Science Olympiad Bridge
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 attemped to make a souped-up version of this bridge after the competition, but my enthusiam 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.
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?
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.
how did oyu make the bride I am in science olypiad in the bridge competition any pointers or helpful tips?
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
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.
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.
i was going to ask i am in 7th grade in we have an upcoming event that our school is doing and i was going to ask one thing i noticed about your brige was that it was not elevated but a regulation at our school is that the bridge has to be elevated to atleast 10-15 cm what do you suggest it needs to hold 15 kg and be as light as it can reply back soon the compitition is on the 23rd
Thanks
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.