Truss Series: K Truss Analysis

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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

21 thoughts on “Truss Series: K Truss Analysis”

  1. 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.

  2. 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.

  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.

  4. I’ve been working on a three chord (in line) truss (not a space truss) and claim it eliminates transverse deflection… it is an arched truss. Clear spans of 3 km perhaps?… maybe more… Should be the lightest and strongest ever and by far the most economical as well.
    In today’s bridges, transverse deflection is the biggest problem… the further you go the worse it gets! Control it and there is far less pressure on everything including the joints so you can use much smaller members… and no need for so much (if any) fly bracing… very very simple and understandable when you can get your head around it!
    Dave 🙂

      • Hi Garrett,

        Sorry I can’t post any pics at this stage as it’s going through patents etc, but it you can reach me through email I guess we can do by confidentiality agreement.

        I’m waiting on a university here to set up a meeting with them for a full analysis but so far I have had one certified for 37 metres using only 50x50x4 top chord… which would easily go to a 50m clear span.

        I can add that all other members, apart from the top chord and struts, are tension members only (25x25x3). No need for a lower compression member as deflection has been eliminated.


  5. So for my project I was thinking of using this Truss design. What is the best place to put the string (braided mason line) to help hold weight? Naturally, the string would be in tension. Also, do you know what the angles are for the bridge and/or the length of the model?


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