TARGET 060712

Tacoma Narrows Bridge Collapse

Bridge buckling
The City of Tacoma and Pierce County Board of Commissioners asked the State of Washington to construct a bridge across the Tacoma Narrows.

The Tacoma Narrows is the single point in the 20,000 square miles of Puget Sound where the Washington mainland and the Olympic Peninsula are close. For years, it had been clear to State officials that the Narrows would have to be bridged in order to open up the spectacular and thinly populated Peninsula. Aware of this situation, the Washington State Legislature created the Washington Toll Bridge Authority in 1937, with a mandate to finance, construct and operate toll bridges.

The legislature appropriated $25,000 to study the request. Satisfied with the results of the study, on May 23, 1938, the State of Washington submitted an application to the Public Works Administration (PWA) requesting funds for construction of a bridge.

Between the time the state legislature authorized the money to study the proposal and the completion of that study, Lacey Murrow, Director of the Washington State Department of Highways, had given Clark Eldridge, a bridge engineer with the department, the green light to design a bridge to span the Narrows. Eldridge’s plan called for a 5,000 foot, two-lane suspension bridge. When completed, the structure would be the third longest suspension bridge in the world.

After examination of Eldridge's plans in May of 1938, the Public Works Administration agreed to finance 45 percent of the construction, provided that the State of Washington retain a board of independent engineering consultants to reexamine Eldridge's design. The State complied and employed the firm of Moran and Proctor to study the plans for the substructure. Furthermore, the State retained Leon S. Moisseiff, the world-renowned suspension bridge builder to examine the plans concerning the superstructure. Both Moran and Proctor and Moisseiff made significant alterations to Eldridge's original design. Specifically, Moran and Proctor wanted an entirely different substructure. As to Moisseiff, he substituted the 25 foot deep open stiffening truss with an eight foot, shallow plate girder, resulting in a much lighter bridge.

Prior to the opening of the construction bids, a group of contractors notified the engineers they could not meet the specifications for the substructure. As a result, Moran and Proctor's plans for the thicker substructure were scrapped, and Eldridge's original plans for the substructure were reintroduced. After consultation with Moisseiff, it was agreed that Eldridge's design for the substructure would be used in conjunction with Moisseiff's plans for the thinner superstructure. This modified plan was approved by the Public Works Administration and bids for construction were opened on September 27, 1938. The Pacific Bridge Company's low bid of $5,594,730.40 was accepted. The Bethlehem Steel Company was an associate contractor that supplied and erected the steel and wire. Work on the bridge began in early 1939 and on July 1, 1940, the $6.4 million bridge opened and the link between the Washington mainland and the Olympic Peninsula was complete.

The first Tacoma Narrows Bridge was revolutionary in its design. Since the turn of the century, suspension bridge construction had valued structural grace and slenderness to achieve an artistic appearance. With its shallow stiffening trusses and slender towers, the bridge across the Narrows was the epitome of artistry in bridge construction.

But its failure on November 7, 1940 marked the end of a trend in bridge engineering towards a maximum of lightness, grace and flexibility.

The slender and graceful Tacoma Narrows Bridge became known for its tendency to sway in windstorms. The shape of the bridge was much like that of an aircraft wing and under windy conditions it would generate sufficient lift to become unstable.

Vertical oscillations of the roadbed occurred even during the construction phase and raised questions about the structure's stability. Some breezes as low as four miles per hour caused oscillations, while stronger breezes often had no effect. Prior to the bridge's opening, hydraulic buffers were installed at the towers to control the stresses. The undulations continued, however, and further studies were undertaken at the University of Washington. Their recommendation of the installation of tie-down cables in the side spans were implemented, but to little effect.

Local folks lost no time in nicknaming the bridge "Galloping Gertie." Fascinated by Gertie, thousands of people drove hundreds of miles to experience the sensation of crossing the rolling center span. The disappearance and then reappearance of cars from the sight of those standing at the ends of the bridge often highlighted the experience. For four months, the Washington Toll Bridge Authority thrived as traffic had trebled from what had been expected. Although concerns about the bridge's stability had been voiced, bridge officials were so confident of the structure, they considered canceling the insurance policies in order to obtain reduced rates on a new one.

But throughout the early morning hours of Thursday, November 7, 1940, the center span had been undulating three to five feet in winds of 35 to 46 miles per hour. Alarmed by this constant motion, highway officials and state police closed the bridge at 10:00 A.M. Shortly thereafter the character of the motion dramatically changed from a rhythmic rising and falling to a two-wave twisting motion. The twisting motion grew stronger with each twist; span movement had gone from 5 foot to 28 foot undulations. This twisting motion caused the roadbed to tilt 45 degrees from horizontal one way and then 45 degrees from horizontal the other way at approximately one third the speed shown below.
Bridge buckling
For about 30 minutes, the center span endured the twisting. At about 10:30 A.M., the windstorm caused severe and catastrophic failure of the bridge's thin deck. Once the structural integrity was compromised major portions of the bridge deck began to fall into the Tacoma Narrows channel. A center span floor panel dropped into the water 195 feet below. The roadbed was breaking up, and chunks of concrete were raining into the Sound. At 11:02 A.M., 600 feet of the western end of the span twisted free, flipped over, and plunged down into the water. Engineers on the scene hoped that once this had happened, the remainder of the span would settle down. The twisting continued, however, and at 11:09 A.M., the remaining bridge sections ripped free and thundered down into the Sound. When this happened, the 1,100 foot side spans dropped 60 feet, only to bounce up and then settle into a sag of 30 feet. As for the center span, it rested on the dark and tide-swept bottom of the Narrows.

When the collapse happened, there was one car trapped on the bridge with its driver and his dog, Tubby. Notice the unfortunate car on the bridge deck in the upper right corner of this picture. The driver finally staggered along the bridge to safety, but the dog could not be coaxed to leave the car, and was the only fatality of the bridge's collapse.

The collapsed bridge

The significance of the first Tacoma Narrows Bridge, to a large extent, is derived from its startling collapse on November 7, 1940. The collapse brought engineers world-wide to the realization that aerodynamic phenomena in suspension bridges were not adequately understood in the profession nor had they been addressed in this design. New research was necessary to understand and predict these forces. The official investigation into the collapse recommended the use of wind-tunnel tests to aid in the design of the second Tacoma Narrows Bridge and resulted in the testing of all existing and future bridges across the country. New mathematical theories of vibration, aerodynamics, wave phenomena, and harmonics as they apply to bridge design arose from these studies.

For a video of the bridge's opening day ceremonies, its normal tendence to "gallop", and its spectacular collapse, click here

For more information on the collapse of the Tacoma Narrows bridge, click here .