TABs were designed to put high shear loads in trees without meeting either of the following definitions of treehouse attachment point failure:
- bending the bolt
- crushing the wood under the bolt
TABs and Bending
TABs are made with 4140 steel which will bend slightly under load, even at the nominal thickness of 1.25" thick shafts. To resist bending in the TAB shaft, they are usually heat treated to a specific hardness level without losing too much of the resilience under cyclical stresses. Over heat treating metal makes it brittle and dangerous. We seek the middle ground: we don't want bending, but we want it to be shock absorbant.
TABs and Tree Wood Failure
A normal 1/2" lag bolt in a tree has 1/2" of tree tissue under the bottom half circle of the installed bolt. If you put enough shear load on the bolt, you will push it through the tree tissue. This is not bolt failure, per se, but tree failure. Either way, it is failure of the Tree Attachment Point. So a 1.25" shaft would still push through most North American Trees around 800 - 2500 pounds in shear, depending on species. That is why TABs have the 3" collar pressed onto the centers. That 3" collar more than doubles the surface area bearing onto the wood. If the collar is longer than 1" (minimum currently produced), it can more than triple the safe loads without irreparable tree deformation.