Model Bridge Design » k-truss

K-Truss bridge for Wake Tech engineering class

bcmiller2 — Sat, 12 Jun 2010 13:22:41 +0000

This bridge is supporting 191lbs of weight. It didn’t break, that was all the weight we could fit in the trash can. I believe this is close to the maximum load it could hold because the platform the jig is resting on was significantly bowed. The rest of the structure was rock solid. The construction of the bridge is popsicle sticks.

Balsa Wood K Truss Bridge

Jonathan — Tue, 12 May 2009 15:32:23 +0000

Here is a bridge using the K truss design made from Balsa wood. This bridges uses a combination of end and lap joints. You can see that where the slanted members of each K join together at the vertical member, there is an end joint. At the top and bottom chords, the joint is a lap joint. This bridge does not have any lateral bracing on the top, which is something I would highlight to change. The performance of this bridge would be greatly increased with that lateral bracing.

What do you think about this bridge? Is there anything you would have done differently?

K Truss Analysis

Garrett Boon — Thu, 15 May 2008 14:49:22 +0000

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

Truss Design

Garrett Boon — Tue, 16 Aug 2005 11:07:14 +0000

Learn all the main types of trusses used in real bridges, and see how to apply them to model bridges. Learn the history of each common truss design. This page is designed to help you make an educated decision about what truss design you should use on your bridge.

Common trusses used in engineering:

Warren Truss

The Warren truss is one of the most simple yet strong designs. This simple design already existed, but what made the Warren unique is that it uses equilateral triangles. Each side of the triangles are the same length. This marked an improvement over the older Neville truss which did not use equilateral triangles.

Go to a more in depth analysis of the Warren Truss.

Pratt And Howe Truss

The Pratt and Howe trusses are very similar. In fact, the only difference is the direction the slanted members are angled. This changes which members are in compression and tension. On the Pratt truss, the shorter, vertical members are in compression. However, on the Howe truss, the longer, angled members are in compression. Because most materials (especially wood) that model bridge builders use decrease in the ability to resist compression the longer they are, I think the Pratt truss has an advantage.

There are more factors to consider, however. The Pratt and Howe trusses also differ in how they spread the load to the top and bottom chords. The Pratt truss has larger forces on the top and bottom chords than the Howe. Thus. you’d have to use bigger top and bottom chords.

Go to a more in depth analysis of the Pratt Truss.

Go to a more in depth analysis of the Howe Truss.

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.

Go to a more in depth analysis of the K Truss.

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.

If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from amazon. It can be purchased online though some simple credit card processing from Flagship Merchant Services. This is a comprehensive study on the engineering principles behind the design of bridges. It is easy to understand and to follow, and is a great fit for students who are just learning, but advanced enough to be a great resource to those with more experience. For more great resources, see this list of other great bridge books.