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Balsa Wood Bridge

  1. Apr 18, 2009 #1

    I am not sure if this belongs here and I apologize in advance if this has been placed incorrectly. Please move it to the appropriate forum.

    I have a Balsa bridge due in about 2 weeks. I have picked up the supplies but I am not sure how to start the process and I want to do it correctly. I have the Balsa wood, cutting tools and glue.

    The requirements for the bridge are as follows:

    The bridge must be freestanding, constructed of no more than 30 linear feet of 3/32” x 3/32” balsa wood (NOT BASS WOOD) cemented with the glue of your choice. The roadbed must be constructed of a single balsa sheet of the same 3/32” thickness. No balsa sheeting may be used below the roadbed. Laminating of the 3/32” x 3/32” stock is allowed.
    The bridge must be able to span a 12-inch river with no supports in the water.
    The bridge roadbed must be a minimum of 5 inches above the water and level all the way across. No balsa structures may be above the roadbed.
    The roadbed must be a minimum of 2 inches wide. The roadbed must be solid from end to end. No holes are permitted in the roadbed.
    The maximum allowable height is 10 inches and the length may be anywhere from 12 to 20 inches.
    A clear channel of 3 x 4 inches must be provided under the center of the bridge.
    The bridge may not be painted or cemented so that the balsa wood grain is hidden.
    Your name and period must be clearly marked on the bridge.

    Anybody have any tips on where to start? I am lost. If anyone can provide a good, reliable design I would appreciate it. Any other information that would be helpful is also appreciated.
  2. jcsd
  3. Apr 18, 2009 #2
    Start here:


    I helped my son build one for a 9th grade class. He won. It may have helped that we baked the bridge in the oven to the point where the wood darkened a bit. This dried out any moisture and may have strengthened the bridge. The rules for his competition were a bridge of fixed size and then the ratio of the breaking weight divided by the bridge weight was to be maximized.
  4. Apr 19, 2009 #3
    Make it an "Arch" bridge.

    Roman bridges are still standing after 2,000+ years. :wink:
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