K Truss Analysis

The K Truss design was a variant from the Parker truss design. The Parker, in turn, came from the Pratt truss. The idea of the K truss is to break up the vertical members into smaller sections. This is because the vertical members are in compression. The shorter a member is, the more in can resist buckling from compression. The K truss, probably because of its complexity, did not became very popular in the United States.

K Truss

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the K Truss is under a load. The first shows the load being applied across the entire top of the bridge. The second shows a localized load in the center of the bridge. In both cases the total load = 100. Therefore, you can take the numbers as a percentage of the total load.

The K truss shows the smallest amount of change from the two types of loads on the top and bottom chords. In fact, there is very little difference between the two for the top and bottom. For the internal members, however, there is a large change. As usual, the concentrated load increases the forces on most members. Interestingly, on the K Truss, some members change from tension to compression. Notice this on the top half of the vertical members.

K Truss and model bridges

I think the K truss, while being more complex and more difficult to build, could be a good option for model bridges. I have not build a K Truss bridge yet, but if I get the chance I would like to try it out. Many of my readers have reported success with this design.

Additional Resources

1. Pictures of Real K Truss Bridges
2. Some K Truss History

9 Responses to “K Truss Analysis”

1. Shafle - April 25, 2012 at 10:54 am

   Pont de Quebec also is a k truss design cantilever bridge

   Reply
2. Arnold - March 12, 2012 at 9:42 am

   I got 1st place

   Reply
3. Seemo Big Pork - February 4, 2012 at 6:34 pm

   I used design for my science project and it held 100 pounds even. It was my first popsicle bridge I have ever made too…homie.

   Reply
   • Brendan - April 11, 2012 at 3:25 pm

     Did you put the weight on the top or bottom?

     Reply
4. Marvinjuarez11 - October 18, 2011 at 7:30 pm

   i tried the k truss and it turned out to be great i won second place in my science class

   Reply
5. Briallia Parratt - February 19, 2010 at 10:50 pm

   I did the K-truss for an architecture class and it turned out great! I won the prize for holding the most weight.

   Reply
6. amanda - January 21, 2010 at 12:51 pm

   what factors were important in the design of this truss brigde ?

   Reply
7. Mackortoyota - October 26, 2009 at 3:36 pm

   For shop class I have to build a 12″ long bridge. I made a modified k-truss. I removed the green beams to save balsa wood. Its made of 1/8″ square balsa beams, and is 2″ tall. It has to hold 42 pounds in the middle. It is about like this, except with a beam on the bottom:
   ____________
   /|/|/|/|\|\|\|\
   / \|\|\|/|/|/ \
   I’ll post the results once it is built and tested. Also I’m using thin CA glue.

   Reply
   • Mackortoyota - October 26, 2009 at 3:36 pm

     Sorry, it messed up the spacing.

     Reply