Tags: howe truss, k-truss, pratt truss, tips, truss design, warren truss
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Truss Design
Learn all the main types of 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
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
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- Pratt Truss
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- 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
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
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
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.
sorry 4r late reply sorry we can not help you tnx
Okay, i am trying to build a Truss brige fr school out of Balsa wood(sp) Would it be okay if i use my initails for a desing?
Austin, I do not understand what you are asking.
he means like an s and p in the frame of the bridge or so you cand see the shape of his intials
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!
Zoe, you might want to consider checking my popsicle stick bridge plans for the different types of trusses. You can look at them here: http://www.garrettsbridges.com/store
Zoe, hon, no one really understands this stuff. I’m in college and I ain’t got a clue!
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
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.
i am building a warren truss with vertical supports, made of popsicle sticks, and wood glue. is this good enough to hold 20 lbs.??
Most likely, depending on how well you construct it.
Hey Garrett,
I’m team leader of my school’s team taking part in a bridge design competition hosted by Northrop Grumman. We will be competing against their top engineers. Considering my team and myself are not nearly as experienced as they, I believe it would be wise to seek some advice. I could share you a more detailed description of the rules (including pictures) via email, but here’s the basic run-down. The bridge will be made out of balsa:
15 Pieces of 1/8” x 1/8” x 24” Balsa Strips
4 Pieces of 1/4” x 1/4” x 36” Balsa Strips
1 Piece of 1/16” x 3” x 36” Balsa Sheet
1 Oz of Glue
bridge must have 14″ span and we can choose to have the load applied directly in the middle, or be applied evenly at two points 3″ apart (this option is probably better). Bridge must also be able to accommodate a 2″ block on the roadway along its entire length
I have been playing around with the truss designer tool on your site and the K-truss looks promising.
Would this be a good design for this application? or Do you have any advice on possible designs we could attempt? Any help would be greatly appreciated. Thanks for your time.
I am building a bridge out of 9 sq. ft. of cardboard for a 3D Design class. It has to hold 25 lbs. in pennies (the weight will be spread out over the span of the bridge). It has to span 30″ and can’t be connected to the tables and we can only use hot glue and cardboard.
I think I will go with the Warren Truss. Is it better to cut out members and then glue them back together or just cut out the sides as one piece and eliminate the need for gusset plates?
Do you have any other recommendations?
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?
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.
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.
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.
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!
Doesn’t matter.
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.
May, check out my popsicle bridge photos:
http://www.garrettsbridges.com/category/popsicle-bridges
That will help give you some ideas of where to start.
which of these designs are best when the weight is loaed by hanging the weights from the botton of the sturcture??
Maggie, it doesn’t matter.
okay, thank you
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
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.
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.
Try the warren im doing that. Though i herd some good coments about the K truss