How do Bridges Support Weight?

By Mohala K.

Last updated: 9/28/11

Different bridges support weight in different ways. For example, a beam bridge supports weight differently than a suspension bridge and a suspension bridge supports weight differently than an arch bridge. In fact, there are so many bridges with so many different ways of supporting themselves that I can’t write them all here. So instead of telling you a little bit of undetailed information on everything, I have picked a few bridges to give you detailed explanations about their support.
Beam Bridges
The very simplest bridge of all is the beam bridge. A beam bridge is a beam that is supported by two vertical supports at each end. Beam bridges were in use throughout the world by 10,000 B.C. beam must be strong so it doesn’t bend or break. A continuous beam bridge has lots of beams and supports and some beams share the same supports. Most highway bridges are continuous beam bridges.
mohalak675_beam_bridge.jpg
A beam bridge diagram



Arch Bridge

The next bridge I’m going to tell you about is the arch bridge. The arch bridge pushes out against its supports. The supports must be heavy or the arch would break them. The deck arch is basically a beam bridge with an arch under it to support it. This makes it stronger than a beam bridge. The through arch is basically a beam bridge held up by and arch. The arch balances the weight on the bridge and keeps the weight even on both sides. The Romans were the first to build arch bridges in 200 B.C.
Arch_bridge_MK.jpg
A picture of a deck arch bridge.

Suspension Bridge

Now for the suspension bridge. The suspension bridge is held up by steel cables hung from towers along the bridge. The steel cables are strong to support the weight of the bridge and the traffic on it. It also is stronger than a beam bridge. The Golden Gate Bridge is a suspension bridge. Old versions of suspension bridges were built in the Stone Age. In 1801 G. Roberts made the first modern suspension bridge in Pennsylvania. All of the longest bridges are suspension bridges because they are lighter weight than other bridges.
Suspension_bridge_M.png
A drawing of a suspension bridge.
Extra Information

Here is some more about bridge support. Arches and columns are compressed by weight and the compression makes them stronger. Cables and bars face tension, but they are strong and don’t snap.

Conclusion

In conclusion, there are many different ways bridges support weight. And this only tells you about a few of them. Other kinds of bridges include the cantilever bridge, which is supported on only one end. And then there’s the bascule bridge, which rises up to let ships pass underneath. It’s pretty impressive that we have been able to sustain the skill to build such amazing bridges over such a long period of time. See if you can identify the different kinds of bridges next time you take a road trip (you’ll probably only see beam bridges until you get out of state).

Citations:
Ardley, Neil. How Things Work. Pleasantville: NY Reader’s Digest, 1995. Print.
"bridge." Compton's by Britannica. Encyclopædia Britannica Online School Edition.

Encyclopædia Britannica, Inc., 2011. Web. 25 Sept. 2011.

<http://school.eb.com/comptons/article-9273351
Reid, Sruan. Invention and Discovery. London: Copyright Usborne Publishing Ltd., 1986. Print.
Glossary:
Arch Bridge: A beam bridge that is supported by an arch underneath or on top of it.
Beam Bridge: A beam that is supported by two vertical supports at each end.
Compression: A squeezing force.
Suspension Bridge: A bridge that is held up by steel cables.
Tension: A pulling force.