Truss Series: Warren Truss

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 “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

74 thoughts on “Truss Series: Warren Truss”

  1. I am doing a science fair project for a bridge project with my friend and we are wondering if a howe, warren, or pratt truss would work best covering a 24 in. span.

  2. I am using this site to help write a report on truss bridges and you have awesome information here. Is there any way I can find the resources that you had used through the making of this site? Thank you.

    • It will not be the railway bridge over the river Severn in the centre of Worcester which has a mid span support but it is an illustration of a classic Warren girder.

  3. Hi there,
    I don’t understand what these numbers mean. Do they mean the newtons of force? And also why is there the number 139 on the bottom closest to centre?

    • Joe,

      The numbers in the truss analysis pictures represent a percentage of the total force. They are not associated with any specific unit, like Newtons. Instead, because they are showing a percentage, you can substitute any unit in their place as long as you are consistent.

      So for instance, let’s say there is a 100 Newton force pushing down in the very center of the bridge. The bottom red member that says “139” now means that there is 139 Newtons of force pulling on that member (red in this case shows tension). How can this be if the total force applied at the top is only 100 Newtons? This is because the design of the truss sometimes multiples the original force in certain members.

      • Hi there,
        Because of you helping with this, the team I am in have won the bridge building competion.
        Our bridge, decided with the help of this website, got a paddle pop stick and hot glue bridge that held up 0.112kg per gram of bridge, and held up 20kg overall!
        We flattened the competition, who’s best was 0.29kg behind.
        So basically thank you.

  4. Hey I need some help on a project and I hope someone can help me out so my question is how much does an average warren truss bridge cost?

  5. I think this information wasn’t that helpful. What sites can I use for background information about Warren Truss Bridge Designs?

  6. Hello there, I am doing some research for a TV programme about the history of the Warren Truss bridge.

    I am looking for a standout example of a Warren Truss bridge that is still in existence. Do you know the oldest/first/longest/biggest Warren Truss bridge in the world? Or another notable one. The only stipulation is, it must be a Warren Truss to be exact.

    Any advice greatly appreciated… thank you!

  7. 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

    • 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.

    • 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.


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