Building a simplified balsa wood model of a bridge is a way to learn about the many issues important to the design of real civil engineering projects. More than just the types of trusses and the importance of triangular supports, it is a way to learn about the inevitable trade-offs and optimization problems that arise whenever scarce materials must be used to achieve a goal in the most efficient way possible--a characteristic of all engineering projects. This project uses a piece of freely available software to help you design a truss-based bridge and determine its failure modes prior to building the real thing.
Design Your Bridge
Follow the instructions on the West Point Bridge Designer website to download and install the program.
Open the West Point Bridge Designer program and select the option "Create a New Bridge Design." Click "next" until you reach step 3, where you will set out the basic guidelines for the bridge design. Leave the default settings in place, as they are correct for a basic truss design and hit next. Select the option for "high-strength concrete" and hit next again to reach the template selection screen.
Determine which template you wish to use (if any) for the bridge design you want to build and click "Finish."
Follow the guidance of the template in assembling the bridge design, but remember to use only two sizes of beam (300-by-300 and 150-by-150, for instance) and solid beams only in order to simulate that the balsa bridge will be construction of two sizes of balsa wood strips.
Test your bridge design using the testing animation in order to determine whether it is strong enough to withstand a load. Decrease the size of the bridge members and rearrange them to use less material until the bridge is just strong enough to withstand the simulated test load. This is the optimal design by way of cost-analysis.
Close the member-analysis panel on the right side of the screen and print out your bridge design.
Build Your Bridge
Lay the design printout on top of the work surface you will use to build the bridge. Lay wax paper over it and use pins to hold the sheets down at the corners (you can use tape if you want to avoid damaging the work surface).
Measure lengths of balsa strip-stock against the structural members in the design and cut them to length with the hobby-knife. Lay the bridge components on the wax paper over their corresponding positions in the plan and glue them together with epoxy, using pins to keep them in place (pins should go on either side rather than through the structural members). Leave the assembly to dry for an hour.
Gently break the completed truss loose from the wax-paper and set it aside. Pin/tape down a new sheet of wax paper after removing the old one and repeat the process above to create a second truss section identical to the first.
Join the two truss segments together, side by side to form the bridge, by linking them with more 1/8-inch balsa strip stock segments wherever there is a joint between two or more pieces in the trusses. The length of these joining members will determine the width of the bridge.
Test the load-bearing capabilities of the bridge by using it to join together two equally high piles of books and laying or hanging weights on it until it fails.
If you have any difficulties with the bridge design software, use the help functions to assist you.