My bridge is 24ft and like you, I wanted boats to be able to go underneath. I called around and consulted with several engineers primarily for the design. I should have also focused on how it would be moved and installed once fabricated. Here are some things I learned that may help:
-Make sure you are able to get the bridge out of the structure where it is fabricated so you don't have to disassemble.
-Have a plan for moving it.
-Know how much it will weigh.
Having never done anything like this before...I learned allot through my mistakes
