This bridge won the 2004 GA State Science Olympiad Bridge Building competition. It weighed 9.47 grams, spanned 35cm, and held 15kg without breaking. The efficiency score was 1584. This bridge was made completely from Basswood. No Balsa or other wood was used.
The top chord was made from 2 sticks of 1/16″ square pieces, laminated to a 1/32″ x 3/16″ piece. I steamed the wood for the top chord before bending or gluing. Learn more about bending wood.
Theory
This bridge had one load point, in the center of the bridge. I tailored the design to maximize this fact. All the truss members held spread the load from the load point to various points on the arch. Interestingly, this design works very well and is pleasing to the eye.
I do not believe I maxed out this bridge. I probably could have shaved off a couple grams and the bridge still would have held the entire weight. I could have taken off most of the weight on the arched chords, but also some on the bottom tension chords. I attempted to make a souped-up version of this bridge after the competition, but my enthusiasm gave out and I never finished it.
I used Basswood on this bridge because at the time, I believed basswood was inherently better than Balsa. Now I know that isn’t true. However, I would probably still use Bass because it is harder to bend Balsa wood. This bridge does have enough of an arch to make bending balsa wood to fit a problem. It could be done, but perhaps it isn’t worth the trouble.
Still good in 2007
Just recently I got the bridge out and decided to test it again just to see how it would do. The bridge did much better than I thought. In fact, the bridge remains unbroken. I ran out of sand at 40 pounds. This gives the bridge a new efficiency score of 1920, which is quite a bit better than before. I was surprised that the glue joints held up. This only furthers my belief that Probond is a really good glue. I now have tested this bridge six times, and it is still going strong. And since the bridge is not broken, I may try and test it further later on. I imagine the bridge probably could hold 50 pounds, and possibly more. It may be able to get a score of close to 2500, which would put it close to the national winning bridge back in 2004.
Hey I have to wonder, being tested so many times, do you think possibly that the joints are weakened? And if you were to rebuild the bridge using balsa (and test it) That i would do much better, considering that it is new built and the structure has not been weakened yet?
Cyrus, you bring up a good point. There is always the potential that testing a bridge multiple times weakens it. However, I don’t think I have come close enough to this bridge’s failure point to cause much damage. I am worried about the age of this bridge. I built it in 2004, and it is now 2009. I don’t know how well the glue holds up over time. I meant to test this bridge again over Christmas break, but just never got around to it. I imagine it can hold 50+ pounds.
It’s been sooo long! You should test it again when you get the chance. I’d like to see how well it does this time!
Yes, it has been a while. Right now this bridge is on display at my house. My wife won’t let me test it to failure though, she thinks of it as permanent decoration.
Do you have any 3 way design views or any design specifications that you could share?
Hey, Did you use a 1 to 6 height to width ratio?
For this bridge I used an aspect ratio very close to 1:6. The bridge was 14.5 inches long and 2.5 inches tall. If you divide 2.5 by 14.5, you get 0.17. This is very close to 1/6 which is 0.167.
I’m new to bridge building, but I noticed that you said the bridge pictured spanned 35 cm. Then you said that it was 14.5 in long. These values don’t match, and I was wondering if the span and length of a bridge were different things. Also, when you say that the bridge is 2.5 in tall, is that vertical height? The previous comment was addressing width, so I’m a little confused. Thanks.
Phoebe, thanks for your question. To clarify, the I define the span of a bridge as the distance between supports. In this case, the bridge spanned 35cm. However, the total length of the bridge was longer, 14.5 inches. This allowed the edges of the bridge to rest on the supports, with a little less than 1cm of overlap on each side.
This bridge was 2.5 inches tall. It was about 2 inches wide. The previous comment asked about the 1:6 ratio, which refers to height:length. Thus, I assumed that they meant length and not width.
The span is defined as the open space that it goes over. The length is how long the bridge itself actually is.
out of curiosity, where was the loading point on this bridge. Was it on top of the bridge or “inside” of it?
The loading point was on the inside of the bridge.
I built a bridge similar to yours for Science Olympiad, but during the competition the loading block fell through the bottom of the bridge. The bridge was fine; only the parts holding the block were broken. I tried reinforcing the center, but the bridge started bending sideways. So what are the dimensions of your base, and how did you get the block to stay in place?
I was wondering if you thought a arched bridge like this was better than a rectangle like shape with a lot of equilateral triangles? i was wondering because i was interested i making a arched bridge. I’ve never made one before but they seem good. go ahead and reply at any time
I have always used a rectangle bridge they weigh alot less
how did oyu make the bride I am in science olypiad in the bridge competition any pointers or helpful tips?
The trick with bending balsa is to use thin strips laminated together to make a curved beam, rather than trying to bend a thicker beam.
My arch bridge used an inverted arch under the load bed for the main beam (smile instead of frown), with warren-like truss members from each load point on the road bed to the beam. This puts the longer arch member in tension and the shorter, straight loadbed member in compression.
I tested 2 to failure and found weak points in the design that surprised me. The final version scored 1834 without breaking, which was good enough to win the CO state 1987.
The following year I used a *very* simple, *very* light simple beam design that scored 2150 even though it always broke under a light load.
My favorite glue is a rubberized cyanoacrylate (CA) that you can find in hobby shops. You do need to be careful with it, though, because it will bond skin like superglue and has the same sort of fumes as superglue.
Im doing a science project am i have to build a bridge out of this thin wood. The nicer looking designs dont leave big enough spaces for the weights. the holes are too small and it is impossible to shove the metal stick through. we need to put those metals in to see how much the brigde can handle. i can not enlarge my scale. Help piease? ideas?
Can you explain a little more about what you mean? What holes are you talking about?
I suggest that you just leave a hole in the center where the metal thing has to go. Just leave an open space where you need it and then do the supports everywhere else. That is what I am doing for my Science O bridge.
Hi Mr. Boon,
How did you get the bridge to be made of only bass AND weigh 9 grams? Your bridge was even strong enough to hold the max capacity! Was there a reason you designed the bottom to be mostly empty? What size was the basswood you used?
My friend and I have been trying to make a bridge that can hold max weight and be at most 15g for an efficiency score of 1000. We’ve only gotten to 578 with an arch balsa/bass bridge that weighed 20g and only held 12 kg.
Zoey,
This bridge was actually overbuilt. If I had to build it again, I would make the bottom chord smaller, and maybe make the top T Beam smaller as well. The national winner from the same year as this bridge was around 7.5 grams if I remember correctly.
The bottom of this bridge is mostly empty because it only needs to hold weight at the very center. You can see the reinforcement at the bottom center. This was to insure that the bridge didn’t twist in the center. The lateral bracing (the X’s) on the top also help keep the bridge from twisting. This is critical to the strength of a light bridge. Similarly, it is very important to build your bridge perfectly vertical, with no lean at all.
Balsa and Bass have different characteristics. I’ve heard of great bridges built entirely from Bass, and others entirely from Balsa. I prefer working with Bass because I find it more consistent.
Keep in mind that the Science Olympiad bridge rules change every year, so the what designs worked well back in 2005 may not work in following years. Have you checked out the SciOly Wiki? It has a wealth of information.
http://www.scioly.org/
Also check soinc.org or your rule book, as they may have differing info from the SciOly wiki.
Hi! I am building a bridge for a a science Olympiad competition. I read that medium viscosity CA glue was really good for building bridges. Which do you think would be better, Probond or medium viscosity CA? Thanks!
CA is very common in Science Olympiad. For most cases, you won’t need the strength of ProBond in SO, so I would stick with the CA glue.
Dear Mr. Boon,
So is the top chord made from 3 strips of bass wood laminated together and then bent or is it a single piece that was steamed and bent? Also what is a T beam, and how does it differentiate from an I or L beam? I’m making a bridge for Science Olympiad and was hoping to make something of this design since it seems to be extremely powerful.
thanking you,
Tupur Lahiri
Tupur, the top chord is made from 3 individual pieces of Basswood. Two were 1/16″ square pieces and the other was a 1/32″ x 3/16″ rectangular piece. These pieces were individually steamed and bent around a jig to make the arch. After each dried, they were glued together to make an T Beam. A T beam is an engineered beam that looks like a T. It increases the stiffness of a beam by having a larger cross section in both the x and y axis while cutting weight compared to a square beam.
It is very similar to an I or L beam, but easier to make. I think it would almost impossible to make an arched L beam in a model bridge.
Hi Garrett, What is the highest efficiency you have heard for a 35 cm span, 5 cm min width, 15 kg max load bridge? Those are the S.O. Class B parameters for 2015. On the S.O bridge building DVD they shows bridges with efficiencies up to 3800, but they it doesn’t say what the max load was that year. To get 3800 with a 15 kg load, the bridge would have to weigh less than 4 g, and from the video the bridges in the video did not look to be that light.
Bill, there have been SO bridges in the 3 gram range. I can’t remember the name of the school offhand, but heard about it from the Science Olympiad Student Center, scioly.org. The max load was 15kg that year. I don’t think it has been changed since I started in 2000. I haven’t been following the past 3 years or so though. The best I had personally was the bridge on this page, although after this I stopped working on SO bridges. I probably could have shaved a gram off this design if I had kept working on it that year. In 2004, the national winner was around 6.5 if I remember correctly.
Hi Garrett,
I am curious about how the loading block can be “inside” the bridge. Do you mean on the second layer on the bottom? Wouldn’t that be extremely weak? What do you mean by inside?
Sam, it is best if the loading block sits on the top of the bottom chord. This bridge had an additional piece of wood that was glued to the inside of the bottom chord (1 on each side) that the loading block sat on. This was not ideal, but it did work. If the loading block sits on something glued to the bottom of the bottom chords, that would be very weak.
Hello Garrett,
My Science Olympiad Bridge team it trying to replicate your bridge on a larger scale (using 50 cm on the bottom), and wanted to consult you to find a good height for it, and how to spread out the supporting pieces accordingly. We cracked some numbers, and came up with a length x width x height of 50 cm x 5 cm x 8.5 cm, with our total arch rounding out at 54 cm and having our supporting pieces every 3 cm apart from each other as they spread out on the sides. Any suggestions?
Bo Rounds, I am trying to do this as well. your numbers seem correct to me. How much weight will this add? Also, is that an L-Beam on the arch? How is this constructed?
Hi Garret
I am trying to rebuild this bridge for my engineering class. I have a few questions about how you built it.How far apart were the lateral bracing in the top? How did you get the Basswood to arch? What size Basswood did you use? Did you only use lap joints throughout he bridge?