After laying the subfloor out, and starting the rafters on the second floor, I noticed that there was a lot of sway in the building, and it moved as much as two or three inches from side to side relative to the tree trunk. I suppose this can't be a huge suprise for a small building whose whole foundations is two big beams fastened to a tree with a mammoth threaded bold!
I decided that the sway was too much, and I would design a simple method to essentially anchor the second floor frame to the tree. A lot of interesting design ideas went through my mind, as the method must somehow be adjustable, in that I want the second floor joists to remain in place, even as the tree trunk grows slowly outwards.
The simplest and most effective solution was to lag screw a short two by four brace to the tree trunk, and then use two lag bolts to attach the closest second floor joist to the. The photos below give a good view of what the end result looked like.
First photo from second floor looking down (with Rudi looking up), with the brace, main lag screw, and two lag bolts visible.
The second photo is from the first floor, with the joist, lag screw (installed using the large hole opened in the joist), and the right hand lag bolt holding the joist to the brace.