Bridges must be able to withstand several types of forces. The two most common to model bridges are compression and tension, pushing and pulling respectively. The other two are torsion (twisting) and shear. Learn what these forces mean so that you can build a better model bridge.

# Forces that Act on Bridges

Bridges must be able to withstand several types of forces. The two most common to model bridges are compression and tension, pushing and pulling respectively. The other two are torsion (twisting) and shear. Learn what these forces mean so that you can build a better model bridge.

## Compression:

Compression is a pushing (compressing) force. The shorter a piece of wood is, the more compression it can hold. The longer a piece of wood is, the less compression it can hold. When you compress a long stick of wood you will notice that it starts to bend. When a piece of wood breaks because of compression, we say it failed from buckling. Typically the top chord of a bridge, including model bridges, will be in compression. Different truss designs spread out the force so that various internal parts will be in compression as well.

Compression

## Tension:

Tension is a pulling force. Wood has the ability to resist a lot of tension. It would be hard to break a popsicle stick if you held both ends and pulled apart. Tension may be applied parallel to the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong in tension parallel to the grain, but much weaker in tension perpendicular to the grain. Also, unlike in compression, the ability of wood to resist tension does not change with its length. A shorter piece of wood should hold the same amount of tension as a longer piece.

Tension

## Torsion:

Torsion is a twisting force. When you wring out a cloth, you are applying torsion to the cloth. If you take a stick pretzel, twist one end, and hold the other end still, it will break very easily. If you do that with a baseball bat, it will not break. However, if you take a piece of licorice and apply torsion to it, the licorice will twist around several times before it breaks. Each of these materials has a different way of responding to torsion. Bridge designers must watch for torsion and try to reduce it as much as possible.

Torsion

## Shear:

Shear is an interesting force. It happens when there are two opposing forces acting on the same point. If you hold a piece of wood with both hands next to each other, and push up with one hand and down with the other, you are applying shear to that piece of wood. Shear usually occurs horizontally, and not vertically.

Leave any questions in the comments below.

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### 42 Responses to “Forces that Act on Bridges”

1. Katie - January 6, 2013 at 4:41 pm

Hi there, this is a great site! I’m participating in a competition for building a drawbridge out of 1/8″ by 1/8″ balsa wood and had a few questions that I would appreciate anyone answering. We will be testing our bridges with a Pitsco tester which applies weight to the very center of the bridge. I understand that the shorter a member the better it handles compression but I was wondering if there was a way to reduce the tension that the members will undergo and if a better truss design to do so would be the Pratt or Howe truss. Thanks again for this great site, it has been a huge help!

2. Brittany - April 15, 2013 at 11:52 am

This website was good but it didnt really explain what affects these forces had on the bridges. that what i want to know more about .

3. smurf - August 13, 2013 at 10:01 am

thanks! great help for sciency stuff!

4. Claudia - December 2, 2013 at 1:03 am

Thank you. It helped 🙂

5. Tj - February 8, 2015 at 8:54 am

Thanks! This helped heaps for my assignment, although it would be nice to have other materials like steel and concrete

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