How Does a Suspension Bridge Work?

The suspension bridge is easy to recognize for its slender, distinctive shape. Most suspension bridges have two long pairs of long, narrow towers with cables hanging gracefully between them. These main support cables are strung with smaller, vertical cables to hold up the main bridge deck. The main cables descend into supports at either end of the bridge, often holding up two smaller decks to the outside of the main towers.

The main cables are strung with vertical cables that attach to the deck. These cables support the deck of the bridge, transferring the weight to the towers at the end. The main cables continue past the tower, and are anchored past the far ends of the bridge. They are carefully balanced so that the force pulling inward on the towers is equal to the force pulling outward. As a result, the weight pulls directly down into the base of the tower. The towers can be fairly thin, since they aren't being pulled to either side. The deck can also be thin, since it is being supported by the cables.

Suspension bridges have a few weaknesses that have caused disaster in the past. One of the most serious problems is the main cable. Early suspension bridges were sometimes made using chain links of heavy steel for the main cable. If only one chain link failed, the whole bridge would collapse, as what happened to the Silver Bridge in 1967, killing 46 people. Nowadays, suspension bridges use bundles of cables. If one or two of the cables fails, the bridge still stays intact.

The Tacoma Narrows Bridge was a more famous and spectacular lesson in design. The deck was not thick or stable enough, and would start to oscillate in a moderate wind. Eventually, in a fairly heavy wind, the bridge shook itself apart. Nowadays, suspension bridges are built with a more thick and rigid design to prevent excessive motion.