Truss Design

By on August 16, 2005 -- Modified on December 14, 2017

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.

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167 thoughts on “Truss Design”

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  1. I want to build one project under civil department. its a competition..
    I have to build a truss structure using Popsicle. But it will not be tested with vertical load but with horizontal force. Actually judges will attach a string on the top of my Popsicle truss structure which will go over a pulley and will carry a load at the end of the string..
    Could anyone plz suggest what structure will be the Best for this very purpose! I need help as soon as possible.. Thnx.

    By Akshay -- January 25, 2009
    • I personally made a warren truss bridge out of popsicle sticks and wood glue. It held over 200 lbs. i cant say that this is the best design to use though. You just need to find which design distributes the wieght most evenly. But i didnt exactly understand how you said the weight would be applied.

      By josh -- January 27, 2009
  2. so which one would hold the most weight??

    By rob -- January 21, 2009
    • that question can really not be answered. there are alot of determinig factors. 1)how the weight is applied. 2)what materials you use. 3)how much materials you have to work with. etc.

      By josh -- January 27, 2009
  3. I have to build a bridge out of spaghetti and glue for a school project. I was wondering which bridge design you would recommend so that my bridge could hold as much weight and possible (the weight is being suspended from the bridge in a bucket and is attached to a piece of wood that will sit on my bridge)

    By Evan -- January 14, 2009
    • Well, i think what you need to do is see which truss design distrubutes the weight most evenly. The more distribution of weight you have the better your bridge would hold up. From what i see it looks like the k truss bridge distributes the weight most evenly. I made a bridge out of popsicle sticks that held over 200 lbs. I used the warren truss bridge but the k truss seems to distribute the wieght more evenly.

      By josh -- January 27, 2009
  4. “The following figures are shown under a load. The numbers represent percentage, where the total load = 100. The numbers are rounded to the nearest 5. For absolute numbers, visit the Bridge Designer.”

    I do not understand what this means. Could someone please explain?

    Also, what is the difference between compression and tension. I understand that tension is like stretching a spring and compression is pushing the two ends together but I dont understand how this relates to bridges. For example: If a truck is driving on a bridge, is it applying tension or compression on the bridge?

    Thank You,

    Petey

    By Petey -- January 11, 2009
    • The total load on the bridge = 100. The loading point is indicated by an arrow. This could mean 100 pounds, or 100% of some unknown load. The figures show how the load is spread throughout the truss with a number. Look at the first color figure, the warren truss. The top piece is in three sections, and each has a number. This number shows the load that is on that section, which is 50 on the outside and 85 in the middle.

      The different colors show the difference between tension and compression. Imagine there is a truck in the middle of each bridge on this page. Any piece that is red is in tension. Any piece that is blue is in compression.

      By Garrett Boon -- January 11, 2009
      • I understand the point about compression and tension, but I am still not understanding the load.

        If there is 50 on the outsides and 85 on the insides wouldn’t that equal over 100?

        I am still confused about that.

        By Petey -- January 12, 2009
        • Basically, the numbers on the top would represent how much of the load is distributed to this point on the bridge. The 85 would, if im correct, mean that this is the point on the bridge that is under the most stress. 50 would mean that this area is under less stress than the middle area of the bridge. Hope i made sense and that this helped you.

          By josh -- January 27, 2009
  5. Your website is really really helpful. I don’t know what I would’ve done without it. I had a question about the trusses. Which truss would be best for a bridge that is elevated by two pole things at each end? I need to have a good efficieny score, so I need to use a truss that won’t weigh much, but hold a decent amount of weight. Once again, thank you so much for making this site.

    By Double A -- December 30, 2008
    • Double A,

      I suggest using the warren truss as it is the most simple to build and can hold a descent amount of weight. Also using vertical members in the warren truss is another good option. So far that is what I have been using. I assume you are doing the elevated bridge event?

      By Cyrus Duong -- January 3, 2009
    • Double A I have made many bridges and one of my most recent is a cross of a howe and pratt. It is a it harder but has a much higher efficiency

      By first timer -- January 25, 2009
  6. Hey, just want to know if you sell plans just for Pratt,Howe,and Warren truss bridges. I really need those for a project.

    Thanks a LOT :()>

    By jacob -- December 21, 2008
    • Jacob, what materials are you building your bridges out of?

      By Garrett Boon -- December 22, 2008
  7. i analized the first two warren a bit, and something is not good about the first one.
    (I checked them with your bridge calculator and manually too.)

    By Gergő Áron -- December 2, 2008
  8. It seems to me there is a calculation mistake in the howe truss.
    If there is no other load how could be the 10 the value of the tension in the first member of the bottom chord, when the top chords first member is compressed by 15?

    By gergőáron -- December 2, 2008
  9. I tried to build using the Bridge Designer, the first example of calculation used in the TRUSS DESIGN page (” On this Warren truss, each of the down arrows represents 50% of the load….). It does not calculate the load distribution:” No solution. Matrix is singular.” If I want to add memebers or nodes the formula relation” member+3=twice the Nodes” is not satisfied. What should I do to get your result?

    By Manuela -- November 29, 2008
    • I can’t really know what went wrong without seeing what you did. Just make sure that you don’t accidentally add an extra node somewhere.

      By Garrett Boon -- November 29, 2008
  10. there is mistake in calculating the forces in members of first warren truss,check it

    By aayush -- November 27, 2008
    • I have double checked the first example and it seems fine to me. You have to remember that changing the dimensions of a bridge will affect its load distribution.

      By Garrett Boon -- November 29, 2008
  11. David if i were you i would check the popsicle bridge area if the website it has kits to help you make popsicle stick bridges.

    Also Peter i agree these fellas should do some reasearch for themselves but the thing is this is the best website to do it at there a myriads of links and resources about almost every aspect of bridge building design etc. ‘

    Also we apologize for any question that we were not able to answer we will try our best to do so.

    By First Timer -- November 14, 2008
  12. All you fellas:

    Do some research yourself mate. Don’t just wait around for someone to reply to a post. I have got a similar project and have realised that you have to do some work! Answers dont just come without a price!

    [edited by admin due to comment policy]

    By Peter Skrzynecki -- November 13, 2008
  13. hey i am building a bridge out of popsickle sticks and i can only use 50 of them and it can only be 5 inches high and 4 inches wide
    could you tell me the best truss design to use?
    thanks man!!!!

    By David fallon -- November 7, 2008
  14. thank you soo much I have been looking at different disigns for a truss bridge im a 8th grade student and Im mix some of the ideas that I’ve seen. I’ve tried to combined the basic ideas of warren, pratt, and howe. I’ve tried pratt and howe. I will also look at the K-truss and see the strengths i think would work best of it because it is a truely unquie disgin that i am glad I’ve found on this web site.

    By jayson -- October 28, 2008
  15. these bridges are awsome they give me lots of ideas of how to build a bridge…… thanj you for your help

    By angel Miren -- October 28, 2008
  16. Daniel, in theory yes. However, they are issues that apply to a roof that don’t apply to model bridges. This website was not meant to provide engineering information about real life scenarios.

    By Garrett Boon -- October 21, 2008
  17. In theory can these designs and calulations be used to design and calculate for roofs? I am studying truss roofs and found your web site. Thank you Dan

    By Daniel Abrams -- October 19, 2008
  18. Thanks GB!!!

    By first timer -- October 18, 2008
  19. i have an assignment and it is graded upon how close i can predict the breaking load for the bridge. I cant do 2d trusses using method of joints and sections.The tensile and compressive forces change on adding the lateral bracing. I was wondering if you could point me in the right direction to sole 3d truss systems

    By MANUEL (another one) -- October 15, 2008
  20. FT, model bridge geeks are hard to come by :P. I am not sure of anyone offhand who you could also interview. The only thing that comes to mind is a forum where you might could find somebody. http://www.scioly.org/phpBB3 That forum has a lot of model builders.

    John, good question. Nothing changes about the truss in an arch form. Whatever piece was in tension before will still be in tension on the arch bridge.

    By Garrett Boon -- October 15, 2008
  21. Also Garret do you know any one else that i could contact for another interview we are required to have 2 and yours was great and because your so reliable and intelligent anyone that you suggest has to aswell

    By First Timer -- October 14, 2008
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