Truss Design

By on August 16, 2005 -- Modified on December 8, 2017

What is a Truss?

A “truss” is what you see when you look at a truss bridge from one of its sides. A truss is typically made up of a lot of triangles, but some uncommon truss designs don’t have any. The purpose of a truss is to help a bridge support a load (car, train, person) from any point along the span of the bridge. Without a truss, you simply have a beam bridge.

Brief History

While trusses have been used for both roofs and bridges for many centuries, there was an explosion of truss advancement in the 19th century in America. The need for bridges to span longer distances in this era, as well as to hold increasing heavy loads brought about many creative solutions, and many truss designs.

Three names stand out as true pioneers in these early truss bridges: Timothy Palmer (1751-1821), Louis Wernwag (1770-1843), and Theodore Burr (1771-1822). These men, along with other bridge builders who followed them, designed and built many bridges, especially in New England. Theodore Burr came up with a design that was used in many iconic covered bridges, and some are still standing today. These men came up with practical solutions for bridge building, and did not know or have access to the theory behind their designs.

These early truss bridges were made primarily from wood. As iron became more available, more truss designs were developed to make better use of this material. The Howe and Pratt trusses in particular were designed to incorporate iron rods in the truss.

This article will help you learn about trusses used in real bridges, and see how to apply them to model bridges. Learn the history of each common truss design. This page is designed to help you make an educated decision about what truss design you should use on your bridge.

Common trusses used in engineering:

Warren Truss

Warren Truss

Warren Truss

The Warren truss is one of the most simple yet strong designs.  This simple design already existed, but what made the Warren unique is that it uses equilateral triangles. Each side of the triangles are the same length. This marked an improvement over the older Neville truss which did not use equilateral triangles.

Go to a more in depth analysis of the Warren Truss.

Pratt And Howe Truss

The Pratt and Howe trusses are very similar. In fact, the only difference is the direction the slanted members are angled. This changes which members are in compression and tension. On the Pratt truss, the shorter, vertical members are in compression. However, on the Howe truss, the longer, angled members are in compression. Because most materials (especially wood) that model bridge builders use decrease in the ability to resist compression the longer they are, I think the Pratt truss has an advantage.

There are more factors to consider, however. The Pratt and Howe trusses also differ in how they spread the load to the top and bottom chords. The Pratt truss has larger forces on the top and bottom chords than the Howe. Thus. you’d have to use bigger top and bottom chords.

Go to a more in depth analysis of the Pratt Truss.

Go to a more in depth analysis of the Howe Truss.

K Truss

K Truss

K Truss

The K truss looks very good on paper. It shortens the lengths of the compression members compared to the other trusses. However, one must wonder if it adds additional weight simply because of the number of members. It is really interesting to note the two green members on the K truss, in theory those pieces could be taken off. However, I had to include them to make the truss design program work. This shows only one orientation of the K truss. If I reversed the direction of the K’s, I wonder how much it would change the forces.

Go to a more in depth analysis of the K Truss.

The one thing I don’t like about this truss is the long vertical compression member in the middle of the bridge. If that one member could be shortened or even eliminated, I think the bridge would become more efficient.The K truss would be the hardest of these trusses to build. This is something worth considering. Making a strong joint that would make the most of the switch between compression and tension of the vertical members would be difficult.

If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from amazon. It can be purchased online though amazon. This is a comprehensive study on the engineering principles behind the design of bridges. It is easy to understand and to follow, and is a great fit for students who are just learning, but advanced enough to be a great resource to those with more experience. For more great resources, see this list of other great bridge books.

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161 thoughts on “Truss Design”

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  1. It sounds like you are in Science Olympiad. If so, then you don’t want the bridge to be any wider than it has to. The bridge should be just wide enough to accommodate the loading block.

    By Garrett Boon -- October 9, 2008
  2. How do you determine the optimal width of a model bridge? We have to make an elevated wooden bridge that has a span of 45 cm, a tower height of 7 cm and a maximum height of 15 cm (including the tower). Pls advise. Thanks.

    By imee -- October 8, 2008
  3. Garret Boon, yes I am talking about lateral bracing. I didnt know what is was called

    By Manuel -- September 29, 2008
  4. Manuel, are you talking about adding a horizontal line in the middle of the Pratt truss? Don’t do it. Try plugging that design into the Bridge Designer program and see what happens. It is basically adding useless weight.

    I am not sure what you mean with the X’s at the beginning and end. Are you talking about lateral bracing?

    By Garrett Boon -- September 28, 2008
  5. thank you so much i think i’m going to ace this assignment

    By Manuel -- September 28, 2008
  6. i’m using a pratt truss what will happen if i add an extra line going straight through the middle and add X’s on top and at the beginning and end.

    By Manuel -- September 28, 2008
  7. It does not matter very much if your triangles are not equilateral. The triangle is a very strong shape period.

    By Garrett Boon -- September 27, 2008
  8. thanks so much man your a genius

    By First Timer 2 -- September 27, 2008
  9. Im building a bridge out of popsicle sticks and im using the truss design. Im not sure which type of truss is stronger because i will be putting bricks on it to test. I was going to do the Warren truss but the way i built it so far makes the triangle on it not equilateral. I was then thinking about a Howe truss…..What is your opinion?

    By First Timer 2 -- September 27, 2008
  10. I think I understand now. Yes, a K truss is a solid way to build a bridge, just really work hard on getting your joints perfect.

    By Garrett Boon -- September 18, 2008
  11. of course by on one plain it means we can not place pieces over the joints to help stability it has to be flat basically but you said a k truss would be good for an arch bridge?

    By First Timer -- September 16, 2008
  12. First Timer, a K truss is a good choice. As long as your construction is done well, it should be a good bridge. However, I am not sure what you mean by your bridge has to be on one plain. Could you explain that for me?

    By Garrett Boon -- September 16, 2008
  13. Arianna you have to build a bridge too 16 grams or less? weird i have to make on for school

    By First Timer -- September 15, 2008
  14. My first question is how can a beginner understand everything about bridges enough so to make a bridge under 16 grams and withstand a lot of weight????

    By Arianna -- September 14, 2008
  15. Hey it’s me againthank you very much for the advice i am going to try the welbonde again but it turns out the bridge i am building has to have an opening so “boats” can pass not cars and i was wondering if i put a k-truss on arches would the bridge be strong or is their a different type of method to building those types of bridges
    thank you again this website is easy to use for all ages

    By First Timer -- September 11, 2008
  16. Arianna, what questions do you still have that I could answer?

    By Garrett Boon -- September 9, 2008
  17. this website is very usefull even though it did not answer all of my questions on model bridges. With that said this site is still the best one that I know of that has good information on how to make model bridges and the materials and tips to make it a good one!!!

    By Arianna -- September 8, 2008
  18. The other point with the green members is the location of the load vectors. They are shown in the ‘dead load’ mode; when the ‘live load’ configuration is considered, there are no green members.

    By Jeffrey Pigden -- August 15, 2008
  19. You asked a really good question about the “green” supports. The program shows that they have no load, so why even include them on a bridge?

    The reason is that the truss design program is dealing with theory only. However, in a real model bridge scenario, the bridge is going to twist and deform under load. Once the bridge starts to deform, then the forces go wacko. The green members might suddenly go red or blue. By leaving the green members in the design, you are putting a small safety factor into the bridge.

    It would be an interesting experiment, however, to see if you can achieve a greater efficiency by leaving the green members out. The bridge might not hold as much, but because it will be lighter it could potentially have a greater efficiency.

    By Garrett Boon -- August 8, 2008
  20. are you 11? what do you think would happen if you reversed the k’s? your bridge would only be as strong as your adhesive since all the weight would pull on the supports rather than sit on them… you’d increase the stress on joints considerably!
    and what would be the purpose of removing the verticle piece? and those green verticles… sort of necessary to hold up the supports. what’s the point in even having that first support? physics… seriously.

    By not impressed -- August 8, 2008
  21. This is a great site. Thank you for taking the time to put it together.

    By Celeste -- July 14, 2008
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