Balsa Wood Bridge

grabcad

If you're in a bridge competition and are limited to the amount of outside help you receive...then don't tell anyone you read past the first paragraph!! And make sure you change enough that no one recognizes your bridge. I understand the concept of people doing their own research, but the most of the balsa wood competition websites are just a bunch of arrogant snobs.They wouldn't even give one guy any hints were to start his research, when all he wanted to know was the basics so he and his teenage daughter could have some quality father-daughter time. I have a daughter, so I know EXACTLY where he was coming from. This is version 5 of a bridge I built for my Intro to Engineering class. So please take the hints and advice I'm giving you, build a few different versions and run a simulation. If you don't have SolidWorks or the simulation addon and you have a .edu email address...Autodesk offers all of their software to students for free. Autodesk Inventor can open some things from SolidWorks, but if you have any problems let me know and I'll see what I can do. Because of a 1/16" mistake when I built it, I had to snap two of the diagonal sticks on one side to fit the weight plate. This of course was failure point. However, from the first crack until a stick actually broke under the weight of testing, there was a very substantial about of time compared to the other 8 bridges. And no other stick even budged. Even with my mistake, we came in 3rd for total weight held and 4th in the weight/strength ratio. The reason why one horizontal stick appears to be higher than the one next to it, is because I taped 1/2 of the horizontal sticks together and carefully used my table saw to make dado cuts, to give my bridge some mechanical adhesion. I did this because in most of the videos I watched on YouTube, the bridges started to lean before they broke. I found the best way to assemble the bottom half was to put packing tape over a 2"x4" and spray it with just a touch of 3M adhesive. If you're patient and do it right, you will not have to sand and of the adhesive off the bridge. And forget getting anything on the bottom half set properly without some tweezers! Buying a balsa wood saw and miter block from Hobby Lobby was well worth the few dollars I spent! I also had some scrap 1"x4" that I cut to make a 2"x1" sanding block that had both a 45 and 90 degree angle. I glued that and an emery board on each side to another piece of wood. Without it you'll sand away your perfect angle that you cut in the miter block. I also cut a 90 degree wedge in the ends of the diagonal sticks to offer some mechanical adhesion as well. If you're going to bend an arch, let the wood soak for 3 hours or so. The more dark brown veins you see in the wood, the more dense it is. If any of your sticks don't have any brown veins at all, save yourself the headache and just throw it away. I used nails to make a jig, but you'll also need some 1/4"-1/2" scrap pieces of balsa wood to pad the nails so they do not make an impression into the wood. I would suggest soaking 4-6 pieces of wood or more, because I broke the first 3. I found it best to bend the arches gradually over a 20 minute period and let them dry over night. If you bend more than one at time, add spacers so the wood can dry. I'm not sure if it helped or not, but I put my sticks in a Ziploc bag with rice for 2 days before I planned to do any gluing. My theory was that more glue would be absorbed by the wood. When your gluing the end grain of a stick, add the glue and set the stick. Then remove the stick and glue it again. I know there are ways to improve this bridge, but I'll leave that up to you to research and figure out. The last hints I'll give you is that when you're adding the weight during the competition, add the weight slowly to reduce the momentum. This will allow the wood fibers to stretch slowly. And build a LEAST one prototype of the bottom half. You'll regret it if you don't!