Truss Design

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.

168 thoughts on “Truss Design”

    • 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.

      Reply
      • 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

        Reply
  1. 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.

    Reply
  2. Pingback: Pasta Bridge Competition | Gene Marie Chagaris
  3. 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.

    Reply
        • 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).

          Reply
  4. 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

    Reply
    • 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.

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  5. 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

    Reply
    • 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.

      Reply
  6. 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!

    Reply
  7. 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

    Reply
    • 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.

      Reply
  8. i am building a warren truss with vertical supports, made of popsicle sticks, and wood glue. is this good enough to hold 20 lbs.??

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  9. 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?

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    • 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.

      Reply
  10. 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.

    Reply
    • 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.

      Reply
  11. 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!

    Reply
  12. 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.

    Reply
  13. 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

    Reply
    • 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.

      Reply

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