Source: Design Squad
Clifton Suspension Bridge
(Video)
Design Squad: Truss Bridge
(Video)
Forces Lab
(Interactive)
Grass Bridge
(Video)
As is true with any engineered structure—whether it's a skyscraper, a tunnel, or a dome—different bridge designs manage the forces of tension and compression in different ways. In this video segment adapted from Design Squad—a PBS TV series featuring high school contestants tackling engineering challenges—a team of students competes in a bridge design and construction challenge that requires them to build a suspension bridge, which uses long sagging cables and towers to support the weight of a suspended deck. The rules of the challenge prohibit them from using power tools and force them to use natural resources. Thus, they use hand tools and tie ropes to trees and wooden posts.
Download and ShareThanks 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.
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