K Truss Analysis

By on May 15, 2008 -- Modified on October 4, 2016

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.

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

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14 thoughts on “K Truss Analysis”

  1. Used this design using the above pictures and won most efficient in my class. Great bridge.

    By Gary -- February 3, 2017
  2. what are the dimensions of each member or the ratio between them ??

    By Kirolos Adel -- November 30, 2016
  3. You can improve this design quite a bit by making the members that come from the fixed/rolling nodes (i.e. the outer most members) vertical instead of slanted. Basically, you add another K on each side instead of the long diagonal member, except the top right (on the right side) and top left (on the left side) nodes will have all zero-force members and you can just remove those two nodes (and the four members that connect to them).

    …Maybe it sounds simpler if I just say you can move the roller and fixed node inwards one node, and delete those outer-most nodes.

    For the original K Truss, it has a maximum of 66% of the load in single member, whereas my revisions ensure a maximum of 50% of the load in a member…. so assuming your material has a 100N compression/tension maximum, the original K Truss could only hold 150N centralized load whereas this can hold a 200N load.

    By Tom -- February 22, 2013
    • Could you send me a picture of your plans, I’d like to modify them to build a crane bridge

      By Grace D -- November 18, 2016
  4. Pont de Quebec also is a k truss design cantilever bridge

    By Shafle -- April 25, 2012
  5. I got 1st place

    By Arnold -- March 12, 2012
  6. 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.

    By Seemo Big Pork -- February 4, 2012
    • Did you put the weight on the top or bottom?

      By Brendan -- April 11, 2012
  7. i tried the k truss and it turned out to be great i won second place in my science class

    By Marvinjuarez11 -- October 18, 2011
  8. I did the K-truss for an architecture class and it turned out great! I won the prize for holding the most weight. 🙂

    By Briallia Parratt -- February 19, 2010
    • What material did you use…and how much weight did it hold?

      By Hannah -- April 1, 2014
  9. what factors were important in the design of this truss brigde ?

    By amanda -- January 21, 2010
  10. 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.

    By Mackortoyota -- October 26, 2009
    • Sorry, it messed up the spacing.

      By Mackortoyota -- October 26, 2009

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