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. how do u calculate the ratio of the weight a bridge can hold?

    By Crystal -- March 14, 2017
    • The easiest way is to load it till it breaks, and divide that weight by 5 or so. Don’t trust it with lives, but you can bs an estimated max weight. To do it right, you need to perform a static analysis of the truss members (look up the joint method on google) to determine the load on each member. Then you need to find the shear and bending moments of each of the members (or so I think, I’m in mechanical engineering, there might be an easier way, probably not, still google that). Find all these values as equations with a weight variable in them (i.e. force on the member ab = 2.1*W). Then you can determine when your welds (whatever holds your members together) will break by googling the welds rated forces (compression, shear, tension) and setting the highest weight ratio equal to the max values then solve for the weight (your max weight before the first weld will likely pop). After, you set the largest bending moment in the bridge equal to the largest recommended bending moment of that material. Reference a material guide, steel beams are by convention rated to a max bending moment which can be found via google, wood can be found too for simple shapes, straws and hollow pipe must be researched on how to calculate it. This determines the highest weight at which the member most susceptible to bending is going to buckle. At last you can check the maximum tension and compression of the materials against your highest member tension and compression in the same way, using the reference probably found in your last google search. The lowest of those weights will shows you what weight the first part of your bridge to fail. Divide that value by 5 for a factor of safety and you’re done, that’s the max rating for your bridge.

      By Jonathan -- April 24, 2017
      • Hi I’m in year 7 and we are doing a truss bridge assignment and I can’t seem to figure out what a comparative ratio or the total load carried for the research assignment and you seem to know a lot about trusses

        By Justin Zeng -- April 28, 2017
  2. Hi, I’m in Year 12 and we need to design a lightweight spaghetti bridge and I’m currently deciding between Warren truss and Howe truss. My teacher in Engineering told us that we should consider pasta stronger in compression than in tension and from what I’ve “researched”, the Warren truss is the best but the Howe truss is good in compression. I’m not sure which one as I’m not sure whether warren is better in compression. Please help me decide.

    By Joseph -- November 19, 2016
  3. This is a very simple design but works very well and is super strong

    By Jayna Loney🐯 -- November 15, 2016
  4. Dear Mr. Boon,
    I am an eleventh grade student and I would like to take this time to thank you for your wonderfully helpful website. My physics class is currently beginning a balsa wood bridge project and your examples of different trusses have been particularly helpful, as well as your tips on types of glue and types of joints. I would also like to take this time to ask a question. For our bridge, we are being given strips of balsa wood that are about a centimeter cubed. Would doubling up the wood be helpful and more supportive to the overall structure, or would it simply make the bridge weigh more and subtract from the bridge’s efficiency? Thank you very much for your time and consideration.

    By CM -- January 28, 2013
  5. actually the pratt and how truss is the best

    By Alexis freshman -- February 14, 2012
  6. thanks for the help(: i got an A woop woop <3

    By Alexis freshman -- February 14, 2012
  7. Which triangle is the best? equilateral or isoceles? and out of these 4 which bridge is the best?

    By Angie hwawkins -- December 4, 2011
    • Equaliteral because you want it to be sturdy and strong and in a right angle to hold more efficant, and im an 8th grader.

      By Ciarra -- December 19, 2011
      • You can’t have an equilateral triangle that is right…

        By Jerome -- January 3, 2012
        • i agree with Jerome.

          By Link Lee Irving -- February 2, 2012
      • Well for it to be a right triangle it must be scalene so but an equilateral triangle would work the best.

        By Laura -- February 6, 2012
        • No, a right triangle can also be an isosceles triangle with two 45º angles and one 90º angle. Equilateral triangles are ideal because they evenly distribute the force among all angles. These work well for trusses, but for your tension and compression beams, it is best to combine them in a right triangle so that you can put more weight on either the tension of compression beams (depending on the tensile and compressive strength of your material).

          By Jared Sindt -- May 9, 2017
    • Equilateral is always the best. The pressure hits the same on all sides. Good luck!

      By Richard -- February 27, 2012
  8. They aren’t equilateral triangles that warren trusses use, They’re isosceles triangles. Equilateral triangles have the length for all three sides, which are not what were used for the example image. I’m not saying they can’t be equilateral, it’s just you make it seem that they can only be made up of equilateral triangles

    By Alex -- September 29, 2011
    • Alex, good point. I made a mistake with those example images for the Warren Truss. A true Warren is indeed made from equilateral triangles. I have updated the images in the article. Thanks.

      By Garrett Boon -- November 7, 2011
  9. the best way is to put a stick on a stick to make an X figure 

    By Pierre -- May 20, 2011
  10. Austin, I do not understand what you are asking.

    By Garrett Boon -- March 18, 2011
    • he means like an s and p in the frame of the bridge or so you cand see the shape of his intials

      By Tyne97 -- May 12, 2011
  11. please help me! it is extremely urgent! i have a blue print due soon and i really need to know how the floor of mt warren bridge should be to make it more supportive! the bridge has to hold at least 1 to 5 kg handing from below it in a cup hanging on ropes from the bridge. i really need fast help, thanks

    By allison -- March 12, 2011
    • Probably dont need to support the floor, all the wieght will be on the sides(because of the ropes).

      I made a bridge that was 496g and 70cm long. It was a combination of a howe and pratt truss(they made Xs like lXlXlXl) with a supporting truss below the stress point(where the ropes will be).
      it hold 200pounds, but we didnt have enough weights to continue, it didnt crack or have any damage on it(other than divets where the ropes rested on).

      You shouldnt have any problem with holding 1 to 5 kgs.

      By Ryan -- March 19, 2011
  12. Hello I’m in 6th grade and I am seeing which design of truss bridge is the strongest. I am doing all of the bridges above, but I was wondering which materials would be the best to use… Please don’t say metal or something expensive because im only in 6th grade. Thanks!

    By zoe -- March 4, 2011
  13. I know that this page is devoted to Trusses, but what are the advantages/disadvantages of using an Arch as opposed to a truss?

    The bridge must span 20″ and have a maximum hight of 10″, performance is based on weight ratio, however I would love to break the school record. Made of 1/8 x 1/8 x 36″ balsa stringers.

    thanks

    By James -- March 3, 2011
    • What truss design do you recommend if there are 5 equal loads placed across the top of the bridge, if the bridge span is 10 inches?

      I was thinking that the warren truss design might be best and that I should double up the thickness of the trusses inside the span. But this is my first time trying this so I don’t know.

      By TIm -- March 6, 2011
  14. i am building a warren truss with vertical supports, made of popsicle sticks, and wood glue. is this good enough to hold 20 lbs.??

    By mitch -- March 2, 2011
    • Most likely, depending on how well you construct it.

      By Garrett Boon -- March 3, 2011
    • Yes, definitely. I built one and it held 34 pounds

      By William Webb -- November 14, 2016
  15. If I’m building a warren truss (basswood), does adding verticals in the triangles make much of a difference in the weight it could hold, or is it just adding unneeded weight to the bridge?

    By Sarah -- January 17, 2011
    • Sarah, the verticals in a Warren truss are quite useful, and can add a lot of strength to your bridge. They break the top (and bottom) chord into smaller sections. This helps because the top chord is in compression. Wood will bend and eventually break (buckle) when under compression. The vertical members help the top chord resist buckling and thus increase the strength of the bridge. Great question.

      By Garrett Boon -- January 18, 2011
  16. Hey, Garrett. I’m building my first bridge (out of basswood). My design’s really simple; I’m doing a Warren truss with vertical sticks in between the sides of each triangle.

    My question is this: I’m gluing the sides of each triangle to the outside of the bridge. Would it be better to glue the vertical sticks to the inside of the bridge, or on the outside as well?

    Thanks.

    By N -- January 16, 2011
    • This is a great question. The answer, according to my knowledge, is slightly complicated. When you take a stick of wood and hold it vertically with one end on a table and the other end in the middle of the palm of your hand, you can push straight down on the stick. Because the ends of the stick are “free”, the stick bends in a perfect arc shape. However, if you were to push down while holding the ends of the stick, it would bend in a more complex shape. Perhaps an S shape or something weird. In the second case it would be harder for the stick to begin bending. Thus, by holding the ends of the stick you actually increase it’s strength.

      However, if you glue a piece in compression on the outside as you say (called a Lap joint), the strength of the joint is in the surface face of the wood. And End joint (see my Bridge Joints page for pictures) doesn’t have that issue. The ultimate joint for a member in compression is a Gusseted joint. It combines the value of End and Lap joints. However, it uses more wood and glue, and thus is heavier.

      By Garrett Boon -- January 18, 2011
  17. Hello! Our team is making a bridge out of basswood and when we test our bridge, weights will be placed on the bottom floor center. We were going for the Pratt or Howe design but we got confused as to which one is going to be able to support the centered weight the most…or does it not matter?

    Thank you!

    By P&E -- January 15, 2011
    • Doesn’t matter.

      By Garrett Boon -- January 18, 2011
  18. Hi, I am doing an extra credit project for my school and I’m am suppose to construct a bridge out of craft sticks and glue. I have been researching for different bridge structures I could try, but I would like to know your opinion in which structures I should try first. Also, this bridge must be able to hold a text book that is about 3kg, this has been a obstacle for my bridge. I would greatly appreciate your input.

    By May -- January 12, 2011
  19. which of these designs are best when the weight is loaed by hanging the weights from the botton of the sturcture??

    By maggie -- January 11, 2011
    • Maggie, it doesn’t matter.

      By Garrett Boon -- January 11, 2011
      • okay, thank you

        By maggie -- January 12, 2011
  20. Garrett, I have a project in which I have to build a truss bridge with pre-cut wood. They are the really thin but long ones made from basswood. The way the weight is applied is by placing a block inside the structure with a pipe from the bottom to pump more pressure. How would you suggest I build the “floor” of the bridge to support the weight?
    Thanks

    By Caleb -- January 5, 2011
    • Caleb, my first recommendation is that you make your bridge just wide enough for the block to rest inside. The block should rest directly on the two bottom chords for maximum strength.

      By Garrett Boon -- January 5, 2011
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