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
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
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
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.

68 thoughts on “Forces that Act on Bridges”

  1. whoever wrote this website must know this is not complete.  you have to write how this effects real bridges and you guys forgot BENDING FORCE!!!!!!

    Reply
  2. Thanks i guess. But i have some questions i would like anyone to answer..
    What things keep a bridge from surviving an earthquake?
    How does a bridge stay standing over water or a road?

    Reply
    • Ashley, great question. Unfortunately my model bridges don’t have to worry about earthquakes, so I have no experience. I do know that a lot of engineering goes into the design and construction of bridges to help prevent damage from earthquakes, especially in places such as California.

      Reply
    • I’m a practising structural engineer, although I’m from Australia so I have never actually had to design for earthquakes (they don’t happen often here), however I can make a few guesses as to what is important. The first is using the assumed properties of a design earthquake (basically what the design standards, local authorities, geologists etc. tell you) such as acceleration and amplitudes of movement to determine the loads on the bridge and designing accordingly. Secondly, making sure that the bridge’s natural frequency (the frequency at which it vibrates) is far enough away from the likely earthquake vibration frequency that the bridge will not resonate and tear itself apart. Finally you need to make sure it has high ductility. A member or connection with high ductility will stretch a lot when it fails, rather than breaking immediately. This means that the structure can re-distribute loads to parts that have not failed yet (allowing the bridge to carry more load in case the earthquake is bigger than expected). High ductility will also ensure the ultimate failure of the structure will be slow and steady allowing people to leave the bridge safely before it collapses.

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      • Thanks Skane, This is a fantastic and simple explanation of the forces acting on a bridge when affected by an earthquake. Very well written.

        Reply
  3. I still don’t quite under stand what shear is. Is it both compression and tension in a single piece of wood??? But then wouldn’t there be a neutral point? Where there is also no tension or compression in a member? Can you explain? Thanks 🙂

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    • I’m glad to help. Shear is a force that causes parts of material to slide past one another in opposite directions. I hope I helped!!! Amazing website by thw way.

      Reply
  4. OMG! this website is awesome.. but i think more info. could help more the people, but although its very helpful, thanks alot, i like alot this website! THANKYOU ALOT AGAIN!

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