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. That’s interesting that the need for bridges in the 19th century of America brought about an explosion of truss advancement. I am looking at getting some building trusses for my home as part of my renovation project. With getting the trusses, I think I’ll work with an expert to get a good design that matches my home.

    By Larry Weaver -- December 8, 2017
  2. This really helped me and will be good for my project.

    By BK -- November 28, 2017
  3. I’m a sophomore mechanical engineering student at Boston University. In my mechanics course we’re analyzing bridge trusses made of straws. Great info on the merits of the different designs.

    By Ben Mason -- November 17, 2017
  4. I’m in 8th grade and we have to make a bridge with spaghetti so this helps a lot. Lol

    By Kyle -- November 14, 2017
    • Whaaaaaat?! At least we get actual wood to work with! I feel so bad for you

      By Ashtyn Lucas -- November 20, 2017
  5. good website, helped me and my friend justin in my POE class in school.

    By Mitchell Drzadinski -- October 16, 2017
  6. I’m in 4th grade and there is this thing where my table group has to do a bridge making contest where we use 100 popsicle sticks to make a bridge. Also, I really like the desines for the bridges. It’s amazing.

    By Mariah -- May 22, 2017
    • I think it’s funny because I am a freshman in high school and we have to do this for a honors geometry class. I wish that our school district was more interactive with engineering designs

      By Alex Vannaken -- December 5, 2017
      • Even funnier; I’m a sophomore in college for mechanical engineering and we just made popsicle stick trusses in our statics class to see who’s design could hold the most weight. Pro tip: I’m a personal fan of the Warren truss (as long as your weight isn’t at a specific point, but is distributed across the whole bridge).

        By Emilie -- December 5, 2017
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