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.
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.