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.”
-
- 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 
I’m in Science Olympiad and I will be competing in the Elevated Bridge event. The rules: http://soinc.org/sites/default/files/uploaded_files/ElevatedBridgeBRulesGraphicv2.doc (this is the general rules, i could not find the specific ones). Please take a look at the rules. We must use 1/4 by 1/4 (inches) balsa wood. Do you have any tips or ideas on bridge designs, density of wood, grade of wood, etc.? That would be very helpful.
right now us the grade sevens have a science fair in january 26 and i have chosen to build a bridge and your website here has helped so much but when those engineers told you that if you had added more diagonal supports your bridge would of lasted was that all they told you..? was that the only flaw you had in your bridge or were there other problems..? I want to have the best bridge possible and i need all the help i can get
Mackenzie, that is exactly what I was told by the engineers at that event. They suggested that the bridge started to twist and therefore failed by torsion. This probably could have been avoided had I included more lateral bracing throughout the truss.
so would this work with tooth picks? there is a project, and i have already attempted to build crazy bridges, but the teacher doesn’t want them… so i need something to beat an efficientce rate of 1500… if this bridge works for toothpicks I might do excellent. he mentioned something about fractals, and walked away, so please help
I was wondering if you used any particular glue, I have to make a bridge for geometry(this weekend cause my teacher is lame and didn’t even give us direction) and we have to be able to have the quarterback of our football team stand on it haha
thanks so much, this was very helpful!!!
Ali, I used two types of glue on this bridge. I used both Probond and Weldbond. You can read about them on my glues page:
http://www.garrettsbridges.com/building/gluetips
Thanks, i’m in the process of building my bridge right now, and 16 inches is prettty small it looks a whole lot bigger in the pictures!! We’re using gorilla glue and uhh something i think its called epoxy?? I don’t know, it’s blue and white and you mix them together haha
we have to only use popsicle sticks, so its very difficult, but your site is soo helpful thanks again!!
I was wondering whether the Fernbank Bridge could be successfully replicated with only toothpicks…also, was the weight attached to your bridge at the top of your superstructure…for my project, I believe the weights are suspended from the bottom of the bridge…do you have any advice for how I could accomplish a strong bridge under these parameters? Thanks!
I probably won’t be making blueprints for this particular bridge. I think I still have my original plans I used to build the bridge, but right now I am in a different country.
would you be willing to make instructions on how to build that?
my teacher bet that i couldn’t hold over 200 lbs but i want to prove him wrong!!
Hi, Your website is amazing and has helped me a lot!
I am, however, stuck between the crossroads of building an arch or truss bridge. Our balsa wood bridge must span a valley of 20 cm and have an inside deminsion of at least 5 x 5 cm. I’m wondering which structure would be more efficient?
Kate, the next best thing after Basswood is Spruce. Balsa will work also, but is just harder to bend and is in general not a consistent wood.
I was wondering what type of material would you recommend for building an arch bridge? I cannot get my hands on any basswood… Would balsa be the next best thing or would there be something better out there?
I have thought about it, but I just haven’t put the time into creating a product. The hardest part for me would be to get the design into digital format. Thanks for the input.
Have you ever considered selling designs for bridges other than popsicle. Like this one or towers?
Triangles seem the strongest in most types of bridges.
Danae, I have built arch bridges before. The problem with saying that one type or design of a bridge is the strongest is that it changes for each situation. An arch bridge might be the strongest for one situation and a truss bridge for another.
I was just wondering if you have ever built an arch bridge? Because I heard that was the strongest bridge design.