We started our bridge program last year in my beginning engineering and manufacturing classes. I wanted to make a bridge that was very complicated with simple rules that required them to have to work on the mathematics of the bridge. I wanted to stress the strength of the truss design, so within the rules, I made it so other types of bridges do not work as well. Since we are a manufacturing program, I wanted to allow for more material options. I created a rule that supports manufacturing but takes away from pre-made materials. As of now, these are the rules.
The bridge rules are as follows:
- Span is 30 inches
- Weight limit is 200 grams
- All materials are allowed except:
- pre-hardened glue (example: no plywood, but creating plywood is acceptable).
- contained gases
- I will supply 3/16 wood sticks of varying materials (right now: fir, mahogany, spruce, cedar). Material outside of this is on your own to get. (balsa is outside of our budget).
- The bridge must hold our 3 inch by 5 inch square that houses the eye bolt and carabineer. The hanger must be within 3 inches of the center of the span.
The students have access to 3D printers, Solidworks, welders, metals, plastics, wood, machining tools, CNC routers, and even composites.
Due to the manufacturing nature of my program, some groups head straight for the most technological solutions and find struggles early and often. 3D printing in today’s world seems like a logical solution to solve this problem, but it is both time consuming and heavier than they think. A few groups have played with carbon fiber and I believe that there are some amazing solutions available in composites for this assignment, these students are using the material in odd ways, which leads back to this assignment is more about engineering than manufacturing. That said, the students that spend the most time manufacturing quality joints do really well.
I push groups to design a truss style bridge out of 3/16 shop made sticks. Then use paper gussets glued with Elmer’s glue. We have learned that the gussets do really well, but they need clamping. We discovered that binder clips make the best “Bridge Clamps.”
Most of our success comes from arch type bridges and truss type bridges. Arches are made easily with 2-4 bridge sticks glued into an arch (laminated). Some of our bridges have held 120 pounds. To achieve success in my grade book, they should strive for 70 pounds. This may not seem like a lot, but the span is impressive. I felt like the common 16 inch bridges could find some success with just a stick across the span. We found that a 32 inch metal beam bridge made that falls under 200 grams limit held about 10 pounds.
My only issue with balsa is cost, in order to run a classroom program with balsa, I would have to spend upwards of 600-800 per year in balsa. A group did go out and purchase balsa and found similar results to our fir wood at a fraction of the cost.
This is how I make the sticks. I find fairly straight grained VG fir and resaw the boards on the bandsaw to 3/16 of an inch. Then I glue the ends of the resawn wood together to act as a form of clamp. Then I resaw the glued boards the other direction. This allows me to make them in bulk both quickly and cheaply. I generally make them at 36 inches as most bridges are 32 inches and with an arch it is easy to get to 36 inches. Students have done experiments with different wood species to varying success. One group discovered that cedar was much lighter, but had terrible compression strength. They used cedar on the tension members and fir on the compression members.
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