1. 42 inch clear span
2. The center must hold 10 lbs, and fail at 15 lbs.
3. Provide a 6x6 loading platform at the top of the center of the span.
4. The maximum is 6x6 that each module can be to build the entire bridge. (To be used in tension, compression, and connections.)

Any ideas or websites that I can refer to?

Thanks in advance. I'm thinking of using triangles, but no particular shape or form is required. Only the requirements above. I do have access to a laser cutter.