I am building a tree house with a bridge from one tree to another. What is the maximum distance I could run a joist for the bridge without supporting it in the middle?
Not what you asked about, but: connecting two trees with a rigid support is a bad idea. Trees move with the wind and the weather, and you'll get an enormous amount of stress in your bridge during a storm as the trees sway. It will probably damage your bridge, and perhaps the trees as well. If you really want to connect two trees (or a tree and a house), consider something flexible like a rope bridge. Of course, designing that is it's own set of questions.