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. 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?

    By luke -- December 26, 2010
    • 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.

      By Garrett Boon -- December 27, 2010
  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

    By Skipper -- April 26, 2010
    • hey can u help me with a project? I think it has to be a specific size but it has to hold more than 150 pounds. help me. please.

      By Rico -- April 26, 2010
      • I reccomend the howe if u are using stronger wood than mine. mine was thiner and smaller than popcicle stix.

        By Skipper -- April 26, 2010
  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!

    By jake -- February 23, 2010
    • 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!

      By Daz -- July 18, 2010
  4. hey, love ur work! love ur song! which is the strongest bridge design? i’m a big fan of ur designs,

    By roy aubertin -- February 14, 2010
  5. 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. . .

    By aishikimiru -- February 11, 2010
  6. Hello!!! im at school trying to win the bridge building competition !!! you guys really helped me!!!!
    thx thx thx!!!
    your Falaviena

    By Falaviena -- January 29, 2010
    • I won a competition at my school too! i wouold like to thank My mom, Garrett Boon, and Michael Jackson for support!

      By brian -- April 29, 2010
  7. 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.

    By paddingtongreen -- January 21, 2010
    • Thank you for your comment. Could you explain more about panel load?

      I will look up Bowe’s notation. Do you know offhand where I might find an example (a picture) of it?

      Thanks.

      By Garrett Boon -- January 22, 2010
      • 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.

        http://wjesus.org/Frame_3.htm

        By paddingtongreen -- January 26, 2010
  8. im making a warren for my elective with balsa wood.
    what would be better a subdivided warren type or a warren with vertical supports?

    By thomas betha -- January 20, 2010
    • I do not know what you mean by a subdivided Warren truss. What does this look like?

      By Garrett Boon -- January 20, 2010
    • The vertical supports on the Warren truss allow the bridge to sustain more weight. =) <3

      By Wandahhhh -- January 20, 2010
    • would that be like the pratt truss?

      By Muutari -- March 11, 2010
  9. 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?

    By Matthew -- January 19, 2010
    • I have the same problem. Can SOMEONE please help us?

      By Lizzy Bell -- January 20, 2010
      • Where must the cubes be located in the span?

        By paddingtongreen -- January 26, 2010
        • In the x and y axis located in the centerfold of the main bridge compartment, DUH!

          By jpa -- March 22, 2010
    • 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

      By Muutari -- March 11, 2010
      • 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.

        By Wayne -- May 13, 2010
    • Try using the london crossover?

      By Jerrifus -- March 22, 2010
    • 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.

      By Micaiah -- August 29, 2010
  10. trying to redo the bridge
    –____________
    , /– /— /– /
    /__/__./__/__

    PRESSURE AT THE DOT

    THX, hope it came out

    By nicholas -- July 6, 2009
    • i’m so silly, this is a U.S. site, all the same, i am in AU, so if you can, is it possible to rely to me in metric units? i don’t really know how to convert it. But if you can’t either, I will use a google converter

      please reply 2 me, i need help!!!

      By nicholas -- July 6, 2009
    • Nicholas, the force distribution is the same if the load is on the top or on the bottom of the bridge.

      By Garrett Boon -- July 6, 2009
      • even though i agree with you, it is the problem of compression VS tension that i am up against.

        By nicholas -- July 6, 2009
      • nicolas is correct.

        By Dave Schwartz -- December 14, 2009
      • so, what bridge would you recommend that will make me win the bridge building contest of 2015?

        By sorrow -- January 19, 2015
  11. 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.

    By Bridget -- May 26, 2009
  12. 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!!!

    By Debby Ade. -- May 26, 2009
  13. 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.

    By E.M.T.A -- May 18, 2009
  14. this is a good webstie for teens and it gives the motives for kids to learn how to build bridges.

    By MRs. Goodfellow -- April 13, 2009
    • is this Mrs. Goodfellow from CMS???

      By Danielle N. -- April 13, 2009
      • Why yes, I am from CMS.

        By MRs. Goodfellow -- November 9, 2009
  15. 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 =]

    By Clarence -- March 9, 2009
    • 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.

      By Wraith0127 -- June 16, 2009
  16. . 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.

    By vinny oddo -- February 10, 2009
  17. 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!

    By Flying Monkey -- February 9, 2009
    • oh yeah, i just noticed that. That’s really cool.

      By Hannah -- February 26, 2015
  18. p.s. thanks for the info, this is very helpful

    By Liam -- February 2, 2009
    • Hey Liam are you in the Snowshoe Ski Team????
      If you read this I will be soooo happy please reply!!!???? 🙂

      By Estelle -- February 24, 2015
  19. 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.

    By Liam -- February 2, 2009
    • Thanks, very much for that insight Liam. It was very helpful and I am very appreciative.

      By Liam's Pal -- December 9, 2014
  20. 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/:( !!!!

    By rose -- January 30, 2009
    • 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.

      By Garrett Boon -- January 30, 2009
    • 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!

      By Kate -- April 3, 2013
  21. 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.

    By josh -- January 27, 2009
    • 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.

      By josh -- January 27, 2009
    • 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.

      By im a judge -- August 10, 2009
      • 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.

        By gjsmo -- February 28, 2010
    • 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 ??????

      By tiana -- October 6, 2009
      • 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.

        By Saria -- November 21, 2009
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