Pratt Truss

How the forces are spread out

Here are two diagrams showing how the forces are spread out when the Pratt 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.

These diagrams bring up several interesting things. Notice that the two end diagonal members do not change. Also, there is little change on the bottom chord between the two pictures. However, there is drastic changes on the internal truss members. The centered load dramatically increases the amount of force that is applied to the internal members of the bridge. Also, the forces are increased on the top chord of the centered loaded bridge.

This seemingly insignificant change in how the bridge is loaded makes a big difference in how your model bridge will perform. If you have the ability to change and set how your bridge is loaded, I’d shoot for spreading the load across the entire span. This pretty much goes for any model bridge design, not just the Pratt Truss.

Pratt Truss for model bridges

The Pratt Truss is one of my favorites. I have used it often for my model bridges, including balsa, basswood, and popsicle sticks. It is easy to construct, and is a solid choice for a model bridge design.

Additional Resources

Pictures of real Pratt Bridges
History of Truss Design

This bridge used 50 popsicle sticks, weighed 75 grams, and held 200 pounds. It’s efficiency score was 1212, which is the highest out of any popsicle stick bridge I have ever built.

Here is a video showing the testing of the bridge:

How this Bridge Broke

The video makes it pretty clear how the bridge failed. The first stick to go was on the left end closest to us (on the first part of the movie). The bridge slowly started to lean to the left as this stick was probably a weaker one, and it started to bend more and more until it finally buckled. As soon as it broke off, the rest of the bridge collapsed. This is sweet to see on this video clip because quite often the reason a bridge broke is not easily seen.

One way to help avoid this from happening on your bridges is to pay extra close attention to the quality of your popsicle sticks. If you have solid popsicle sticks that make up your bridge, then your entire structure is more likely to be solid.

Plans for this Bridge

If you want to build this bridge, I am offering the blueprints I used to build it. You also get step-by-step instructions with the purchase which clearly explain everything you need to know about building this bridge. Check out these plans today:

Click here to see the new and improved version of this bridge

This is a popsicle stick bridge I built using the Pratt Truss design. This bridge is 13.5 inches long, 4 inches tall, and 3.75 inches wide. This bridge weighed about 70 grams and held 73 pounds before breaking. I used 46 popsicle sticks to build this bridge. That gives it an efficiency score of 474.

Pratt Truss Analysis

The last picture shows the truss analysis from the John Hopkins Bridge Designer program. If you watch the movie, you will notice that the front left corner and the back right corner started to bend a lot right before the bridge failed. The truss diagram shows that these popsicle sticks had the most weight on them, which probably contributed to their bending. To improve this design, I will double up the last diagonal popsicle sticks to account for the extra amount of force they have to withstand.

Also, the movie shows that the lateral bracing popped off right before the bridge broke. The bridge had been slowly leaning towards the left. I wonder if my standard method of just having one angled popsicle stick glued on each end for lateral bracing is sufficient. I think that having a full X pattern would be better. I will also make this change and see how it affects the strength of the bridge.

I found pictures of some old trusses I built for Science Olympiad bridges. Most of these were built in the 2003-2004 school year. I thought I would post them to give examples of how these trusses could be used in model bridges.

As you can see, I’ve modelled it on the Pratt Truss. Although, I didn’t do any “real” plans, just rough drawings (no force/load drawings).

Let me know what you think!

Cheers, 

Joe.

This is a Pratt truss bridge that looks very well built. It is much larger than most of the models here. Notice the guys loading it are not wearing safety glasses, which is not recommended. I wonder why the bridge failed. It looked as if the right side of the bridge, the last two diagonal members were bending. That probably could have been fixed by making the beams into an L or T beam shape.

Here is the email Bill sent me:
I started making bridges about a year ago and have pictures to share with you. For the first bridge I used a set of plans that I found free on the internet from a guy who was in the army. The bridge plans were designed to be made 100% from manilla folder paper. The plans were to fold the manilla cardboard into the different post diameters and use them to make the trusses.

I decided to use dowels from my local Lowe’s instead. However, I did use the manilla cardboard to reinforce some of the joints and to give thebridge a real look.

You can find the plans for this bridge here:
Manilla File Folder Bridge

This truss bridge uses the Pratt truss model for its design. The Pratt truss is one of my favorites because the longer diagonal members are in tension. This allows the shorter vertical pieces to be in compression.

Common trusses used in engineering:

Warren Truss

Warren Truss

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

K Truss

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.