Truss Series: Truss Design Overview

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.

Overview of Terms

Let’s define a couple terms to help you understand how to study truss design.

Shown here in red is the Truss Frame. The frame is the outermost parts of the truss.

The frame is made up of several parts: Top chord, bottom chord, and two end posts. This diagram shows the frame in an expanded view so you can easily see each part. Practically, you might use different sizes or shapes of wood for each of these parts due to the force being put on each part is different.

Now we will add the truss members, which are shown in black in this diagram. The truss members are simply an arrangement of triangles (most of the time) that transfer the force/s put on the bridge to the ground. The way these triangles are arranged or shaped is the essence of truss design. You will see examples of the most common designs further on this page.

These terms will be helpful to keep in mind as we talk more about truss design. Now let’s take a quick look at the history of truss design, particularly in the United States.

Brief History of Truss Design

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 increasingly heavy loads, brought about many creative solutions in the form of new 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.

Interestingly, building bridges in the 18th and early 19th century was more about quality of construction. Skilled carpenters were needed, and most of the engineering was practical and not theoretical. Wood was the primary material available in these early years, but iron and then steel came along and changed everything.

With iron and steel, and the expansion of railroads that carried heavier and heavier loads, new bridge designs were needed. The Howe and Pratt trusses in particular were designed to incorporate iron rods in the truss. These two designs, which you can see from the original patent images, do not look exactly like the truss designs that we associate with those names today. This is a bit of a mystery to me, but you can see semblances of the original designs in the modern depictions. Both the Pratt and Howe patents were very much concerned about methodology of construction more so than the actual design.

Bridge history is fascinating, and there is so much more to learn. This short section is meant to whet your appetite, but now we turn to the application of truss design to model bridge building.

Common trusses used in model bridge building

Each of the following truss designs are very common in both real and model bridges because of their sound engineering and ease of construction. As I mentioned earlier, the key for us model builders is how these designs transfer forces throughout the bridge and eventually to the bridge supports. Each of these designs does that in a different way.

Take some time to read up on each of these designs before deciding on one to use for your bridge. Perhaps you will end up not using any of these designs but creating something on your own based on the principles of force transfer.

Warren Truss

Learn about the Warren Truss.

Pratt Truss

Learn about the Pratt Truss.

Howe Truss

Learn about the Howe Truss.

K Truss

Learn about the K Truss.

I have chosen to highlight these four examples of different trusses to get you started with some very solid examples that you can easily use on your bridge. There are other, more complex, designs that aren’t shown here. You can do a web search for truss design and see many more examples. I’m a fan of keeping things simple, but it is possible that your unique bridge project would benefit from one of the more exotic designs.

If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from 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.

189 thoughts on “Truss Series: Truss Design Overview”

  1. hello i am doing a bridge building contest in illinois at my school, Fenwick High School. The bridge must be able to hold 50 kg if i am to have any chance at winning this contest. What design would you recommend?

    • Luke, this is the type of question that I just cannot answer. You have to consider what materials you can use, the span of the bridge, and how dedicated you are to winning and learning what it will take to win. The Pratt, Howe, and Warren trusses are great designs. They have been used for real bridges for over 150 years. Choose any of them. But it is not so much what design you pick, but rather how you build it. The care you take in selecting your materials, and the effort you put into the construction are key aspects to the performance of your bridge, just as much so as the actual design.

  2. I think that the strongest one is the Howe. Mine held 150 pounds when it was mad of very thin wood. anyone need help? ask me… Skipper

  3. i recently made a k-truss brigde in tech-ed class that held over 350 pounds i used hot glue and wood glue and about 100 or so popsicle sticks it was pretty cool because once it broke i didnt just falter ubder the wait it exploded!

    • Hey Jake,
      How did you make the K for the K truss using the wood? HOw did you make the < stay on the | ? Get what I mean? Please reply quick!

  4. I think the pratt truss is better.
    someone told me that the compressional members are more efficient if in an I shape or in a T shape. . .

  5. Hello!!! im at school trying to win the bridge building competition !!! you guys really helped me!!!!
    thx thx thx!!!
    your Falaviena

  6. Reference the zero force members. In real life, the loads on bridges are applied at the panel points, so those members would carry a load, even if just a panel load.

    You would make member identification much easier if you were to use Bowe’s Notation. With the external forces shown in place, number all the spaces between the forces. From left to right, label the interior spaces with alpha characters, starting with A. Each force can then be identified by thespace number before and after; each member can be identified either by a number-alpha or an alpha-alpha designation.

      • The panel points are the points where the load is applied, usually the joints on the top and bottom chords. For a bridge, beams, carrying the roadway or railroad, would span between two trusses and apply their loads to those points. For a roof truss, the purlins land at the top chord joints.

        I found a site for you that shows Bow’s Notation, slightly different than I described because he needed to number the joints for the graphical solution that he describes. The graphical analysis is very accurate, before computers and calculators came on the scene, I used to check my calculated results that way.

  7. Hi I have to make a bridge out of straws and masking tape that spans one meter in between two desks plus it has to hold two 5cm cubes full of sand which i calculated and can be no more than 20grams. The most efficient bridge able to hold this weight wins (least amount of straws). Any suggestions on what type of design i should use?

    • im doing a similar project, but with hot glue and popsicle sticks and im having trouble deciding a truss type. it need to support 50 lbs of weight,be at least 18″ long, 4″ wide, and a max of 12″ high. any ideas

      • I just built a Warren Vertical Support Truss Bridge out of toothpicks and regular white Elmers glue tha supported 121.7 pounds of sand. This design is typically made for railroad bridges because of the extreme amount of weight a train weighs fully loaded. Good luck.

    • one thing you should do to make the joints stronger is cut vertically up the straw and stick it in the end of the other straw and just use a little take to make sure it stays.

  8. trying to redo the bridge
    , /– /— /– /


    THX, hope it came out

  9. thanks for the tips. i have to make a balsa wood bridge that will support 15 kilos in the center and the truss designs really helped.

  10. The school I got to St.Peters grade 6s had to build a bridge for Design+Technology and this website has really helped alot thank you!!!

  11. I love this site…it gave me a lot of help for our project. Thanks soo much. I just hope that the bridge that my group makes wins. Um, our bridge is meant to be 40 cm long and we have 150 paddle popsticks to complete it. Anyway, thanks a lot again.

  12. hi, im in yr 12 and im currently doing an assignment on bridges

    ive built the warren truss bridge with vertical members,
    ive had a look at your diagram with the load factors, but im quite confused, you say that the load factors add up to 100% but i dont see how the numbers come together to make 100%
    i can only see that when 85% + 15% = 100% at the bottom line, but with the 60% lines i cant figure out how they would add up to give 100% please explain to me =]

    • Actually, he meant to say that the two black arrows above the bridge represent a single weight, distributed evenly between those two arrows (thus, 2 points at 50% each = 100%), the numbers on the bridge lines are the force amounts(not percentage) most likely, in Newtons. Hope that helps explain things better. There are many free online bridge building games for you to try your own designs and see how they work. I tried a K-truss design in one program from West Point, but the forces were completely reversed. It might have been the program’s physics, but I’m not sure. I had lots of compression at the top and tension at the bottom.

  13. . It is the simplest design of the Truss Bridge, with two sides, a bottom, and the railing in simple triangle forms. The bottom of the side of the bridge is 11 inches in length and the top of the side is 10 inches in length. The little rails on the inside of the left and right side of the bridge are 2 inches in length. The bottom of the bridge…… Some of the things I learned in making this bridge were the different jobs involved in the process of actually building a real bridge. You need to have an engineer, an architect, a scientist, and a mathematic. You would need an engineer to help design the bridge. You would need an architect to oversee the construction of the bridge. Also, the architect would prepare information on the structure’s design and specifications, materials and equipment, estimated costs, safety, and construction time. They also make scale drawings and make sure that the bridge meets the building codes and laws. Also, to build a real life bridge, you would need a mathematic. They would make sure that all the little details in the bridge were exact and specific so the bridge was safe, and also look right.

  14. Has anyone else noticed that the K Truss looks like it has arrows in it as well as Ks? Its like an optical illusion! Great Website, and thanks for the tips!

  15. For a science fair i have recently done, i had the warren, pratt and howe trusses all trusses.
    the efficiencies in weight held over bridge weight were:
    warren – 385.6 reoccurring
    pratt – 425
    howe – 340
    These are not averages of several bridges, and they were only 1 foot long (cause thats what the stores had in stock.)
    In this test, they were weighed by hooking a rope to the bottom beam, putting a bucket on the rope, and adding coins to the bucket (and a ten-pound weight).
    I hope this answers some questions on which is best (the pratt)
    Of course i had 30 pounds of coins.

  16. i have to biuld a bridge for science fair do you think that popcile sticks would be the way to go , but i also need the bridge to break at some time with in 5-10 minutes. anyone can answer to this i just need some kind of answer. 🙂
    i would also like it if you just posted here on the website!!
    please someone respond soon!:D
    thank you
    p.s. i think that this website could use some more information on the different kinds of bridges and not just on your experiments you should have a viriety of appinions so that people will see two or more sides of the story!! OKAY? DO NOT take this as a insult i just think that it would be better that way , i thnk it would guve you more viewers than what you have now !! just my apinoin but i bet other people agree with me on this matter!! *****
    sorry if that sounded wierd :D/:( !!!!

    • Rose, part of the reason I have the option to leave comments is so people can share their opinions about my bridges or projects they have worked on. I also have an option for people to upload photos of their own bridges, so this site is not just my own work.

      Any bridge can break within 5-10 minutes. It depends on how strong it is and how fast you load the bridge.

    • Yeah I’m doing the same thing but we have to use toothpicks which is so much hard so yeah if any of you have advice I would like to hear it but it will be hard!

  17. i made a truss bridge that was 40 cm long. i used the warren truss design. i added vertical suports also though. it was made from popsicle sticks and wood glue. the key is distrubuting the weight evenly. my bridge held over 200 lbs. depending on the materials and how much you have to work with, you should be able to do the same.

    • Also a determining factor would be exaxtly how the weight is going to be applied to your bridge. i also forgot to mention that my bridge was 10cm wide.

    • 200 pds wow if u want us to believe u say something reasonable 40X10 cant hold 200 pds it would have to be 50 cm high considering u used normal wood glue.

      • Sorry, but yeah it could certainly hold 200 lbs. He most likely beefed it up with a lot of popsicle sticks. Wood glue is good enough if used properly. I saw a vid on youtube with a simple king post that held so much, they were running out of things to use as weight. It was REALLY strong.

    • hey josh im tiana and im in yr 7 and we are making bridges and how do you know if it is the write bridge and there is other choices what do you do ??????

      • When you stated that for a Howe Truss you would have to use larger compression members, were you referring to the diagonal members or vertical members? Thank you for your clarification.


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