Warren Truss

By on January 4, 2011 -- Modified on November 28, 2016

The Warren Truss is a very common design for both real and model bridges. It’s exact history and origination is a little muddled, however. James Warren patented a design in 1848 (in England), which many attribute the name “Warren Truss”. His patent was more about the methodology of building rather than a a “design”. Regardless, the Warren Truss has been around a while and has been very popular. Examples of it can be found everywhere in the world.

The Warren Truss uses equilateral triangles to spread out the loads on the bridge. This is opposed to the Neville Truss which used isosceles triangles. The equilateral triangles minimize the forces to only compression and tension. Interestingly, as a load (such as a car or train) moves across the bridge sometimes the forces for a member switch from compression to tension. This happens especially to the members near the center of the bridge.

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the warren 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.

Interestingly, there is a significant difference. When the load is concentrated on the middle of the bridge, pretty much all the forces are larger. The top and bottom chord are under larger forces, even though the total load is the same. Thus, if you want your school project bridge to be able to hold more weight then try to spread out the force across the top of the bridge.

For a real life Warren Truss bridge, the forces often will be very localized and not spread out along the bridge. Thus, engineers must calculate how strong to make each member of the bridge and build accordingly. Unfortunately, not many Warren bridges are made anymore.

Warren Truss for model bridges

I have definitely used the Warren truss design for many balsa and basswood bridges. I have also used for some popsicle stick bridges. In fact, you can get a learning kit using a Warren Truss from my store. I think the Warren is a very solid choice when designing a model bridge. If you do not know how to start designing your own bridge, I would recommend the Warren, or the Pratt or Howe trusses.

The Warren truss is easy to use with Lap Joints, which are very strong joints. Find out more on my Bridge Joints page. All you have to do is lay down your top and bottom chords, and glue on the truss members directly on top of the top and bottom chords. The example bridge that I build in my 5 Steps to Building a Model Bridge ebook is a Warren Truss design.

Additional Resources

Pictures of real Warren Truss Bridges
In depth history of the Warren Truss

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50 thoughts on “Warren Truss”

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  1. we are trying to find something that will help with our science fair project. Our question is what size truss on a thirty centimeter bridge suits the bridge the most. if anyone could help we would appreciate it

    By cole -- December 9, 2017
    • hello cole, im glad to know that you are working so hard on your science fair project. To answer your question cole A thirty centimeter bridge should have a number that is easily divisible by thirty, in order to have a even amount of trusses that will cover the whole bridge. Thirty is a good number to pick, because all the numbers divisible by it are good picks for an experiment.

      By spencer -- December 9, 2017
  2. what do all those numbers represent in the diagram and why are they of that value/ how do they add up??

    By george -- November 13, 2017
  3. How much weight can a warren truss bridge hold if it is made out of straws?

    By Caydence Draper -- November 3, 2017
  4. will the amount of trusses on a warrent bridge that is the smae length affect how much it can hold?

    By cole -- October 1, 2017
  5. How much weight can a warren truss bridge made out of popsicles hold?

    By miah -- September 5, 2017
    • A lot! A Warren truss is a great design and works well in model bridge made out of popsicle sticks and other types of materials.

      By Garrett Boon -- September 7, 2017
      • It works well made out of balsa wood, too. Over 20 lb!

        By RandomGuy -- November 22, 2017
  6. The Warren Truss is amazing!

    By Mariah -- May 22, 2017
  7. does the warrent bridge have a solid road bed

    By kathryn -- May 12, 2017
  8. Could you tell me why do you always apply forces at the top joints? In real life, as I understand, only bottom joints are loaded.

    By Peter -- February 12, 2017
    • Peter, this has to do with the ease of loading from the top verses loading from the deck of a model bridge. The theoretical load distribution doesn’t change based on top or bottom loading.

      By Garrett Boon -- February 13, 2017
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