We see bridges almost everywhere but rarely think about how they’re made or how they work. Learning about bridges is an excellent way to explore engineering, math, science, and art. It all comes together with bridges.
The problems we face today when building a bridge are the same as the problems ancient Romans, Sumerians, and the first humans had to solve. The problem: how do we get across that river? Besides simply crossing a river or a valley, bridges must also be able to support the people and their cargo as they cross. There’s a lot of thought that must go into the design.
Civil engineers build bridges. The term “civil engineer” goes back to 1750 and John Smeaton, a British engineer who designed and built bridges, canals, harbors, and lighthouses. Smeaton devised the phrase to distinguish between engineers who worked on civil projects for communities, like bridges and canals, military engineers who worked on armaments, and defensive projects like forts. The professional association for engineers, IEEE (“I triple E”), has a lot of resources you can use to explore bridges and other fun engineering topics. Go to https://tryengineering.org/teacher/bridges.
Suspension Bridges
Suspension bridges have one or more towers with cables, but the cables don’t support the deck directly. Instead, the cable has vertical cables that drop down from the cable to hold the deck. San Francisco’s Golden Gate Bridge and the Brooklyn Bridge are classic examples. In their simplest form, suspension bridges also have been used for thousands of years with rope and wooden slats for a deck.
Cantilever Bridges
A cantilever bridge uses structures, called piers, to hold horizontal decks one after another across a space. Generally, they have three spans, with the first and third span anchored with an abutment and the second span levered by two piers. Cantilever bridges are less durable than other kinds.
Arch Bridges
Some bridges built by the Romans thousands of years ago are still standing. Made of stone and concrete, they have a deck on top of a set of arches that hold the weight of the deck. The heavy stones withstand wind. Roman concrete is more durable than ours because they used volcanic ash which healed cracks before they spread. This is an arch bridge from 1827 in England.
Cable Bridges
Some bridges have one or more towers with cables that support the bridge deck. The earliest cable bridges date back to 1595 when designs were published in a book, Machinae Novae. Modern cable bridges have thin towers that look like chopsticks. They’re used when a bridge needs to be longer than a cantilever bridge but not as long as suspension bridges. This is a cable bridge in Washington, USA.
Truss Bridges
Truss bridges are built from a defined structure, for example, wood or steel arranged in a repeated pattern like triangles. The pattern provides a strong structure to hold the bridge’s weight and resist wind or other stresses. These bridges use materials very efficiently. This is a truss bridge in Massachusetts, USA.
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