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Narrows Bridge is awesome, both times around
After only four months of life, the bouncing suspension bridge succumbed to winds of only 40 miles per hour, which caused a convulsive, twisting and waving movement that finally destroyed it.
When it was built, "Galloping Gertie" was the third longest suspension bridge in the country, connecting Tacoma and Gig Harbor on state Route 16 over a mile of water.
The first design, proposed in 1938 by Washington Department of Highways engineer Clark H. Eldridge, called for a two-lane suspension bridge with 25-foot deep open stiffening trusses for the sidewalls. The Public Works Administration, however, decided to follow the cost-cutting revisions of the prominent suspension bridge engineer Leon Moissieff, which used shallow, solid sideplate girders as stiffening members instead of the open trusses.
Moissieff's bridge would be two lanes with 5,000 feet of suspension structure, two 1,100 side spans and a 2,800 main span. The narrow deck measured 26 feet from curb to curb, with five feet of sidewalks. The towers were 425 feet above the piers and taper from 50 feet at the base to 39 feet at the top. His theory was that the plate girders would supply the stiffness and the cables would control movement.
The result was an awe-inspiring structure that would cost $6.4 million.
Though engineers said the galloping motion was "perfectly safe," work began right away to subdue the roller-coaster-like waves of the bridge deck.
Frederick B. Farquharson of the University of Washington engineering department was called in to study the bridge movements. Farquharson set up cameras to record the bridge, and began testing the aerodynamics of a model of the bridge in a wind tunnel.
To the surprise of the engineers, the tests showed no correlation between the bridge movement and the velocity of the wind. Farquharson suggested stringing cables from the side spans to cement anchors - which snapped right away. His other suggestion - drilling holes in the girders to let the wind pass through - was never tested.
On the day the bridge fell, sustained winds of 38 to 40 miles per hour were recorded at about 10 a.m. The bridge began to gallop violently - then it began to twist. One cable would slacken as the other tightened, and the bridge deck rolled into a corkscrew. At 11 a.m., about 600 feet of the bridge fell from midspan, unzipping the girders as it went and buckling the side spans. Engineers who later saw the famous video of the bridge collapse said it looked as if the bridge had been made of rubber rather than steel.
From this defeat rose what is now the Tacoma Narrows Bridge, the first suspension span to be build after its predecessor fell. While the first bridge was designed and built in 26 months, the second bridge would take 10 years.
This time, engineers looked at the aerodynamics of the bridge, using trusses instead of shallow girders for greater stiffness, along with grates between the lanes of traffic to let wind pass through.
Principal engineer Charles E. Andrews and design engineer Dexter R. Smith worked with Farquharson's group, together pioneering the study of aerodynamics in bridge design.
Bethlehem Pacific Coast Steel Corp. and John A. Roebling's Sons Co. were hired to construct the bridge. The towers were built atop the old piers, but with wider bases for better load distribution. The roadway was wider, 49 feet compared to 26 feet.
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