Treehouse design & bolt questions from Merritt in Washington:
1. TABs for western hemlock? — Yes, they will work in hemlocks. Here on the east coast, the Hemlocks (Tsuga Canadensis) are under attack from a tiny woolly adelgid. I don’t believe that’s a problem with the western Hemlocks (Tsuga heterophylla) that you are probably working in. We have successfully used Treehouse Attachment Bolts in Eastern Hemlock trees.
2. Is it really a good idea to perch a cross beam on TAB/bracket assemblies or would it be better to hang the beam from a properly designed TAB/hanger assembly? My tree house will be built in four trees with 8’ and 10’ cross beams at opposing ends. Two 16’ longitudinal (6×12 No.1 DF) beams supporting the deck and house floor will float on top of the cross beams. The point of greatest concentrated load will need to support 3000# total design load. I am concerned that the cross beams will be subject to rollover if perched rather than hung. — Rollover can be an issue, but you can address that by connecting the beams to the crossing joists with brackets or angled pieces of wood. Or you can suspend them in hangers if you prefer, but we never use that method unless the tree is leaning outward.
3. How does one go about estimating the anticipated lateral movement at each attachment point? The smallest of my four trees is a western hemlock with a 13” D at the planned location of attachment which is 12’ above grade. I expect that allowing for 4” of movement will be plenty but don’t really know. The other three trees are a much larger hemlock and two larger Douglas-fir. — You can build a simple temporary ground supported structure next to or around the tree. Then go out there during a thunderstorm and measure how far the trunk actually moves at that height. Or you could develop a safer measuring process… My guess is that it won’t move more than 1″ at 12′. The way to allow movement is with floating treehouse brackets.
4. I realize that TAB producers are reluctant to provide design values and understand the numerous variables that can affect the actual capacity of each individual installation. Nonetheless designers and builders need a standardized reference value on which to base their assumptions. E.g. Values for Zll and Z _l_ and withdrawal in dry lumber of a given specific gravity as is provided for lag screws in the NDS would at least give a basis for design. Without that it’s a total WAG and that’s not a good situation for safety nor for potential loss of investment. — The primary reason why these numbers aren’t available is that there aren’t enough sales of these parts to warrant investing in testing that would cost over $25,000. If you have $25,000 or so to invest in such a venture, or know someone else who does, then please let me know. But as far as helping you out, most professional treehouse builders will review your plans and suggest hardware for $100-250, and that way you can make use of our experience and judgement, in liu of numbers, although I agree that the numbers would be more satisfying.