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The long inverted arch of the suspension bridges was not only economic, it possessed inspiring beauty. But that very beauty blinded some builders, who wanted to create an even slimmer bridge by cutting down on the depth of the stiffening girders. Such a bridge was the Tacoma Narrows Bridge, built out over Puget Sound in 1940. Motorists crossing the bridge often noticed that the car in front appeared to sink into the roadway or even vanish for an instant. Nobody was alarmed at first, and engineers and drivers alike enjoyed explaining the advantages of "Galloping Gertie's" flexible suspension design. Then, four months after the bridge was opened, Gertie galloped herself to pieces in a high wind. Gertie's extremely narrow, slender and flexible design was strong enough to withstand foreseeable forces. But the wind that killed the bridge came at more than 40 m.p.h. across and under the bridge, and started the span on a vertical oscillation, which so fed itself that the deck was whipped clear of its supporting cables. The bridge, ruled the experts, was "aerodynamically unstable."
Concrete. Gertie's final gallop convinced bridge builders that they did not know everything about bridge building. Back to school they went to learn more about aerodynamics, stresses and strains. The new technology produces far more than just better suspension bridges. One of the most ingenious uses of prestressed concrete is in the $21 million floating bridge across the Hood Canal in Washington's Puget Sound. Carried on 23 concrete pontoons, the bridge has retractable center sections that slide into the main body of the bridge, allowing waterborne traffic to pass through instead of under. The greatest use of prestressed concrete is in the 51-mile bridge over Venezuela's Lake Maracaibo—the longest prestressed concrete bridge in the world.
By necessity, since nearly all of their big bridges were destroyed in World War II, some of the busiest users of the new technology are the Germans. They are also some of the most inventive. Nearly all the steel bridges built in Germany today use a German-developed steel plate called orthotropic. On a conventional bridge, the concrete roadway is supported on steel stringers. Not on an orthotropic bridge, which has instead of a concrete slab a half-as-heavy steel deck serving both as roadway and stress-carrying component of the bridge spans.
Bridge building is almost as frenzied in other parts of the world. Britain's new bridges include the majestic Firth of Forth suspension span (3,300 ft., longest in Europe), soon to be completed. Already under construction in Portugal is the even longer (3,323 ft.) Tagus River span, scheduled for completion in 1967.
Biggest. But nowhere on earth is there such a surge of bridge building as in the U.S., which already has 500,000 bridges. So far the most spectacular new span is the masterwork of George Washington Builder Othmar Ammann (now 85)—the Verrazano-Narrows Bridge across the main entrance to New York Harbor. Nearly everything about the bridge is the biggest: it cost $325 million, it outspans Golden Gate by 60 ft., it hangs from 145,000 miles of cable wire. Its twelve traffic lanes will carry 48 million cars a year between Brooklyn and Staten Island.