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 🙂
I was just wondering if you have ever built an arch bridge? Because I heard that was the strongest bridge design.
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.
Triangles seem the strongest in most types of bridges.
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?
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.
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…
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?
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!!
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.
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. 🙂
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
This is absolutely amazing. Your website is so helpful and cool!
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
I am pretty sure this bridge was 4 inches tall.
NIce bridge guys!!!!!! 😀
We are making a bridge for our class at West Finchester Memorial Academy of Engineering- New England
wow i am building a bridge for a university project and this design has helped to an extent amount 🙂
Thanks! That helped a lot
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.
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.
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.
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
This would be classified as a truss bridge.
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!
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.
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
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.
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.
Should I glue the support beams on the sides of my bridge or put it on top of the base beams?
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.
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.
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?
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?
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.
I was wondering, how was the bridge loaded?
This bridge was loaded from the top with a metal plate that was attached to a hydraulic loading machine.
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
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.
What kind of truus did you use? A howe, pratt, or warren?
This bridge used a hybrid truss design. Mostly it was a Subdivided Warren, but went to the Howe Truss on the ends.
What kind of popsicle sticks did you use, just regular, or a certain kind. Also, what kind of glue?
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.
If this design was used with popsicle sticks would it work?
Probably, although you might have make some minor modifications.
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.
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.
Do you have a blueprint we can use to make a bridge like this one?
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.
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?
Hi Margie,
I don’t have the measurements of this bridge anymore, so you will just have to wing it.
Is it alright with you if I use this bridge design or a similar one in my school project. Keep in mind I will credit you for the design.
do you have the plans for the bridge if so could you send it I will credit you
Unfortunately I do not have the plans for this bridge currently.
Mr. Garett Boon, thank you for posting this well-documented bridge. I am an AP Physics student, and for our final project, we are required to construct a bridge with the best strength to weight ratio possible. I understand that, based on your previous comments, you do not have any schematics or diagrams; however, I was wondering if you had any additional pictures? Anything would be helpful in our attempt, with your permission, to recreate your magnificent design.
Thank you,
Alec
Hey Mr. Garett, we have to do a cardboard bridge project in my Physics class and our teacher is incompetent and has neither given us any parameters for grading or any idea how to build it. The only thing we can use is the cardboard and Elmer’s glue. I have no idea where to start, but I saw your bridge design and if those little wood pieces can hold 346 pounds, imagine if it was in cardboard. This is just me asking a competent teacher for some help. If not, thank you for your time and I wish you the best with future bridges!
Miriam,
Cardboard is actually much weaker than the wood on this bridge. If I were you, I would try to use corrugated cardboard, the thick kind that has extra stuff on the inside. I would also consider laminating any pieces that would be in compression, since cardboard will buckle very easily.
Check out my truss design page for ideas on bridge designs to use.
Hey Mr. Garrett, what bridge style is this bridge exactly?
This one is a mix of designs. The main section is a subdivided Warren truss, while the ends use the Howe truss.
Is there a blueprint for this bridge
No, unfortunately not.
Do you know how much wood you used exactly
and if so what were the withs
Mr. Garrett, I am not sure if you still update this site, but I was wondering if you think it matters for the diagonal members and the vertical members to be on the same layer/plane. The design I am going with has a similar truss system compared to your bridge however, I planned on placing the horizontal and vertical members level, forming something like an upside down T, and then adding the diagonal member on top of the gussets of the upside down T. Do you see any issues with this?
John, that is an interesting idea, and I’m not sure I’ve seen it before. But as I think about it, I don’t see any real issues with it. I’d be interested to hear how it works out for you.
do you think it would be a good idea to use an inverted bridge in a competition? as you only need to build your bridge to hold up a point load at the bottom of your bridge, or does the load have to go on the deck of your bridge?
Luke, an inverted bridge can be just fine.
what type of truss is this
this is the howee truss
This website is so helpful for my daughter.She is in 7 th grade and she is doing a science experiments on what type of bridge holds up the most weight. So on this website she is learning what types of bridges there are and what they look like. Thank you so very much!!🙂
yeah this site is awesome i am a Yr7 and it has helped me soo much with my maths assessment for the strongest and longest bridge
warren truss actually