October 20, 2012 - Bridge Day

Have you ever wondered how people build bridges? How do you start? Do you build from the bottom up, or from the top down? How do you start to reach from side to side over treacherous waters or a deep chasm?

The Niagara Falls Suspension Bridge is an early example of a bridge that allowed pedestrians, carriages, and trains to cross a wide river—as a matter of fact, it is the world's first working railway suspension bridge. And that first step—the laying of the first line across the wide and turbulent Niagara River—was accomplished by flying a kite!

The builders were stumped because the water was so turbulent that the rapids were called Whirlpool Gorge. They didn't think a boat could take the first wire across the river to start the bridge. Someone suggested shooting a rocket attached to a wire, or some sort of bombshell with a wire, shot out of a cannon. But one many suggested offering a cash prize to the first child who could fly a kite over the gorge.

A 15-year-old boy named Homan Walsh won the prize (either $5 or $10—accounts vary) by flying a kite from the Canadian side of the gorge. Eventually, when the winds subsided, his kite descended and got caught in a tree on the American side. The day after this contest-winning flight, a stronger line was attached to the kite string; a rope followed, and then a cable made of wire.

Next, the builders created a temporary suspension bridge across the Niagara River. They used that temporary bridge to build a better, stronger, permanent bridge.

I've always wondered how bridge builders create those supportive towers in water. I found out that, in the olden days (such as the days of the Roman Empire), huge boulders were piled up in the water, and the bridge was built on and secured to the boulder piles. Later, builders would lower a caisson, which is a huge cylinder made of steel and concrete, into the water and onto the river-, lake-, or seabed. The water was then pumped out of the caisson. At this point, the caisson became a sort of circular dam that kept water away from workers and equipment. Since the 1930s, bridge builders generally float enormous metal caissons to the designated spot and then sink the caissons by gradually filling them with concrete and water. Divers are on hand to make sure that the sinking caissons settle on the right bit of ground.

To learn more about building bridges, check out the Worsley School Building Bridges page. Also, check out the Official Bridge Day website.

• Also, this earlier post deals with a bridge disaster, 
• this earlier post discusses cantilever bridges, and 
• this earlier post links to some fun bridge experiments. 

Also on this date:

Architect Christopher Wren's birthday 

Baseball great Mickey Mantle's birthday