Truss Design
Posted on August 16th, 2005
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.
7,071 views |
Print this Post
| 
(2 votes, average: 4 out of 5)
Loading ...
July 14th, 2008 at 2:40 pm
This is a great site. Thank you for taking the time to put it together.
August 8th, 2008 at 12:02 am
are you 11? what do you think would happen if you reversed the k’s? your bridge would only be as strong as your adhesive since all the weight would pull on the supports rather than sit on them… you’d increase the stress on joints considerably!
and what would be the purpose of removing the verticle piece? and those green verticles… sort of necessary to hold up the supports. what’s the point in even having that first support? physics… seriously.
August 8th, 2008 at 8:03 am
You asked a really good question about the “green” supports. The program shows that they have no load, so why even include them on a bridge?
The reason is that the truss design program is dealing with theory only. However, in a real model bridge scenario, the bridge is going to twist and deform under load. Once the bridge starts to deform, then the forces go wacko. The green members might suddenly go red or blue. By leaving the green members in the design, you are putting a small safety factor into the bridge.
It would be an interesting experiment, however, to see if you can achieve a greater efficiency by leaving the green members out. The bridge might not hold as much, but because it will be lighter it could potentially have a greater efficiency.
August 15th, 2008 at 3:14 pm
The other point with the green members is the location of the load vectors. They are shown in the ‘dead load’ mode; when the ‘live load’ configuration is considered, there are no green members.
August 17th, 2008 at 11:11 pm
loving it..