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:

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.

Hybrid Truss

Arched truss

Arched truss

Pratt Truss

Simple Truss

Arched Warren Truss

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).

Here’s a picture to show the scale:

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.

Warren

Pratt

Howe

K Truss

The following figures are shown under a load. The numbers represent percentage, where the total load = 100. The numbers are rounded to the nearest 5. For absolute numbers, visit the Bridge Designer.

Red represents tension, blue represents compression, and green is no load.

On this Warren truss, each of the down arrows represents 50% of the load. Notice how the two middle pieces have no load. Notice the load is quite a bit less on the ends of the top and bottom chord than in the middle. When you build for efficiency, keep in mind that for a bridge loaded in the center, the ends can be smaller than the middle.

Now examine the same Warren truss, but with added vertical members:

The added vertical members serve to break the top chord into smaller segments, making it stronger. However, the percentage of the load has increased both on top and bottom. Also notice where the green members are now.

Pratt Truss

I want you to notice the difference between the Pratt and Howe trusses under a load. The Pratt has bigger numbers on both the top and bottom chord, but its middle compression members are shorter, and hold less than those on the Howe. But the load is still concentrated in the middle, and gets less further to the ends on both.

Howe Truss

It is a trade off between the Pratt and Howe. For the Pratt, you’d have to use bigger top and bottom chords, while on the Howe you’d have to use bigger compression members.

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.

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. 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.