I want to attach a 2×12 beam to a treehouse attachment bolt bracket, and I want to double up the beam, as shown (the view is from underneath). The bracket is 2" wide, and apparently designed to accommodate a doubled beam – the bolt holes are offset toward one side, to go into one board only.
I'm also considering adding 1/4" plywood spacers between the boards for drainage (as shown), which would push the outside board even further from the bracket support – essentially cantilevering most of the outside board out from the bracket.
I'd been assuming I'd attach the beams through the plywood with bolts plus construction adhesive, but I'm open to other suggestions given the rest of the situation. Assuming the two halves of the beam are securely attached, will load from the outside board still transfer properly through the attachments if only 1/4" is supported? If not, what's the best way to make sure it does?
More information: I don't have a final plan for the treehouse, but the beam will cover a 12' span, and will be one of two identical beams on either sides of a tree, supporting joists in a 10'x10' treehouse platform between three trees. Knee braces will be involved. The treehouse will probably be single-story, made of wood, shaped approximately like a cube, and won't be bigger than the platform. I assume it will weigh about as much as a wooden shed, so somewhere between half a ton and a ton and a half. It will occasionally have live loads of as many as four kid-sized humans, with varying weights and wind profiles.
More Information II: the number and sizes of the children will vary. I'm assuming standard gaussian distribution of child weights and heights, per CDC Growth Charts for the United States, Methods and Development Series Report 11, Number 246. 201 pp. (PHS) 2001-1696[1], with a target rating of .04 children per square foot, per designation RA4V (100 sqft Conference Room) of the State of New Mexico Space Standards section 2.5[2]. Surface areas and wind profiles of children assume a cylindrical child.
Edit: I've also contacted the bracket manufacturer, who assures me that the diagram below will be fine. I'd like a second opinion.
Final Edit: It was difficult to choose which answer to select as they were all so similar, but I'm convinced that such a small bearing surface isn't ideal, so I selected the first one to say "don't do it." Ultimately I decided on a different configuration which will allow centering the beam on the bracket.
Best Answer
The short answer is that no, 1/4" of bearing doesn't have value. Any substantial load would cause the soft SPF lumber you're likely using to shear off. In modern construction you'd be expected to provide at least 3/4" of bearing.
Therefore, you're depending on the attachment mechanism between the two joist members. If you were to use a suitable number of heavy bolts near the end you'd eliminate the need for full bearing. That's about as specific as I dare get with the information at hand.