What's the world's longest suspension bridge?

Inventing on the Fly: A New Method of Bridge Building

Why did it take 40 years to finish Japan's Akashi Kaikyo Bridge? The answer is surprisingly simple, yet astounding. Some of the bridge's materials and construction techniques had to be invented along the way. And it all began with the first phase of construction -- the bridge's support columns.

Placing preformed concrete towers (the traditional method) in the deep, fast-moving waters of the Akashi Strait was out of the question. Instead, engineers had to come up with a way to build the towers on site. Giant, double-walled steel cylinders were constructed and towed into the strait. While an army of tugboats held the cylinders in position (not an easy task, given the racing currents), massive built-in water spigots were intermittently activated to fill the cylinders' walls. Over the course of eight hours, the cylinders slowly sank onto foundations that had been leveled on the sea floor, hitting their intended targets within a pencil's width of center.

Unfortunately, this was only a temporary solution. The cylinders required concrete innards to stay in place, but ordinary concrete would dissolve when poured into seawater. Japanese engineers were forced to invent a new type of concrete, one that was thick enough to spread under its own weight without washing away in the seawater, yet could be pumped into the sunken cylinders and then harden despite watery conditions.

Then came the next obstacle ... and the next. Even though they'd just come up with an entirely new form of concrete, it wouldn't work for the bridge's support towers. Instead, engineers needed a material that would sway -- not snap -- when the ground shook. This earthquake-proof material, expected to withstand episodes with a magnitude of more than 8, was a special steel alloy. Block by block, the towers were forged and assembled like titan Erector sets until they each rivaled the height of the Eiffel Tower [source: Ryan].

The towers would soon be tested. On Jan. 17, 1995, an 8.5-magnitude earthquake originated just kilometers from the bridge. While the bridge didn't sustain major damage, its supports did shift away from each other. This resulted in the bridge becoming a couple of meters longer than originally planned [source: National Geographic].