up and down. All who watched knew that the bridge would not resist this kind of movement for long.
At around 10:30, the first pieces of asphalt started to fall. At 11:02, the cables holding the center span began to fail and the western section capsized and crashed into the water. By 11:09, the remainder of the central span was also gone. Gertie galloping was gone. Only the side bays and towers remained.
So the question haunted the engineers: what happened to their bridge? An article of this magnitude cannot explore all the physics and aerodynamics issues involved, but the first hypothesis was that the resonance of the wind destroyed it. Subsequent engineering tests and models showed that this was probably not the case.
Indeed, the failure of the Tacoma Narrows Bridge was a combination of many factors, including design flaws. It used shallow plate girders to reduce weight instead of deep open stiffening trusses, which made the bridge less stable. Furthermore, the structure was, quite simply, too long for its width. A wider bridge might have survived, but the roadway was too narrow to withstand the other stresses.
Aerodynamics and a strange-sounding phenomenon called “self-excitation” also played a role. As the span began to undulate and became more unstable, the instability fueled more instability. Therefore, when the span began its torsion motion, it also fed the flames of instability, until the structure collapsed. “Self-excitation” means that one thing leads to another, looping back on itself until it collapses. Film of Gertie’s collapse is available online, as are images of the undulating, winding roadway. Many documents, even online, can be consulted for further answers to the question “why”.
World War II and the resulting steel shortage delayed construction of another Tacoma Narrows Bridge. The second span, of the steel-rope suspension type, opened on October 14, 1950. This time, the engineers studied and modeled the designs for four years before finalizing their choices. This would not have collapsed. As a toll bridge, it has paid for itself and currently features the fifth longest span in North America.
Gertie’s gallop cost Washington state millions of dollars, but her failure revolutionized the bridge-building profession. Designs now undergo rigorous testing and modeling before being built, saving money and lives.
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