On 7 November 1940, the big news of the day was of the total failure and collapse of the Tacoma Narrows Bridge in Washington state. The bridge quite simply tore itself apart and fell into the waters of Puget Sound. How and why it happened are questions that have only recently been adequately answered.
The Tacoma "narrows" mark the closest point on Puget Sound between the Washington coast and the Olympic Peninsula. A bridge across the narrows meant the state government could open up yet another section of the state for expansion, so they 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 weight, slimmest designs. As a result, the Tacoma Narrows Bridge, when finished, was 5,939 feet (1,810.2 meters) long and a stick-like 39 feet (11.9 meters) wide. It was also lightweight, due to the shallow plate girders used in construction. Graceful? Undoubtedly. It opened for traffic on 1 July 1940.
The public didn’t know that the bridge had been manifesting some unusual behavior in the construction phase, but they soon got the hint that all was not well with their tall, spindly bridge. Experts had noticed unusual linear movement along the roadway when the wind rose, as it often does through the Narrows. Engineers attempted to compensate for some of this “oscillation” by installing hydraulic buffers and steel “tie-down” cables to stabilize it. Still, the center span had a habit of undulating up and down, much like a roller coaster.
Drivers soon discovered that cars in front of them seemed to disappear in "valleys," reappearing at the crest of the hill. People came from far and wide to drive on the roller coaster bridge. Locals quickly nicknamed the structure "Galloping Gertie," and it stuck.
On 7 November 1940, the state police and highway department closed the bridge to traffic at about 10 a.m., after reports of undulations 3 to 5 feet (1 to 1.5 meters) high had been reported, with winds exceeding 35 mph (56.3 kph). Fortunately, only one person was still on it, and he managed to get off before the center span collapsed. As the gathering crowd watched in fascinated horror, the center span began twisting from side to side, as well as moving up and down. Everyone watching knew the bridge could not long withstand this kind of motion.
At about 10:30 a.m., the first chunks of pavement started to fall. At 11:02, cables holding the center span started to give way, and the western section flipped over and crashed into the water. By 11:09, the remaining part of the center span was gone, as well. Galloping Gertie was no more. Only the side spans and towers remained.
So the question haunted the engineers: what happened to their bridge? An article of this scope cannot explore all the physics and aerodynamics questions involved, but the first hypothesis held that wind resonance destroyed it. Later testing and engineering models showed that this was probably not the case.
In fact, the failure of the Tacoma Narrows Bridge was a combination of many factors, including design flaws. It used shallow plate girders to decrease the weight instead of deep open stiffening trusses, which rendered the bridge less stable. Also, the structure, quite simply, was 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 fed more instability. Therefore, when the span started its twisting motion, it also fed the instability flames, until the structure failed. “Self-excitation” means that one thing leads to another, turning back upon itself until collapse. Film of Gertie’s collapse is available online, as are pictures of the undulating, twisting roadway. Many papers, also online, can be perused for more answers to the "why" question.
World War II and the resulting shortage of steel delayed construction of another Tacoma Narrows Bridge. The second span, of the steel cable suspension variety, opened on 14 October 1950. This time around, engineers studied and modeled designs for four years before finalizing their choices. This one wasn’t going to collapse. As a toll bridge, it paid for itself and currently features the fifth longest span in North America.
Galloping Gertie cost the state of Washington millions of dollars, but its failure revolutionized the bridge building profession. Designs now undergo rigorous testing and modeling before they are ever built, saving money and human lives.