Definition
A suspension bridge is a bridge built by draping heavy cables between huge towers and hanging the roadway (the deck) straight down from those cables. It is the best design for crossing a wide gap, like a bay or a deep valley, in one long span where building many supports is hard.
How Giant Cables Hold a Bridge Up
Picture yourself holding a heavy bag in each hand. Feel how your arms go tight? A suspension bridge works the same way. Instead of propping the road up from below with supports, it hangs the road in the air by pulling it up with steel cables. That's how a bridge can leap across several kilometers without packing the water with supports.
At each end of the bridge stands a tall tower called a pylon. The thick cable that runs over the tops of the towers and down to the ground on both sides is called the main cable. From the main cable, many vertical cables drop down and tie to the deck, the part where cars drive.
The weight of cars on the deck first travels up the vertical cables. Then it moves along the curved main cable and spreads into the towers and the anchors buried in the ground at each end. Rather than holding things up from underneath, this clever design uses the pull of strong cables hanging from above.
Sharing the Work: Pulling and Pushing
Every material handles force in its own way. A steel bar resists being pushed from above, but a long one bends easily. A thin wire is the opposite: it crumples if you push it, but it can hold a huge weight when pulled tight from both ends.
A suspension bridge pushes this idea to the limit. The tall towers take on the heavy pushing force, called compression, and pass it down to the ground. The long main cable and the vertical cables handle only tension, the pulling force that tries to stretch them tight.
Steel cable is very strong against pulling compared to its own weight. That's why the bridge doesn't need a lot of heavy piers in the middle. By cutting its own weight so much, the bridge can safely connect very long distances.
A Little More Precisely: The Giants Underground and the Wind Problem
No matter how strong the cables are, the bridge will collapse if they aren't held firmly at both ends. That's why each end has an anchorage: a giant block of concrete weighing tens of thousands of tons, or a firm hold on solid bedrock. Each anchorage of the Golden Gate Bridge weighs about 60,000 tons. Like the person at the back of a tug-of-war who digs in with everything they've got, the anchorage holds back the enormous tension of the cables with the weight of concrete or the strength of rock. Some bridges, called self-anchored suspension bridges, fasten the cable ends to the ends of the deck instead of the ground.
As a suspension bridge gets longer, its biggest enemy isn't heavy trucks. It's wind. Because the bridge is a flexible structure hanging from cables, wind can make it sway and ripple like a flag.
If the bridge twists, the force of the wind on it changes, and that force can twist it even more. This feedback loop is called flutter, and it can wrench a bridge apart. That's how the Tacoma Narrows Bridge in the U.S. collapsed in 1940. So modern suspension bridges use a streamlined, airplane-wing-shaped deck that lets wind slide smoothly over and under it, or they stiffen the deck with an open truss frame that lets the wind pass through, to stop the swaying.
Common misconceptions
A suspension bridge stays up because its deck is thick and heavy.
It's actually the opposite. The weight of the deck is mostly carried by the main cables above, and by the towers and anchorages that hold those cables. The deck isn't designed to hold up weight so much as to be stiff enough not to be shaken by the wind.
Where you meet it
The Golden Gate Bridge, the symbol of San Francisco, is a classic suspension bridge.
In Korea, long bridges across wide stretches of sea, like the Yi Sun-sin Bridge and the Ulsan Bridge, are built as suspension bridges.
In one sentence
A suspension bridge hangs its deck in the air from steel cables tied to towers and anchorages, so it can cross very long distances with almost no piers in the middle.