Strong winds forced the Tacoma Narrows Bridge to oscillate in an unexpected, twisting fashion that ripped the bridge asunder and dumped the central span, a car, and a dog into the sea. Studies of the collapse led to the development of the science of bridge aerodynamics.

When it was built, the Tacoma Narrows Bridge was the third-longest suspension bridge in the world (the Golden Gate Bridge was the longest, and the George Washington Bridge was the second longest). Leon Moisseiff had participated as a consulting engineer in the design of nearly every major suspension bridge built during the previous twenty years, and he intended the Tacoma Narrows Bridge to be the most beautiful bridge in the world and the capstone of his career. Indeed, the bridge was breathtaking: a thin ribbon of roadway suspended from cables gracefully draped from two towers that soared 436 feet above the water. More than a mile long, it connected Tacoma with the peninsula across a narrow arm of Puget Sound. The nature of the seafloor dictated where the two support towers could be placed, and once they were established, engineers determined that the length of the central span would be nearly 2,800 feet. The fast tidal surges that flowed through the channel four times a day at a depth of nearly 200 feet made the construction of the towers’ foundations even more complicated. However, after only nineteen months of construction, the bridge officially opened on July 1. It was in use for only four months. [kw]Tacoma Narrows Bridge Collapses (Nov. 7, 1940)
[kw]Bridge Collapses, Tacoma Narrows (Nov. 7, 1940)
Engineering;bridges
Tacoma Narrows Bridge
Bridges;Tacoma Narrows
[g]United States;Nov. 7, 1940: Tacoma Narrows Bridge Collapses[10330]
[c]Engineering;Nov. 7, 1940: Tacoma Narrows Bridge Collapses[10330]
[c]Physics;Nov. 7, 1940: Tacoma Narrows Bridge Collapses[10330]
[c]Urban planning;Nov. 7, 1940: Tacoma Narrows Bridge Collapses[10330]
Eldridge, Clark
Farquharson, Frederick B.
Moisseiff, Leon

As Clark Eldridge, the bridge’s original designer and chief engineer, drove across the bridge at 8:30 a.m. on November 7, 1940, the wind was blowing at 38 miles per hour. Eldridge experienced the notorious undulating motion of the roadbed as it rose and fell, making humps and valleys like the body of a sea serpent. When the wave motion was at its worst, cars in different valleys actually lost sight of each other. The bridge’s history of sinuous dancing in the wind led to its being referred to as “Galloping Gertie,” a nickname also used for the Wheeling Suspension Bridge in West Virginia, which shook apart on May 17, 1854. Eldridge was not overly concerned by the Tacoma bridge’s movement on that morning: The bridge had already survived a storm with winds of 50 miles per hour and had been designed to withstand the push of winds at 120 miles per hour. Eldridge calmly returned to his office.

Frederick B. Farquharson, professor of civil engineering at the University of Washington, drove onto the bridge around 9:30 a.m. that same day. He was responsible for designing mechanisms to reduce the bridge’s motion, so he began filming and measuring the roadbed’s waves. He had already had strong anchor cables attached between the roadbed and the shore, and other cables with dynamic dampers (piston-in-cylinder mechanisms similar to automobile shock absorbers) attached between the roadbed and the suspension towers. When the anchor cables snapped, the dampers could do nothing because their seals had been breached when the bridge was sandblasted prior to painting. Farquharson’s recommendation to attach wind-deflector panels to the sides of the roadbed had only just been approved; these panels would have given the bridge a more streamlined profile in order to reduce wind effects.

About 10:00 a.m., Leonard Coatsworth, a news editor for the Tacoma News Tribune, drove onto the bridge with his family’s dog, Tubby. The wind had increased to 42 miles per hour, and the deck rose and fell 3 feet every 90 seconds. Suddenly the bridge began to twist about its center line, a type of motion that had never before occurred. Although at first the twisting motion was small, within minutes it grew so large that the roadbed tilted at a 45-degree angle, pushing the left sidewalk about 28 feet below the right sidewalk. Then the left sidewalk rose and the right sidewalk fell, and so on, in 5-second intervals. Coatsworth’s car was thrown against the curb in the opposite lane. Coatsworth crawled out a window, but the terrified dog would not come with him.

Not able to stay on his feet, Coatsworth clung to the curb as he crawled toward the east tower, nearly 400 feet away. As he neared the tower, he met Winfield Brown, a college student who had earlier walked across the bridge to experience the roller-coaster effect. As they neared the tower, they were able to stagger the 1,575 feet from the tower to the toll plaza at the eastern end of the bridge. At about the same time, near the west tower, the driver and passenger of a Rapid Transfer Company van were able to jump out of the vehicle just before it tipped. With the help of some workmen, the van’s occupants reached the western end of the bridge safely.

At 10:30 a.m., a large chunk of concrete dropped from the bridge’s central span. The wind subsided, and the bridge steadied somewhat. Howard Clifford, one of several photographers who had arrived, tried to rescue the dog, Tubby, but could not reach him. Farquharson was able to reach Tubby, but the dog bit him on the finger. As Farquharson struggled to safety, the air was filled with the shriek of twisting steel girders, the sharp gunshot sounds of breaking cables and popping bolts, and the grinding of concrete against concrete. One of the two suspension cables slipped from its saddle atop a tower, and at 11:02 a.m., a 650-foot-long section of the roadway tore loose from the central span and dropped 210 feet into Puget Sound. Other segments fell soon after, as did Coatsworth’s car (with Tubby still inside). By 11:10 a.m., most of the central span was gone, and the bridge was quiet.

The basic principles of bridge building seem simple enough: Each segment of the bridge must be able to sustain its own weight plus the weight of any load it might carry. Further, the forces on a bridge segment must be passed on to support towers or to the shore, but for a long suspension bridge it is difficult to calculate these forces accurately. As a substitute, engineers use an accepted design theory that incorporates both assumptions and experimental measurements in order to create estimates for the unknown forces.

Eldridge’s original design cost $11 million and called for 25-foot open-grid-work trusses that would stiffen the roadbed, but eastern financiers would not lend construction money unless a suitable engineer was allowed to review and change the plans. Moisseiff was selected to do this, and he used the relatively new deflection theory to guide his calculations. He proposed using solid-plate trusses that were only 8 feet high because this allowed him to design a more visually appealing bridge that used less steel and therefore cost less. His design was accepted. The bridge was built with a $6.4 million loan plus $1.6 million in toll revenues. Unfortunately, Moisseiff had pushed deflection theory into untested territory, as the shallow trusses and narrow roadway made the Tacoma Narrows Bridge three times more flexible than the Golden Gate Bridge, which had been the most flexible bridge up to that point.

Farquharson’s wind-tunnel tests of a carefully constructed scale model of the bridge and his measurements during the final hours before the bridge’s collapse showed that the structure’s failure was due to torsional flutter. When the wind rammed into the plate truss on the bridge’s side, the wind current divided, blowing over and under the bridge in unequal amounts. This caused the roadway to tilt, and that tilt caused the winds to be even more unequal, which caused more tilt, and so forth, until finally the weight of the roadway pulled it back to a horizontal level. The roadway’s momentum then carried it through the horizontal plane, tilting the roadbed the opposite way, and so on. The model showed that this cycle repeated every five seconds, just as Farquharson had measured. In order for a bridge to be safe, the bridge’s dynamic response had to be taken into account. Farquharson helped develop the tools to do that, and ten years later a new and successful Tacoma Narrows Bridge was opened. Engineering;bridges
Tacoma Narrows Bridge
Bridges;Tacoma Narrows

• Billah, K. Yusuf, and Robert H. Scanlan. “Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks.” American Journal of Physics 59 (February, 1991): 118-124. Shows that physics texts focus on the wrong mechanism for failure and gives the correct mechanism. Includes numerous references. Article is rather technical; see Feldman reference below.
• Feldman, Bernard J. “What to Say About the Tacoma Narrows Bridge to Your Introductory Physics Class.” Physics Teacher 41 (February, 2003): 92-96. A simplified version of the Billah and Scanlan article cited above.
• Freiman, Fran Locher, and Neil Schlager. Failed Technology: True Stories of Technological Disasters. Vol. 2. New York: Gale Research International, 1995. Investigative reports of technological disasters, including the Tacoma Narrows Bridge failure and the loss of the Challenger space shuttle in 1986. Very interesting and easy to read.
• Scott, Richard. In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stability. Reston, Va.: American Society of Civil Engineers, 2001. Uses nontechnical language to trace the development of suspension bridges from the failure of the Tacoma Narrows Bridge to a possible bridge spanning the Strait of Gibralter.

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