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In the late 1800s, a railway bridge across Scotland's Firth of Tay swayed and collapsed in the wind. Seventy-five passengers and crew on a passing night train died in the crash. It was the worst bridge disaster in history. So when engineers proposed bridging the even wider Firth of Forth, the Scottish public demanded a structure that looked like it could never fall down. They got it.

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Chief engineers Sir John Fowler and Benjamin Baker came up with the perfect structural solution: a cantilever bridge. The Firth of Forth Bridge is made of a pair of cantilever arms, or beams "sticking out" from two main towers. The beams are supported by diagonal steel tubes projecting from the top and bottom of the towers. These well-secured spans actually support the central span. This design makes the Firth of Forth Bridge one of the strongest -- and most expensive -- ever built.

But not everyone liked the design. The poet and artist William Morris declared it "the supremest specimen of all ugliness." Ugly or not, the Firth of Forth is a safe bridge. Even today, the highest winds barely shake this enormous structure. This is exactly what the people of Scotland needed after the Tay Bridge disaster. Unfortunately, a cantilever of this size comes with a hefty price tag. This is why very few like it have ever been built again.

Firth of Forth Bridge 8,276'

• The Firth of Forth Bridge may not have been a bridge at all -- engineers considered building a tunnel, but abandoned the idea because it seemed too risky.
• The Firth of Forth was the first bridge built primarily of steel.
• It took 54,000 tons of steel; 194,000 cubic yards of granite, stone, and concrete; 21,000 tons of cement; and almost seven million rivets to build the Firth of Forth Bridge.

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