I first raised ground with hoe and built the bridge with heavy foundations so they are at least light green, I don't trust orange even though it usually doesn't break. After building I used pickaxe to make the ground go as far down as I could before my character started swimming. After that the bridge just drops around 50% hp like everything made of wood without roof and that is pretty much it no need for maintenance. If you see enemies hitting the bridge that is when you need to repair them. Good luck it is pretty fun and challenging to build bridges.