Truss Design
Common trusses used in engineering:
Warren
Pratt
Howe
K Truss
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.
Red represents tension, blue represents compression, and green is no load.
On this Warren truss, each of the down arrows represents 50% of the load. Notice how the
two middle pieces have no load. Notice the load is quite a bit less on the ends of the top and bottom chord than in the middle. When you build for efficiency, keep in mind that for a bridge loaded in the center, the ends can be smaller than the middle.
Now examine the same Warren truss, but with added vertical members:
The added vertical members serve to break the top chord into smaller segments, making it
stronger. However, the percentage of the load has increased both on top and bottom. Also
notice where the green members are now.
Pratt Truss
I want you to notice the difference between the Pratt and Howe trusses under a load. The Pratt has bigger numbers on both the top and bottom chord, but its middle compression members are shorter, and hold less than those on the Howe. But the load is still concentrated in the middle, and gets less further to the ends on both.
Howe Truss
It is a trade off between the Pratt and Howe. For the Pratt, you’d have to use bigger top and bottom chords, while on the Howe you’d have to use bigger compression members.
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. 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.
76,204 views |
Print this Post
| 
(112 votes, average: 4.17 out of 5)
Loading ...
June 14th, 2007 at 11:35 am
I was wondering which truss design would support the most vertical force if the force was place in the center of the bridge?
December 17th, 2007 at 10:41 am
How would you know what the diffrences are between the diffrent bridges? If you would not mind would you type me a 3 page paper about the diffrences and how they work. that would be highly appreciated….
Erickson Brown
December 17th, 2007 at 2:07 pm
What is the best overall bridge design to build?
December 19th, 2007 at 7:42 pm
I was thinking about the vertical supports, does that make it much more effective and stronger then haveing arches?
December 20th, 2007 at 5:36 pm
I was wondering if there was a bridge design, that if a verticle weight was hung from the bottom of the bridge, would be most efficient?
Is it even necessary to build a top portion or will a simple horizontal base be suffuicient?
December 27th, 2007 at 2:51 am
I would like to see the actual design of concrete bridge.
December 27th, 2007 at 4:45 pm
The K truss holds the most vertical force for a bridge loaded in the center (which is the case for most bridge design contests). Go to the bridge designer and compare force distribution on a K truss to what you see with the Pratt or Howe. You will find that the K truss distributes the load much more evenly and efficiently, unlike the Pratt and Howe trusses which frequently have individual members supporting as much as 115% of the load. The bridge I’m currently building uses a K truss design and no member has to support more than 50% of the load.
December 28th, 2007 at 5:19 pm
Thanks for pointing that out, Spencer. BTW, sorry for the delay in getting your comments up. My comment SPAM filter was blocking a lot of comments and I didn’t realize it.
January 2nd, 2008 at 10:41 pm
What bridge is best to use for a science fair project to test the strength?
January 3rd, 2008 at 8:30 pm
We are doing a bridge project in technical drawing. We have 8- 600 mm peices of balsa wood that are about the width of a plastic bendy straw. Probably a tiny bit bigger. The bridge has to be 250-300mm in length, 50-75mm in width, and less than 100mm tall. The roadbed needs to be 200-300mm in length, 50-75mm in widthe and less than 50mm tall and an underhang is opptional (this will depend on the type of truss obviously) The underhang must be less than 180mm in length, 50-75mm wide and less than 20mm tall (this includes the overall height of the bridge). To test the strength, our techer is putting a disc in the middle with a hollow rod in the middle of that. The pole is attached to a bucket and weight is going to be added to the bucket untill it breaks. Anyways, I am the only girl in the class and I feel like I am constantly needng to prove myself (eventhough half of the guys in there are complete idiots). I know that general triangles are the best shape to use. Do you have a recommendation on what type of truss I should use? I was thinking about a pratt or a howe truss but feel free to suggest something else. Thank you so much!