Canyon Physics Bridge Project
This is a practical scaled-bridge construction project. The object is to build a bridge from specified materials with as high of a strength-to-weight ratio as possible. Students will work in teams of two. Each team will compete with teams from all Physics and AP Physics classes.
Materials list for Bridge Project:
The following materials will build one bridge:
(20) 1/8 x 1/8 x 48 inch balsa sticks
(1) 1/8 x 2 x 16 inch balsa sheet
(1) 1/16 x 3 x 8 or 1/16 x 4 x 6 inch balsa sheet (24 square inches)
Glue (gap-filling cyanoacrylate “instant”), 1oz.
(1) Single edge razor blade
(All dimensions are nominal)
Mr. Lyle purchases these materials in bulk and assembles them into kits for students who wish to avoid having to procure materials on their own. These kits are generally much less expensive than materials bought piecemeal. If you decide to procure materials yourself they must conform to the list above.
Each group will also have to provide a standard-sized cardboard box of the size used for copy machine paper in which to store your project while it is being completed. Obtaining a correct storage box is the first step in compliance with the bridge project specifications and materials kits will not be distributed to groups without proper storage boxes!!
A reminder: Do NOT carry cutting tools on the Canyon High campus! All cutting tools will be provided by Mr. Lyle. No cutting tools are to leave the Physics classroom and lab area.
Using a minimum amount of specified materials, design and construct a scale model bridge which will support as much load as possible. The bridge may be tested by placing a load at any point along the roadway.
Make absolutely sure that your bridge meets all specifications! If a bridge fails to meet even one specification, the maximum grade it can be awarded is 60%. You are responsible for understanding all specifications. If you are in doubt, ask for clarification.
Note: dimensions are given in English units, as all available materials are in these units. Life is full of these little hassles. Live with it!
- The roadbed must be constructed of a 16 inch length of 1/8 x 2-inch balsa sheet.
- The roadbed must be smooth and allow unobstructed passage of a vehicle 1-3/4 inches wide by 1 inch high. Be careful, this specification causes lots of problems!
- The roadbed upper surface must be less than 1 inch above the bridge support surfaces.
2. Truss Structure:
- All other structure must be constructed of 1/8 x 1/8 inch balsa sticks.
- A minimum of 50% of the 1/8 x 1/8 wood must be used above the roadbed.
- A maximum of 24 in 2 of 1/16 balsa sheet may be used only for gusset plates to reinforce glued joints.
- Adhesives are strictly the choice of the builder. The only adhesives recommended are carpenter’s wood glue and cyanoacrylate (“instant,” “Hot stuff,” “Zap-A-Gap”) wood glue. The adhesive supplied in the material kits is cyanoacrylate If you use cyanoacrylate glue, be sure that it is intended for use with wood. Carpenter’s glue is much slower-drying than the cyanoacrylate glue, so the latter is recommended.
4. Bridge Dimensions:
- The bridge must be 16 inches long. If the roadbed is slightly shorter than this it is acceptable.
- The bridge must span a 14-inch-wide opening with no intermediate supports.
- The bridge structure must not extend more than 6 inches below the support surface.
- The bridge structure must touch the test fixture only on the fixture’s top surfaces.
- The bridge structure must not extend more than 10 inches above the support surface.
- The bridge structure must not be wider than 3 inches at any point.
- The bridge structure must not exceed 11 inches from its topmost to bottommost point.
- The bridge must not exceed 115g in mass.
(The wood provided in the materials kit has a mass of about 80g)
For a printable drawing showing these dimensions, click here
5. Construction Methods:
- The balsa wood may be laminated to construct beams, slabs, etc.
- The balsa wood may be cut and/or split in any way desired.
- Coatings (paint or glue) must not be applied to outside surface of the wood. You may use marking pens, if desired, to mark the wood.
- Adhesives may be used only for joining wood. It may not be used as a coating.
- Bridges may be made only of balsa and adhesive. No other materials may be used.
6. Testing provisions:
- There will be sufficient openings in the truss structure to allow a 1/2 inch rod to be lowered vertically at several points along the roadbed surface to apply the testing load. The load will be applied to a 3 x 1-3/4 x 1 test block placed on the roadbed at a location to be determined by the teacher.
- Each bridge will be weighed before being tested.
- Each bridge will be loaded until it fails. The maximum load reading attained will be the reading used in scoring.
- The overall score consists of the maximum load divided by the weight of the bridge.
- The highest score will be assigned a grade of 100, while the lowest will be assigned a grade of 70. Intermediate scores will be assigned proportionally.
- Any project not conforming to the specifications will be assigned a score of 60.
- All bridges must be complete and ready for testing by the last regular day of class. Groups with seniors must have their bridges ready to test at the beginning of the period on the senior's last day of class. Late bridges will be assigned a score of 60. Pay attention to the deadline! Better to be a couple of days early!
- Use of short lengths of 1/8 x 1/8 balsa glued beneath the roadbed will prevent splitting of the roadbed.
- Maximize the surface area of glued joints. You may use gusset joints to accomplish this. Butt joints are very weak.
- Most bridges fail at the roadbed-truss connection region. You should strive to spread the load out as much as possible in this region. Concentrated loads will fail the wood at the joint long before large loads occur in the structure. Gusset plates are one good way to distribute the load. Many builders also choose to construct a box structure beneath the roadbed to which to attach the trusses.
- Planar truss structures are rigid as long as they are kept in a plane. If they warp they will buckle and fail. You can prevent this by providing adequate cross-bracing between the trusses of the bridge.
- Long members under compressive loads tend to buckle. You can prevent this by cross-bracing or by increasing the cross-sectional area.
- Keep in mind that wood is a very directional material! Tensile strength is much stronger in the direction of the grain than it is across the grain. It is particularly important when cutting gussets to make sure that the loads are not carried across the grain.