How Do Bridges Support Weight?
Michael F., 2012
Introduction
Bridges are structures that help people cross over an obstacle or a body of water. But how do they support the amount of weight that they do? I will try to answer this question and tell you more about how bridges work in this report.

Types of Bridges & Their Spans
Different types of bridges hold different amounts of weights and will span differently. Span means “the difference between two bridge supports.” When an engineer is going to build a bridge, he must first decide what the best type of bridge to build would be. For example, if the bridge was going over a small or short obstacle, the engineer would probably build a beam bridge because beam bridges can’t support as much weight over a long distance and they would probably be easier to build and cost less than other more complex types of bridges. If the bridge needed to go over a long or big obstacle, the engineer would probably decide to make a suspension bridge since they span over the longest distances. A bridge must be made out of strong materials so it can support the middle part. There are four major types of bridges. They are called the beam bridge, the arch bridge, the truss bridge and the suspension bridge. These four types of bridges usually span at different distances because of two forces. These forces are called compression and tension.

Compression and Tension
Compression happens when a force is making the object that it is acting upon compress or get shorter. An example of compression is when you push down on a spring
Compression and Tension MAF.gif
Figure 1: Compression and Tension
and the spring pushes together and gets smaller. The act of compression is what makes it smaller. Tension happens when a force is expanding from the object that it is acting upon. An example of tension is when you let the two ends of a spring go and the spring goes back to normal. Tension is what causes this. All bridges deal with compression and tension. They must be able to handle these forces without breaking. The best way to do this is by either transferring or dissipating the weight. When you dissipate force, you spread it out. When you transfer force, you move it from a weak area to a stronger area of the bridge. An arch bridge can handle more weight than a beam bridge because the weight is dissipated over the arch.




Beam Bridges
Beam bridges are horizontal bridges that rest on two structures; one at each end. The beams on the sides must be very strong so they can hold up the middle part of the bridge. The farther apart the beams are, the weaker the bridge will be. The beams are usually made out of concrete or steel. The weight on a beam bridge pushes down. Compression pushes down on the top part of a beam bridge which causes tension in the lower parts of the bridge. A beam bridge usually spans the shortest out of the four major types of bridges at around 200 feet.


Beam Bridge MAF.jpg
Figure 2: Beam Bridge


Arch Bridges
Arch bridges are structures that are semicircular with columns at the end. Arch bridges are always under compression. The compression on an arch bridge is pushed outward near the curve of the arch on the bridge. The amount of tension on an arch bridge is reduced by the semicircle of the curve. It is reduced because the curve of an arch bridge dissipates force outward along the arch. Arch bridges are able to span longer distances than beam bridges at between 800 and 1,000 feet.

Compression bridge MAF.jpg
Figure 1 Diagram: Compression and Tension on Arch Bridge


Truss Bridges
Truss bridges are made out of many triangular shaped figures. They are usually made out of straight, steel bars. Cantilever bridges are more complex versions of truss bridges. Two types of trusses are “through trusses” that are above the bridge and “deck trusses” that are below the bridge. Large truss bridges can
Through truss.gif
Figure 4: Through Truss
span at around 1,000 feet.
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Figure 3: Deck Truss

Suspension Bridges
A suspension bridge has cables, ropes or chains that help hold it up. Compression pushes down on a suspension bridge’s main body or roadway but the cables are able to make the force go to the towers, and the towers make the compression dissipate into the earth. Suspension bridges can span much longer than any of the other major types of bridges at between 2,000 and 7,000 feet. The Brooklyn Bridge was the first cable-wire suspension bridge ever built. I am going to tell you a little bit more about this suspension bridge.

Brooklyn Bridge
The Brooklyn Bridge’s construction started in 1869 and ended in 1883. It was the first cable-wire suspension bridge, so there were a lot of accidents and mishaps. About 20 people were killed during the construction of the Brooklyn Bridge. It took a while to build this huge bridge at 1,595 feet long and when the construction was finally finished, a lot of people celebrated because this was such a huge achievement. The Brooklyn Bridge is now still standing in East River in New York City. This bridge is important because it connects Manhattan and Brooklyn. Like all suspension bridges, the Brooklyn Bridge is supported by cables and tall towers through which the cables are attached. The cables transfer the compression to the towers which dissipate the compression into the earth.


Brooklyn Bridge MAF.jpg
Figure 5: Brooklyn Bridge








Glossary
Beams- The metal structure on the sides of a beam bridge underneath the roadway.
Compression-A force that makes the object that compression is acting upon get shorter.
Deck Truss- A truss that is below a bridge roadway.
Dissipate- To make a force break and then disappear.
Span- The distance between two ends of a bridge or the length of a bridge.
Tension- A force that makes the object that it is acting upon get bigger or expand.
Through Truss- A truss that is above a bridge roadway.
Truss- A metal structure that supports the roadway of a bridge.



Citations
Bryan, Kim. “Ask Me Anything.” London ; New York, DK, 2009

"Bridge." Britannica Elementary Encyclopedia. Encyclopedia Britannica Online School Edition.
Encyclopædia Britannica, Inc., 2012. Web. 14 Sept. 2012.
http://school.eb.com/all/elementary/article?articleId=352881.

“Brooklyn Bridge." Compton's by Britannica. Encyclopædia Britannica Online School Edition.
Encyclopædia Britannica, Inc., 2012. Web. 14 Sept. 2012.
<http://school.eb.com/all/comptons/article-9318119>.

"Brooklyn Bridge." Brooklyn Bridge 2012. Encyclopædia Britannica Online School Edition. Encyclopædia Britannica, Inc. Web. 26 September 2012, <http://school.eb.com/eb/article-9016640>.

“Building Big: Bridge Basics.” PBS Online 2000. Web. 24 September 2012. <http://www.pbs.org/wgbh/buildingbig/bridge/basics.html>

Brain, Marshall. “More How Stuff Works.” New York, Wiley Pub., 2003.