For this project, my partner and I had to make a model bridge that has the greatest strength to weight ratio in competition against other teams in our class using limited supplies. The span to be bridged was 11 13/16". We had to follow the guidelines from the official bridge building competition (http://www.pspe.org/lincoln/bridge.shtml).
Materials given:
The bridge shall be constructed from basswood sticks with 3/32 inch square cross section, which may be notched, cut or laminated in any manner.
The bridge shall be constructed using the basswood sticks and glue. Any commonly available glue may be used, but the basswood sticks must very obviously be the major component. (Bridges built primarily of epoxy or other glue with sticks as secondary members are not permitted. Wood bridges encased in epoxy or glue are also not permitted.)
No materials other than glue or basswood may be used. DO NOT paint the bridge.
Constraints:
The bridge mass may not be greater than 30 grams.
The bridge must span a gap of 300 mm (11-13/16 in).
The maximum allowable bridge dimensions are: i. Length: 450 mm (17111/16 in) ii. Width: 80 mm (311/8 in) iii. Overall Height: 140mm (511/2 in) iv. Height from Supports to Deck (Load Point): 40mm (119/16 in)
No part of the bridge may extend below the support surfaces.
The bridge must allow for mid-span loading a 9.53 mm (3/8 inch) diameter loading rod will be attached from below to the center of the loading plate.
In the pictures below you can see the bridge that my partner and I created. Our bridge weighted a total of 26 grams and fit all criteria. We were not given much time to complete this project, therefore much of this project was done outside of school. We used Elmer glue and basswood for our bridge. Our bridge came in 3rd place holding 16.4 lbs. Originally we were in second place but another teams bridge was not ready the day we all tested; therefore, getting an extra day to make improvements to their bridge, when they tested their bridge, they passed the amount our bridge held. One thing that would have made our bridge stronger was putting beams at the top of the bridge. If we were able to do this project over again, that is the one thing I would have changed about our bridge. Otherwise, I believe our bridge did very well, it looked very clean and professional in the way that it was put together.
My major take-a-ways from this project were that following criteria is very important and that it is better to be safe than sorry. Also time management is something that I learned from this project, especially with the time that we were given to construct our bridges. This project took a lot of time and preciseness. Cutting the wood perfectly was important so that all the pieces went together the way they should.
Here is the video of breaking our bridge (sorry for the conversations in the background): https://drive.google.com/open?id=0B06zojxZFzZGdzNRQnM2WEhVZ3c
Here are some pictures of designing and constructing our bridge: