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.

## Post Information

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

1. isaac - November 19, 2008 at 12:43 am

thanks very much this helped so much

2. kaitlin - December 1, 2008 at 4:12 pm

this website didn’t really help me to pass my quiz in tech.put more stuff on it. oh yeah, thanks alot.

• Brooke - December 3, 2013 at 8:00 pm

Well, dont look at the site if it isnt gonna help
it would be time wasting

3. Ashley - December 1, 2008 at 9:17 pm

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!

• brad - September 22, 2010 at 4:44 am

this website has really helped me and i enjoyed reading it.

4. Billy Joe - December 23, 2008 at 11:15 am

Good website. helped me to build my toothpick bridge

• Annika - November 19, 2014 at 4:47 pm

Cool! I’m building a toothpick bridge too

5. Cyrus Duong - January 3, 2009 at 1:52 pm

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

• Koula - January 31, 2010 at 3:25 pm

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.

6. bill rosendale - January 10, 2009 at 5:15 pm

7. bigmike09 - January 26, 2009 at 2:27 pm

helped a lot xD

8. heyhom1234 - February 12, 2009 at 10:35 am

this helped immensily great site i now have an a in science!!!

9. ricky gladu - April 27, 2009 at 7:38 am

this is a great site but i cant read so maybe try more videos
thanbsk a lot tho its really helping me out

10. dima - May 5, 2009 at 12:30 pm

the basic info that you need for bridges. thnx

11. Kevin - May 6, 2009 at 1:10 pm

Shearing makes no sense

12. Haggis - September 4, 2009 at 10:15 pm

thank you, this has all the info we needed for our assignment!

13. Aaron - November 28, 2009 at 10:22 pm

How do you make a Model bridg. With straws so that the weight is pulling on the straws not pushing?

14. justin - March 23, 2010 at 7:52 am

this website helped me finish a project on bridges! awsome!

15. hay-hay - April 4, 2010 at 8:58 pm

Thank you so much could use a little more info but it was awesome!!

16. mina - June 16, 2010 at 1:55 pm

thanks, dis really helped me in my physics project on beam bridges.

17. James Seator - August 29, 2010 at 10:57 pm

thank you so much this is an amazing website <3

18. JBoogie - November 13, 2010 at 2:30 am

thnx, can u include some stuff about stress pionts pionts and how they vary from different bridge designs

19. Ekram Rakib - February 3, 2011 at 6:38 pm

It was really good but can you show an image for shear?

20. Tiffany Tibits - March 11, 2011 at 4:36 pm

Kewl site it helped me for my bridge project!

21. ashley - April 5, 2011 at 10:56 pm

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?

• Garrett Boon - April 6, 2011 at 4:01 am

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.

• skane - April 23, 2011 at 2:59 pm

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.

22. Goldrush - April 7, 2011 at 3:47 am

omg this helped me a lot on my bridge project thanks

23. mmkay - April 27, 2011 at 7:31 pm

im doing homework on forces and this helped quite a lot, all i got was gravity!! xx

24. Lozergallardo - May 16, 2011 at 6:02 pm

it helped me in my report in a way that im thankful for

25. kiki - May 19, 2011 at 12:02 am

This probably saved my life! THNX

26. Lozzyc - June 1, 2011 at 3:59 am

A great and informative website!

27. Vjdubbsv - June 7, 2011 at 8:13 pm

thanks great info, i got an A on my paper thanks to this site, i put it in the bib

28. Superlily76 - June 21, 2011 at 4:09 pm

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!!!!!!

• Garrett Boon - June 21, 2011 at 11:27 pm

This website is specifically for model bridges, and does knowingly leave out many factors that engineers of real bridges must consider.

29. Epdorbu - September 14, 2011 at 6:37 pm

Thank you for this information. How would I cite this article? Is this a blog, or what?
I don’t want to plagiarize this website, so I want to know as much as  possible at the details. Also, I am writing a weekly “essay” for my Physics class. Is there any site like this?

• Garrett Boon - September 15, 2011 at 1:25 am

I wouldn’t necessarily call this a blog. Probably just follow the standard for citing an article published online. You can use the “Last Modified” date for the publishing date.

• ImperatorPavel - October 8, 2011 at 3:38 pm

I think this would be the correct citation, in MLA format:

“Forces that Act on Bridges”. Garett’s Bridges. Garett’s Bridges, 20 April

2011. Web.  Oct. 2011.If you have to use a different format (APA, for example), then I can’t help you.

30. Fudge - October 18, 2011 at 6:09 am

With the shear Force, Where would it actually occur on a bridge?
Which part of a bridge would be affect by it?

Also how do bridges usually fight that force?

31. Cocoa - November 6, 2011 at 10:14 pm

I was looking for how to stop the effect of torsion and shear, but you didn’t say how to. You should include how to stop the effects of the forces when you update it!

32. aj - January 31, 2012 at 3:43 am

how to calculate tension and compression?

• Garrett Boon - February 1, 2012 at 9:57 pm

AJ, check out the Bridge Designer

33. Amy - February 14, 2012 at 5:46 pm

You should talk about where on suspension bridges that these forces work!

34. Jessica - March 3, 2012 at 2:46 pm

Could you maybe show me a better demonstration of shear and where it might be seen in a truss bridge?

35. Deanna - March 17, 2012 at 8:27 am

Do you know where compression, tension, torsion and shear might happen on a bridge? Or when?

By the way, this is really good.

• Garrett Boon - March 18, 2012 at 9:07 am

The top chord is usually in compression, the bottom chord in tension, and the truss members vary depending on where the load is and what type of truss. Torsion might exist if the load isn’t centered and you do not have good lateral bracing. Shear could break a bridge at the point where the bridge leaves the anchor point if that member is not substantial enough.

36. 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!

37. 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 .

38. Leah and Joelle - May 15, 2013 at 8:52 am

For the shear force, it is improper grammar to write “a interesting” instead of “an interesting”.

39. hamed - May 15, 2013 at 9:18 am

i am civil student from iran . i want to know how much is the allowable normal stress in pasta bridge ?

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

thanks! great help for sciency stuff!

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

Thank you. It helped

42. 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

43. Chamal sri lanka - December 4, 2015 at 5:40 am

Thank a lot buddy ..i dnt knw who da owner of thiz blog but its helped me to pass my assignment in science and materials. . Hope u doing well for long time

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