Fernbank Project: Strongest Bridge Design

By on October 9, 2005 -- Modified on October 4, 2016

I built this bridge for the 2005 Fernbank Science Center bridge design event. This is the strongest bridge I have ever built. The bridge spanned 16 inches, weighed 37 grams, and held 346 pounds. That puts its efficiency over 4200! I had never even come close to getting this amount of strength out of a bridge design before. I was really surprised. One person said, “So if this bridge weighed one pound, it could have held up my car.”

The first time the bridge was tested it did not break. The testing machine was set to only apply 250 pounds. This bridge was too strong for that! Here is the bridge after the second testing:

The secret of a strong bridge design

As you can see, the bridge stayed mostly intact. I talked to a couple engineers at the event and asked them what they thought made my bridge break. One suggested that the bridge failed in torsion, as I did not have any diagonal braces in the bridge. That is definitely a design flaw I will fix if I do this again. I would love to try and break an efficiency of 5000. That would be a strong bridge indeed!

For more information on this competition, see Atlanta Toothpick Bridge Competition. I encourage anyone in the Atlanta area to try and go to this event. It is free and open to all, both young and old. Try your hand at making the strongest bridge. And when you do, send me photos of your bridge 🙂

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59 thoughts on “Fernbank Project: Strongest Bridge Design”

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  1. Do I need a protractor to get exact measurements or can I just wing it? If so, what are the measurements for the angle of the trapezoid shape?

    By Margie -- September 27, 2016
    • Hi Margie,

      I don’t have the measurements of this bridge anymore, so you will just have to wing it.

      By Garrett Boon -- September 27, 2016
  2. Do you have a blueprint we can use to make a bridge like this one?

    By Jana -- September 10, 2016
    • Jana,

      I do not have blueprints for sale for this bridge right now. At some point in the future I hope to have them, but not currently.

      By Garrett Boon -- September 10, 2016
  3. Hey Garrett, I’m Sam, and I was wondering what a good ratio of height to length to width ratio would be for a truss bridge, in general. If you need me to be more specific, then, as an example, let’s assume that we have a Pratt bridge that’s 25 inches long. Do you have any insight on how high or wide it should be then? I’m no expert, so just a general idea would really help me out; if you have enough time.

    By Sam Witwer -- December 2, 2014
    • Sam, I’ve had good success starting with a 1:6 height to length ratio. In general a wider bridge will be more stable but also adds weight. If you are testing the bridge with a load that won’t move around lateral, then a more narrow bridge is fine. But if you are planning on standing on the bridge, you probably want to make it wider. The popsicle stick bridges I make that are 25 inches long are about 3.5 inches wide. This is mostly because of the limitations from using popsicle sticks, but it does seem to work out well.

      By Garrett Boon -- December 3, 2014
  4. If this design was used with popsicle sticks would it work?

    By Tessa -- November 7, 2011
    • Probably, although you might have make some minor modifications.

      By Garrett Boon -- November 7, 2011
  5. What kind of popsicle sticks did you use, just regular, or a certain kind. Also, what kind of glue?

    By Tessa -- November 7, 2011
    • Tessa, I did not use popsicle sticks on this bridge. I used Basswood. I used two types of glue, Weldbond mostly and a little Probond.

      By Garrett Boon -- November 7, 2011
  6. What kind of truus did you use? A howe, pratt, or warren?

    By Balsa, Balsa Wood...ha -- April 12, 2011
    • This bridge used a hybrid truss design. Mostly it was a Subdivided Warren, but went to the Howe Truss on the ends.

      By Garrett Boon -- April 12, 2011
  7. what do you mean by diagonal braces? and how long did it take u?
    i tried to copy your design except with popsicle sticks didnt turn out very well. was kinda crooked and twisted.
    hope i can still beat my class in most durable bridge

    By Steve Amdhome -- March 22, 2011
    • Good question. Diagonal bracing here refers to any pieces that would have connected the sides of the bridge on a diagonal. So if I had a piece that was glued on the bottom of the left side and sloped up to the top of the right side of the bridge.

      By Garrett Boon -- March 22, 2011
  8. I was wondering, how was the bridge loaded?

    By Saga -- March 15, 2011
    • This bridge was loaded from the top with a metal plate that was attached to a hydraulic loading machine.

      By Garrett Boon -- March 16, 2011
  9. what truss design did u use?

    By Mike -- March 15, 2011
    • This bridge used a hybrid truss design. Mostly it was a Subdivided Warren, but went to the Howe Truss on the ends.

      By Garrett Boon -- March 16, 2011
  10. We do bridges every year in math and they weigh under 50 g. and hold in the 1,000. my bridge held around 1,200 last year.

    By Derek -- March 4, 2011
    • Hey Derek, what unit is the weight that your bridges held in?

      By Garrett Boon -- March 4, 2011
  11. I saw a bridge today at the TSA Regional conference that held OVER 496 LBS.THe bridge was made of Balsa wood and had a very strong structure.

    By Ryan -- February 22, 2011
    • Ryan, that sounds like a sweet bridge. Do you know anything more about the bridge? How long was it etc?

      By Garrett Boon -- February 25, 2011
  12. Do you reccommend binding the pieces? I am using bass wood, but the pieces are quite thin from my perspective. Should I glue two strips together to make it stronger? And if so, are there any parts in particular on the bridge that might not be good to do this?

    By Yenny -- January 5, 2011
    • Yenny, this is called laminating the wood. Yes, it does make it stronger, but usually not more efficient. This means that it adds more weight than it does strength. I’d recommend you glue your strips of wood into L, T, or even I shapes which will increase their efficiency if done well. The L, T, and I beam shapes are useful for any members of your bridge that are in compression, such as the top chord. Often enough members in compression do not fail because the member itself broke, but rather because the joint failed. I would make sure you are using good joints.

      By Garrett Boon -- January 5, 2011
  13. Hi, I have to build a balsa wood bridge for my class. I just had one question.

    I have decided on a Howe Truss bridge, but the slanted members that are on wither end of the roadway. How do I make them? Specifically, how do I make them angled? And strong?

    I know my question may not be clear, its hard to describe on words. If you do know what I am talking about, do I attach them directly to the top of the roadway? Or the side of it? Is it possible to use gusset joints on this type of bridge?

    spans 30cm

    By Abhinav Mishra -- November 8, 2010
    • Abhinav: If you make your diagonal members on the ends of the bridge connect to the bottom and top of your truss with End Joints, adding a gusset, I think that would work well. This would allow all your other joints to be natural Lap Joints. In your words, attach them to the top of the roadway.

      By Garrett Boon -- November 8, 2010
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