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Howdy Dudes,Elders,Brainstormers,and Bullshooters! Well to get to it; I have a client who wants a treehouse on the “back 40” ( more like the “back 20,000)for her two young sons. The floor span will be approx. 17’x31′, floating, as it were, between 4 large trees; one at each corner. So I plan to use sealed glue lams on the 17′ runs, and then possibly tji’s (sealed);probably doubles, to do the 31′ span. The attachment points on the trees, I imagine, to be custon fabricated fat-boy joist hangers. What I am looking for, is some way to avoid going to an engineer (engineer-phobia??)by whipping out some functional blackmarket formula, and sipherin’it out for myself. Actual Questions: How many lag bolts, and what size,per J-hanger’ what thickness of steel. Also any comments welcome. Thanks DCWebb PS You guys aren’t members of any splinter groups, are ya?
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One word:
Engineer.
Sam
*you will find some here who are splinter groupies - beyond that, project seems unreasonable - four large trees move independent of each other in the wind - racking forces will cause something to give - better set it up on posts among the trees -
*Ah, the naysayers! While it may seem unreasonable to some, I would point out the following: 1000-foot buildings in Manhattan seemed unreasonable to many. 300-foot cathedrals with no steel seem even sketchier and yet they remain standing after 600 years. And I have a friend that lives in a 12x20 treehouse supported by. . . guess what. . . a medium-sized tree in each corner.You can read about dog musher Gus's treehouse in Peter Nelson's book "Home Tree Home" (available at amazon.com as are his other 2 books). They used one 7/8" lag bolt with 6" of penetration into each tree. I used two 5/8" lag bolts, 8" into each of two trees for my sons much smaller treehouse (way overkill). Another option would be graded bolts all the way through the tree. Then it is very easy to look up the shear strength.Maybe the most relevent thing in all of Nelson's books is the point that even 10 feet feels pretty tall when you are up there.Regarding concerns about wind: I've been in Gus's (8' up), my son's (12') and one in Costa Rica (80') and while they definitely move around in the wind and a windstorm is exciting, it does NOT compare to even a wimpy earthquake. I've been 70 miles from a 7.2 and pretty close to a 6.3. I know people who've been right there for a 7.2 (Loma Prieta), 7.7 (SF '06), and 9.2 (Good Friday). If a well-constructed wood houses survive those, a little wind is a minor concern.I would build to modern codes as you would off of any four points of attachment but with an eye towards high strength-to-weight. No drywall, for instance. Use 3/8" ACX on the inside or 1/8" door skin 4x8 mahogany plywood ($12 a sheet last time I bought some). Don't use terra cotta roofing tiles. You know, stuff like that.If you want to minimize horizontal racking (your joists going from rectangles to parallelograms), you could run diagonal steel rod diagonally from opposite corners - forcing the trees to move together more of the time. The gluelams and I-joists would already do some of that, but not across opposite corners. Glue and nail the subflooring, it's a shear wall like vertical walls usually are.Do you have a building department to deal with? And sorry. You have not avoided engineers. -David (Civil P.E.)
*Why not make a "hinge connection" at each tree. Cable suspended off of eccentric connection to tree (belt and supenders) that is connected to floor system and would allow for independent movement of trees. (There would be eccentric loading involved and since I am not engineer it is up to you to evaluate.)
*Some guys are a few bricks short.What does a 17'x30' floor with walls weigh? What is the bearing strength of live wood? What is the impact strength of the wood both live and green and dead and dry?Pay an engineer it is cheap compared to the kids.
*Jeez, 17x30, that's almost as big as my whole house! What kind of kids are these? they would have more fun in an 8x8 with a 4' roof than that monster. Makes the ghost stories scarier.
*Good advice, David. I've built a few small structures in trees - single trees, between two trees, and once in a tree that, instead of a two-way fork, had a three-way fork. One thing I might add; trees grow (no shit!). This might not seen to be a big consideration when you're building, but 10 years (or more) from now it'll be a big consideration. When trees grow, they go their own way, no matter what's attached to them, or how they're encumbered. Check out the tree-lined sidewalks in a city sometime. Also, in time, they will grow completely around whatever you use to attach your structure to them (bolts, etc.). That's why sawmills won't cut a yard-grown tree without a "blade deposit". I once heard a story in NJ about a sawmill cutting into the crotch of a very large, forked, white cedar tree, and hitting the barrel or a muzzle loader that had been completely enveloped.Don't make your tolerances close to the tree too tight, or you may find that the trees start to crush your structure. Happened to the braces of a porch swing that I mounted between two oak trees once.
*It seems to me that a year or more ago there was another similar thread on tree houses with some good discussion and some links. I didn't find it but didn't look very hard, a diligent search of Breaktime may locate it. A couple of web sites:http://www.cruzio.com/~seaweb/corbin/treehouse.htmlhttp://grandoakstreehouse.homestead.com/Grandoakstreehouse.htmlThe second site also has some links to other treehouse web sites.
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Howdy Dudes,Elders,Brainstormers,and Bullshooters! Well to get to it; I have a client who wants a treehouse on the "back 40" ( more like the "back 20,000)for her two young sons. The floor span will be approx. 17'x31', floating, as it were, between 4 large trees; one at each corner. So I plan to use sealed glue lams on the 17' runs, and then possibly tji's (sealed);probably doubles, to do the 31' span. The attachment points on the trees, I imagine, to be custon fabricated fat-boy joist hangers. What I am looking for, is some way to avoid going to an engineer (engineer-phobia??)by whipping out some functional blackmarket formula, and sipherin'it out for myself. Actual Questions: How many lag bolts, and what size,per J-hanger' what thickness of steel. Also any comments welcome. Thanks DCWebb PS You guys aren't members of any splinter groups, are ya?