Pratt Truss

By on January 11, 2011 -- Modified on October 4, 2016

The Pratt Truss was designed by Thomas and Caleb Pratt in 1844. It became popular for railway bridges because it made good use of iron. The Pratt has many variations, most with their own unique name. For instance, the Baltimore, Pennsylvania, and the Parker are all based off the Pratt.

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the Pratt Truss is under a load. The first shows the load being applied across the entire top of the bridge. The second shows a localized load in the center of the bridge. In both cases the total load = 100. Therefore, you can take the numbers as a percentage of the total load.

These diagrams bring up several interesting things. Notice that the two end diagonal members do not change. Also, there is little change on the bottom chord between the two pictures. However, there is drastic changes on the internal truss members. The centered load dramatically increases the amount of force that is applied to the internal members of the bridge. Also, the forces are increased on the top chord of the centered loaded bridge.

This seemingly insignificant change in how the bridge is loaded makes a big difference in how your model bridge will perform. If you have the ability to change and set how your bridge is loaded, I’d shoot for spreading the load across the entire span. This pretty much goes for any model bridge design, not just the Pratt Truss.

Pratt Truss for model bridges

The Pratt Truss is one of my favorites. I have used it often for my model bridges, including balsa, basswood, and popsicle sticks. It is easy to construct, and is a solid choice for a model bridge design.

Additional Resources

Pictures of real Pratt Bridges
History of Truss Design

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25 thoughts on “Pratt Truss”

  1. What angles/dimensions for the triangles would help to support a concentrated load? (There’ll be a small force plate used on the middle of the deck)

    By Will Adams -- December 13, 2016
  2. Hi. I have to build a toothpick bridge that is 11 inches long and I’m interested in the Pratt model. I see that there are isosceles triangles, but do you have any angle measurements? Are smaller or larger angles better in the triangles?

    Also, do the variations of the Pratt design better in any way? I wouldn’t really mind building either type but I would just like to know if there are any benefits associated with the other Pratt designs. Thank you!

    By Anita -- November 26, 2016
  3. You said, “If you have the ability to change and set how your bridge is loaded, I’d shoot for spreading the load across the entire span. ”
    Could you please explain how I could go about ‘spreading the load’ for my model bridge?

    Also, if I were to add counterbraces, where/how would I do that?

    Thanks!

    By Tara -- November 26, 2016
    • Tara,

      If you have the ability to use a larger loading block to spread the force evenly across the bridge, the bridge will hold more weight. That is what I meant. Otherwise, it is the job of your truss design to help spread the load throughout the bridge.

      By counterbraces, do you mean adding to the truss design? Or do you mean lateral bracing?

      By Garrett Boon -- November 26, 2016
  4. We used your blueprints to build a Warren, Pratt, and Long Howe truss bridge. We tested how much weight each bridge could hold using the hanging bucket method with sand and water. The Howe’s bridge held much more weight than the Pratt. The Howe broke under 54 kg while the Pratt held only 28 kg. This seems like too big of a difference considering how similar these bridges are. Also, I read that in the Pratt, the vertical members are under compression and therefore should be able to withstand more force. Do you have any input about these results?

    By Jessica -- November 21, 2016
    • Jessica, that is certainly an interesting difference. Were you able to tell what failed first on each bridge? In my experience, sometimes small differences in build quality or materials can make a big difference in how much a bridge can hold. If there is just one weak joint on the Pratt, that could have caused the early failure. Or perhaps there was a weak popsicle stick. This variances are one of the reasons a single test doesn’t provide enough information to make a conclusion about which design is better or not. More testing has to be done. But I am surprised by your results.

      Yes, in the Pratt design, the vertical members are in compression. Because the vertical members are shorter than the slanted, and shorter pieces of wood hold more force in compression, theoretically the Pratt should do better when everything else is the same.

      How did the Warren do compared to the others?

      By Garrett Boon -- November 22, 2016
  5. what are the blue en red en green lines meaning

    By linus -- November 13, 2016
  6. Any chance you would be able to create a diagram showing compression and tension members in detail;

    By alissa -- October 30, 2016
    • Alissa, what more detail are you looking for?

      By Garrett Boon -- October 30, 2016
  7. Any chance you might have plans for a bridge that crosses a 300mm span , under 30grams in weight,with a 60mm. Load bearing

    By HJdad -- November 7, 2014
    • We do not have plans for a bridge with those specifications at this time.

      By Garrett Boon -- November 8, 2014
  8. My daughter is building a bridge out of Bass Wood. Would wood glue or super glue jell work better.

    By HJdad -- November 7, 2014
  9. am i write in saying that both bridges use steel and iron in modern construction of bridges. because the only disadvantage i found for the Howe truss was that is included wood and was heavier. my question is why is the pratt truss better other then the two reasons i mentioned above.

    By Ajaipal -- November 10, 2013
  10. Hi what will happen if I changed the angle of inclination to about 45 degrees? will it make the structure stronger? or weaker?

    By Sabine -- January 21, 2012
    • Yes this will make it far weaker in the sense that greater force will be transferred onto the members. Although reducing the height would increase torsional strength/sideways stability, a lower angle of inclination will have an adverse effect on the bridge. This can be seen using sin and cos ratios, where 45degrees increases the magnitude of the horizontal forces acting on the member.

      By XXSkux -- May 12, 2015
  11. Would adding counterbraces to this model help it support more weight?

    By Jenny -- December 30, 2011
    • yes it will help support the weight !!!

      By anaya -- May 2, 2012
  12. I need to make a Truss bridge for my Physics class and i wanna use this design but i need to know, does the Height of the Bridge have any serious effects on the Pressure distribution for this design?

    By Patrick -- August 15, 2011
    • Yes. Changing the height of the bridge changes its aspect ratio which affects the forces on the top and bottom chords.

      By Garrett Boon -- August 15, 2011
  13. im making a pratt truss bridge, but i haveee NO idea if it can ave only 4 crosees?

    By Flunky Star -- April 9, 2011
    • I am not sure what you are asking.

      By Garrett Boon -- April 10, 2011
    • yes but the bridge wont be as sturdy

      By Tyler -- April 15, 2013
  14. I am building a bridge model at 1:100 scale. I wanted to know how to scale the full size desing loads down for the model.

    By Jeremy Zuger -- March 11, 2011
    • size to strength ratio is s:s^2
      therefore if you’re bridge’s ratio is 1:100 then it’s strength capacity will be 1:10,000 in proportion to the original bridge

      By A. -- March 22, 2011

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