The twisted Tacoma Narrows Bridge
The twisted Tacoma Narrows Bridge
In early 1940, engineers and construction workers put the finishing touches on what was to be the the longest man-made span in the US, and the third-longest in the world: the Tacoma Narrows bridge. The half-mile-long structure linked Tacoma, Washington to Gig Harbor, and its completion inspired statements such as "a triumph of man's ingenuity," and all other manner of gratified exuberance. But as workers finalized the construction, a curious behavior became evident. Although the bridge had been designed to withstand winds up to 120 miles per hour, observers noted that even a mere breeze would occasionally cause wavelike ripples to travel up and down its length. Many workers had to chew on lemon wedges to suppress motion sickness.

Experiments with a scale model produced no clear solution to the unwanted movement. Though the gentle wave motion didn't put the massive bridge in any structural distress, it was clear that motorists would find it disconcerting as cars ahead of them bobbed in and out of view. In spite of the complication, the bridge was opened to the public on 1 July 1940. It did not remain open for long.

The Tacoma Narrows bridge was originally designed by an engineer named Clark Eldridge, with some later revisions implemented by Leon Moisseiff, the renowned designer one of the designers of the Golden Gate in San Francisco. The final design departed from the tried-and-true conventions of suspension bridge-building in several ways, including the use of "I" beams rather than lattice-style deck supports. This modification, among others, preserved the structure's strength, but at a decreased cost.

Opening day
Opening day
After opening, the new bridge shortly came to be known as "Galloping Gertie," so named by white-knuckled motorists who braved the writhing bridge on windy days. Even in a light breeze, Gertie's undulations were known to produce waves up to ten feet tall. Sometimes these occurrences were brief, and other times they lasted for hours at a time. Numerous travelers shunned the route altogether to avoid becoming seasick, whereas many thrill-seeking souls paid the 75-cent toll to traverse Gertie during her more spirited episodes.

Immediately after the problem was first observed, a number of engineering professors were hired to devise a method to reduce these movements. Tie-down cables and hydraulic buffers were employed with limited success. The matter wasn't considered terribly urgent because the winds were causing longitudinal waves along Gertie's center span-- waves which traveled back and forth along the length-- which did not put undue stress on the roadbed. The structure was not at risk, nor did it create unsafe driving conditions. It was largely a problem of motorists' comfort. But on the morning of 7 November 1940, four months after the bridge was opened, something new happened. While enveloped in a steady 42 mile per hour wind, Gertie abandoned her usual rippling action in favor of a never-before-seen twisting motion which increased in intensity at an alarming rate.

As crowds gathered on either end to watch the structure thrash about, a college student named Winfield Brown paid the 10-cent pedestrian toll and ventured onto the bridge on foot:

"After walking to the tower on the other side and back, I decided to cross again. It was swaying quite a bit. About the time I got to the center, the wind seemed to start blowing harder, all of a sudden. I was thrown flat. A car came up about that time. The driver got out, walking and crawling on the other side. We didn't have time for any conversation.""Time after time I was thrown completely over the railing. When I tried to get up, I was knocked flat again. Chunks of concrete were breaking up and rolling around. The knees were torn out of my pants, and my knees were cut and torn. I don't know how long it took to get back. It seemed like a lifetime. During the worst parts, the bridge turned so far that I could see the Coast Guard boat in the water beneath.""As soon as I got off the bridge, I became sick. So, I went to the home of a cousin and laid down for a while. I've been on plenty of roller coasters, but the worst was nothing compared to this."

Another man, a newsman named Leonard Coatsworth, became stranded on the bridge while driving across it:

"I drove on the bridge and started across. In the car with me was my daughter's cocker spaniel, Tubby. The car was loaded with equipment from my beach home at Arletta. Just as I drove past the towers, the bridge began to sway violently from side to side. Before I realized it, the tilt became so violent that I lost control of the car. . . . I jammed on the brakes and got out, only to be thrown onto my face against the curb.""Around me I could hear concrete cracking. I started back to the car to get the dog, but was thrown before I could reach it. The car itself began to slide from side to side on the roadway. I decided the bridge was breaking up and my only hope was to get back to shore. On hands and knees most of the time, I crawled 500 yards or more to the towers . . . . My breath was coming in gasps; my knees were raw and bleeding, my hands bruised and swollen from gripping the concrete curb . . . . Toward the last, I risked rising to my feet and running a few yards at a time . . . . Safely back at the toll plaza, I saw the bridge in its final collapse and saw my car plunge into the Narrows."

Unable to withstand the increasingly brutal torsional twisting, a number of Galloping Gertie's suspender cables snapped, shifting the weight of the deck onto the remaining cables. Under the increased load and enormous strain, the cables broke one by one until there were too few to support the massive roadbed. After weathering over an hour of the punishment, a huge portion of Gertie's steel-and-concrete center deck broke off and fell 195 feet, plunging into the Tacoma Narrows with a terrific crash. A column of dust rose into the sky, and sparks crackled from broken powerlines. Clark Eldridge, the bridge's original designer, was present to witness the destruction:

"I was in my office about a mile away, when word came that the bridge was in trouble. At about 10 o'clock Mr. Walter Miles called from his office to come and look at the bridge, that it was about to go. The center span was swaying wildly, it being possible first to see the entire bottom side as it swung into a semi-vertical position and then the entire roadway."I observed that all traffic had been stopped and that several people were coming off the bridge from the easterly side span. I walked to tower No. 5 and out onto the main span to about the quarter point observing conditions. The main span was rolling wildly. The deck was tipping from the horizontal to an angle approaching forty-five degrees. The entire main span appeared to be twisting about a neutral point at the center of the span in somewhat the manner of a corkscrew.""At tower No. 5, I met Professor Farquharson, who had his camera set up and was taking pictures. We remained there a few minutes and then decided to return to the east anchorage warning people who were approaching to get off of the span. At that time, it appeared that should the wind die down, the span would perhaps come to rest and I resolved that we would immediately proceed to install a system of cables from the piers to the roadway levelin the main span to prevent any recurrence...."

"I was then informed that a panel of laterals in the center of the span had dropped out and a section of concrete slab had fallen. I immediately went to the south side of the view plaza. The bridge was still rolling badly. I returned to the toll plaza and from there observed the first section of steel fall out of the center. From then on successive sections towards each tower rapidly fell out."

Though no human life was lost when Gertie fell, Tubby the cocker spaniel did not survive.

Much of the catastrophic collapse was caught on motion picture film thanks to Barney Elliott, the owner of a local camera shop. Examination of the film, the bridge's remains, and tests with scale models determined that resonance was responsible for Gertie's demise. Due to the design decision to replace the lattice supports with "I" beams, the wind was unable to pass under the structure as readily as it it passed over it, causing a difference in pressure much like an airplane wing. Once there was sufficient sway to tilt the deck slightly, aerodynamics caused the roadbed to twist to the point that it sprang back, causing a repeating cycle of back-and-forth twisting. During this process, the steadily blowing wind added more and more energy to the vibrations.

The public was shocked that the beautiful state-of-the-art bridge-- designed by one of the most respected bridge engineers in the world-- could fail so spectacularly. The six million dollar structure had been approved by federal and state experts, and suspension bridges were not a new technology. Ultimately the bridge was not built from substandard materials, nor was it under-engineered; its designers merely overlooked the physical phenomenon of resonance. Though the suspension bridge had been designed to sway and to withstand some longitudinal waves, it had not been built to withstand the torsional punishment. Today, Galloping Gertie's spectacular self-destruction stands as a cautionary tale, retold to countless students of civil and structural engineering.

The State of Washington collected on Gertie's insurance-- except for a small portion which had been unpaid due to fraud-- and went on to build a replacement bridge ten years later. The harsh lesson of Galloping Gertie was not lost on the new designers, and the new span was built with resonance in mind. Its new design introduced openings for air to pass through, and scale models were thoroughly tested in a wind tunnel. "Sturdy Gertie" has stood since 1950, and will soon be joined by a sister bridge to accommodate increased traffic.

Much of the original Gertie's wreckage still resides on the floor of the Tacoma Narrows today, forming one of the world's largest artificial reefs. Though some of the steel was pilfered and reused for the war effort in the years following the collapse, what's left of her remains are now protected as a historical site.

Written by Alan Bellows, posted on 04 January 2007. Alan is the founder/designer/head writer/managing editor of Damn Interesting.
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