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
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.
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
Thanks GB!!!
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
FT, model bridge geeks are hard to come by
. 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.
How would the tension and compression be split up if it was made into an arch.
Like take the first colorcoded warren truss for example. If i made an arch with warren truss patterns inside, would the tension and compression still have the same affect on the same pieces of wood? Like, would the ones slanting inward in relation to horizontal ground be under compression and the ones slanting out in relation to horizontal be tension? Or would the ones slanting inward and outward in relation to its attachment to the arch be compression and tension, respectively?
Does that make sense?
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
It sounds like you are in Science Olympiad. If so, then you don’t want the bridge to be any wider than it has to. The bridge should be just wide enough to accommodate the loading block.
How do you determine the optimal width of a model bridge? We have to make an elevated wooden bridge that has a span of 45 cm, a tower height of 7 cm and a maximum height of 15 cm (including the tower). Pls advise. Thanks.
do you know how to figure out mathematically, the forces applied to a truss if there are like 3 diamonds in the middle with a right angle triangle on the side? pretty much the same as the K truss, but just with a diamond where the 2 K’s are
what will happen if i take the “green” lines off the sides for the pratt bridge
Garret Boon, yes I am talking about lateral bracing. I didnt know what is was called
Manuel, are you talking about adding a horizontal line in the middle of the Pratt truss? Don’t do it. Try plugging that design into the Bridge Designer program and see what happens. It is basically adding useless weight.
I am not sure what you mean with the X’s at the beginning and end. Are you talking about lateral bracing?
thank you so much i think i’m going to ace this assignment
i’m using a pratt truss what will happen if i add an extra line going straight through the middle and add X’s on top and at the beginning and end.