Tags: howe truss, k-truss, pratt truss, tips, truss design, warren truss
19,821 views in 2012
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
-
- Pratt Truss
-
- 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
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)
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.
My student are building a suspension bridge from balsa. This is a first for me as well. I am curious if the bridge should attach to the tower at each end of the bridge, or be totally suspended using the string.
“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
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.
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.
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.
heyyy love the bridgess
im at school right now and we’re doing a project on it and your website is very helpful!!!!!!1111
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.
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?
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
cool designs ièll try them all!!
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
)>
Jacob, what materials are you building your bridges out of?
Im currently doin a research assingment on the k-truss bridge, can anyone hrlp with finding internal stresses when you have 3 unknown values at a point? ive tried metod of sections but cant cut the members with less than 4. please help
I need help building the pratt truss bridge. I don’t know how to do it. If you could help me that would be nice.
Make the page smaller and people will like to look at it
I was wondering.. I have to choose a truss design and I’m not sure…how would a warren hold up under a beam bridge? We’re putting weight on models to see whose model can hold up to the most weight without collapsing… And since we’re using balsa wood… a suspension bridge or an arch bridge seems pretty complicated.
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.)
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?
Hey, I have to build a bridge for my Honors Physics I class. We will be testing them by suspending weight 5cm from the center of the bridge on both sides. I was wondering which bridge design would be capable of holding the most weight before failing.
um…im doing a science fair and i would like to use a topic about bridges
could you please help me find one?
oh and I’m in China so I can’t really buy wood…