Fernbank Project: Strongest Bridge Design

By on October 9, 2005 -- Modified on December 8, 2017

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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68 thoughts on “Fernbank Project: Strongest Bridge Design”

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  1. we have to do this for 3rd semester science project, only with smaller measurement, and i was wondering what type of glue as used?

    By Anne -- March 3, 2010
    • Anne, I used two different glues on this bridge, Probond and Weldbond. Both are good choices.

      By Garrett Boon -- March 4, 2010
    • Should I glue the support beams on the sides of my bridge or put it on top of the base beams?

      By Juan Cruz -- December 16, 2017
  2. 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.

    By Sara -- March 2, 2010
    • Sara, this bridge was made out of Basswood. I do not have the plans anymore.

      By Garrett Boon -- March 2, 2010
  3. Unreal! I’d like to try building it myself; I realize this was made in 2005, but if you still have the plans or remember how it was built, could you email me instructions? I can’t tell what size the wood members are.

    Personally, I”d love to take that bridge to an efficiency of over 5000 as well.

    By Joey -- January 18, 2010
    • Hi
      i think that it looks very cool
      i am building one for my class and i was wondering if you know how far apart each vertical suport was placed
      and are the top l beams two peices of 1/2 inch glued together
      plz reply

      By nick -- February 8, 2010
  4. I’ve created a bridge out of balsa using butt joints,cardboard gussets,(and some joints with pva glue,some with hot glue guns) before.It was a complete wuss when it came to testing it. It only held 18lbs before completely collapsing.Compared to your bridge,it would be like an elephant standing next to a pea!

    By TheNocturnalOne -- May 26, 2009
  5. umm this iss alll soo cooolll !! i kant belive it held out soo muchh.. im gonan try to make one simliar to this but i have one question…. “what type a bridge is this”?? u noe is it a beam bridge, a suspension bridge,and arch bridge ?? or lyk wuttt … it woud be a greatt HELPPP if u kan answer tht.and give me a few more tipz on this…… plz reply ASAPP !! <3 <3 <3 thannxx

    By lily -- January 30, 2009
    • This would be classified as a truss bridge.

      By Garrett Boon -- February 1, 2009
  6. I designed the Fernbank Bridge with the idea in mind that it would hold a lot of weight, unlike the SciOly bridges. Therefore, I did some things on this bridge I would not do on a SO bridge. The cross bracing on the bottom was mainly for “insurance”.

    Unfortunately I have been out of practice building bridges for long enough that I do not remember how to compare the weight and density of the pieces you mentioned off the top of my head.

    I would question your statement that butt joints are better for compression. I would suggest that a gusset or double gusset would be the strongest joint.

    Good job with your tech class bridge. That is quite impressive.

    By Garrett Boon -- January 23, 2009
    • i made a bridge out of posicle sticks that held 280 lbs. i dont remember exactly how much it weighed. i do know that it was 40cm long by 10cm wide. it looked very similar to this one though.

      By josh -- January 27, 2009
  7. Working on SciOly elevated bridge. Using 1/8 square sticks (1.75-2.0 grams per 36″ length). Joined top and bottom chords with butt to butt joints (also 1/8 square but 1.3 1.5 gr per 36″). Noticed that your bridge has face to face joints. Any thoughts? I know face to face is good for tension but butt to butt is better for compression. If I used face to face I could go with 1/32 stripped to 1/8 widths. Also on diagonal bracing for the top is it necessary to make that an “x” or can you zigzag. That eliminates 1/2 the wood. Looking at your bridge, is there a reason for the cross-bracing on the bottom chord? I made a ten inch bridge in technology class last year and put in the cross-bracing but I glued it on to faces to the top of the chord and side of the upright. It weighed ten grams and held 93 lbs. which is also about 4200 efficiency.

    By StampingBoy -- January 21, 2009
  8. Thanks! That helped a lot

    By Prakhar -- January 14, 2009
  9. NIce bridge guys!!!!!! 😀

    By Maynelljah -- January 14, 2009
    • We are making a bridge for our class at West Finchester Memorial Academy of Engineering- New England

      By Maynelljah -- January 14, 2009
    • wow i am building a bridge for a university project and this design has helped to an extent amount 🙂

      By Bob O'Brien -- October 21, 2009
  10. Hey Garret,

    Im new to this so can u tell me how tall this bridge is. I’m doing a similar bridge for SciOly
    and would love any pointers.
    Thanks

    By Prakhar -- January 12, 2009
    • I am pretty sure this bridge was 4 inches tall.

      By Garrett Boon -- January 14, 2009
  11. This is absolutely amazing. Your website is so helpful and cool!

    By Sam -- January 4, 2009
  12. Your brigdes efficency was pretty amazing, and congragulations on achieving such a feat. A few subtle improvements could be made, at least from my angle. Triangles are the strongest non-complex shape for bridges, and you clearly harnessed MOST of their potential in your bridge. Your bridge might have withstood more abuse if the triangle was within your frame, rather then being glued to the externally to the frame. A final point of possible improvement would be the plane of your bridge. Your plane resembles a train track, with two long lines parallel to each other and smaller lines intersecting the long ones perpendicularly. This is smart if the bridge is layed on the ground, but most bridges are suspended in the air, making this platform at a disadvantage. In my unprofessional opinoin i would have used a triangle pattern for the plane, or a double triangle (making the appearance of an X). This tends to be the strongest platform since the squares or rectangles that you made usually cant hold much weight effiecently. Your bridge, once again, is amazing and 4200 effeciency rating is much better then my highest of 3900ish, but that was a default because the judges ran out of wieght to put on, lol. Best of luck to you and everyone else in the future

    By Phil Estrin -- December 26, 2008
  13. 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. 🙂

    By Jennifer -- December 19, 2008
  14. 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

    By mackenzie -- December 17, 2008
    • 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.

      By Garrett Boon -- December 20, 2008
  15. 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!!!

    By Ali -- November 29, 2008
    • 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

      By Garrett Boon -- November 29, 2008
      • 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!!

        By Ali -- November 30, 2008
  16. 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?

    By Mike -- November 6, 2008
  17. 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.

    By Garrett Boon -- November 3, 2008
    • Hi. your bridge is amazing…and I think my teacher saw your work and ask as to build our own bridge out of the ff materials; barbeque sticks, glue sticks,5 sheets of bond paper. He gave us the ff:
      span: 30 cm, height- 10 cm.weight,10 cm

      I believe you can pretty help me with how iam going to do my project…What type of bridge would be the best for these materials?

      Please help…i dont know what to do…

      By Shannen -- August 21, 2014
  18. 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?

    By Kate -- November 2, 2008
  19. Triangles seem the strongest in most types of bridges.

    By Signowurl -- October 1, 2008
  20. 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.

    By Garrett Boon -- September 6, 2008
  21. I was just wondering if you have ever built an arch bridge? Because I heard that was the strongest bridge design.

    By Danaë -- September 5, 2008
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