Resource: Design Squad: Suspension Bridge

Thanks to an engineer's masterful grasp of physics, the properties and availability of various building materials, and innovative construction techniques, it is possible today to build bridges that carry heavier loads, span greater distances, and use less material than ever before. San Francisco's Golden Gate Bridge and Japan's Akashi-Kaikyo Bridge are marvels of modern bridge engineering.

All bridges, including the two suspension bridges named above, are subject to forces that push and pull on them. Whether made from modern materials and construction equipment, or traditional techniques and accessible natural resources, bridge designs that successfully balance out the opposing forces of compression (pushing) and tension (pulling) will stand. Those that cannot manage these forces will eventually fail.

A suspension bridge's signature main cables and rising towers work together to support the weight of a suspended deck and the load, that is, the vehicles and pedestrian traffic it is designed to carry. As illustrated in the video segment, the deck is connected to smaller, vertical suspension cables, which are in turn connected at regular intervals to the long main cables that are laid over the towers and run from end to end. Between the towers, the main cables hang in a curve called a catenary. At each end of the bridge, they are anchored, usually into solid concrete blocks or rock, which pulls them taut. The ground on which the bridge's feet stand pushes up to balance the downward force moving through the towers.

In this video segment, the student design team had to rely on pre-industrial materials and techniques to construct a suspension bridge across a 20-foot span. Unable to purchase ready-made ropes and concrete, the team members recognized that tying hand-braided main ropes around rooted tree trunks and larger branches provided adequate anchoring to resist the tension being carried by these ropes. The main ropes were simply run over both natural and artificial towers—trees and posts, respectively. The downward force from the weight carried by the ropes secured the trees and the posts in place.

To watch another team of students construct a different bridge design, check out Design Squad: Truss Bridge.

To learn more about how suspension bridges balance the forces that act on them, check out Clifton Suspension Bridge.

To learn about suspension bridges built using traditional techniques and natural materials, check out Grass Bridge.

To learn more about the forces that engineers must consider in their structural designs, check out Forces Lab.

• Is one type of bridge best for all conditions? Why or why not?
• Make a simple drawing of the suspension bridge. Use arrows to describe the forces on each element of the bridge.
• The engineering design process describes a series of steps, including testing the designed object and making changes and revisions. Are these steps evident in the video? What kinds of changes did the design team make?
• Can you find a bridge in the area where you live? Make a sketch. What parts of the bridge are in tension? In compression?