Tags: 16 inch span, 300+ pounds, basswood bridge, fernbank bridge
22,297 views in 2012
Fernbank Project: Strongest Bridge Design
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 commenter said, “So if this bridge weighed one pound, it could have held up my car.”
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- The award I received
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 
what truss design did u use?
This bridge used a hybrid truss design. Mostly it was a Subdivided Warren, but went to the Howe Truss on the ends.
What happens if the span of the bridge is 14 in. not 16 in. Would it hold the same amount of weight?
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.
Hey Derek, what unit is the weight that your bridges held in?
may i know how can ibuild a very strong bridge structure model only with satay sticks and straws without glues?
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.
Ryan, that sounds like a sweet bridge. Do you know anything more about the bridge? How long was it etc?
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?
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.
Can you tell me where on a bridge a lap joint, end joint, notched joint, and gusset joint would be used
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
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.
that was a cool bridg and i might bield a brige like that
Hi I was wondering what the dimensions of your basswood were.
1/2 inich, 1/4 inch?
what were your dimmensions on this bridge design
can you explain more about the size ,shape about the bridge .so it will be useful for science projects
this bridge is awsome!! I’m gonna try your idea for this project i’m making.
I hope it works as good as yours!
we have to do this for 3rd semester science project, only with smaller measurement, and i was wondering what type of glue as used?
Anne, I used two different glues on this bridge, Probond and Weldbond. Both are good choices.
We wanted to use this for our experimental design structure for science olympiad, and I was wondering if you still had the plans, and what you made this out of. please respond.
Sara, this bridge was made out of Basswood. I do not have the plans anymore.