Can you imagine what it would have been like to explore the unknown frontiers of America with Lewis and Clark? What would you have thought the first time you laid eyes upon the mighty Mississippi River?
Since there were no bridges across the Mississippi back then, you might have thought, “How am I going to get across this thing?" Of course, that's what boats were for way back when. Once technology advanced enough to make automobiles popular, though, the need for bridges across waterways increased dramatically.
One of the most popular types of bridges is the suspension bridge. If you've ever seen pictures of the Golden Gate Bridge in San Francisco, you know that the function of suspension bridges can be exceeded by their beauty and elegance.
Suspension bridges get their name from the fact that the roadway is suspended by cables from two tall towers. Engineers design the cable support systems in such a way that most of the weight of the bridge is supported by the two towers, which in turn dissipate the compression forces from the cables directly into the ground.
On a suspension bridge, smaller cables called suspenders run vertically from the bridge deck up to the main supporting cables. The suspenders transfer the bridge deck's compression forces to the towers via the main supporting cables, which create graceful arcs between the towers and down to the anchorages on each end.
The towers of a suspension bridge can be fairly thin, since the forces at work on the bridge are carefully balanced on each side of the towers. The force of the bridge deck pulls inward on the towers. At the same time, the main support cables extend beyond the towers to anchorages on each end, which are usually solid rock or massive concrete blocks anchored underground.
The anchorages pull outward on the towers with an equal force to that of the bridge deck. This balances and centers the weight of the bridge on the tower, which dissipates that force into bedrock deep underground. Because of their unique design, suspension bridges can easily span distances as great as 7,000 feet or more.
Early suspension bridges suffered from a couple of design flaws. For example, early suspension bridges that used chains for the main cables were susceptible to collapse if one link in the chain broke. This problem was solved, however, by creating main support cables out of bundles of high-strength steel cables. Several individual cables can fail without affecting the structural integrity of the bridge.
Some early suspension bridges also failed because their bridge decks were thin and unstable. When heavy winds hit these bridges, the bridge decks would shake themselves apart. Today, however, suspension bridges feature thicker, rigid bridge decks undergirded with a deck truss system that stiffens the deck and prevents swaying.
Even though suspension bridges seem like marvels of modern engineering, experts believe the first suspension bridges may have been made of twisted grass and built by the Incas over 500 years ago. Spanish explorers discovered such suspension bridges spanning more than 150 feet over deep mountain gorges in Peru in the early 16th century!