Have you ever needed to cross a body of water? We bet you have! Maybe you’ve flown across the ocean in an airplane. Perhaps you’ve rowed across a lake in a boat. You’ve probably crossed a river by driving over a bridge.

Can you imagine how hard it would be to cross a river without these structures? Take the Mississippi, for example. Many people wouldn’t be able to swim across. Even taking a boat would be difficult! Bridges greatly improved people’s ability to cross waterways.

One of the most popular types is the suspension bridge. Have you ever seen pictures of the Golden Gate Bridge in San Francisco? If so, you know that these structures aren’t just useful. They can also be beautiful and elegant.

Suspension bridges get their name from the fact that the roadway is suspended by cables from two tall towers. Most of the weight is supported by the two towers. They, in turn, pass the compression forces from the cables directly into the ground.

Suspension bridges also have smaller cables called suspenders. These run vertically from the deck up to the main supporting cables. The suspenders move the deck’s compression forces to the towers through the main supporting cables. This creates graceful arcs between the towers and down to the ground.

The towers of a suspension bridge can be fairly thin. That’s because the forces at work are carefully balanced on each side of the towers. The force of the deck pulls inward on the towers. At the same time, the main support cables extend beyond the towers to anchor each end. These are usually solid rock or heavy concrete blocks secured underground.

The anchors pull outward on the towers with an equal force to that of the deck. This centers the weight of the bridge on the tower. Today’s suspension bridges can span distances as great as 7,000 feet or more.

Early forms of this structure had design flaws. For example, some used chains for the main cables. These could collapse if one link broke. This problem was solved by making main support lines out of bundles of high-strength steel. Several parts of the bundle can fail and the bridge will stay standing.

Some early suspension bridges also had thin and unstable decks. When hit by heavy winds, they would shake themselves apart. Today, the structures have thicker, rigid decks. They’re unlikely to sway.

Suspension bridges seem like marvels of modern engineering. But the first ones were built by the Incas over 500 years ago. These were made of twisted grass and often spanned over 150 feet. They were used to cross deep mountain gorges in Peru!

Have you ever crossed a suspension bridge? Have you ever seen one from a distance? They’re both beautiful and useful. Maybe you’re interested in helping build one someday!