Structural integrity of a hanging tent platform 1

[treeclimberr]

I am planning on building a tent platform that will hang from four trees. My plan is a 10'x10' platform constructed with 2x8s with 15" center to center joist spacing. Flooring will be the lightest plywood I can find. Will this platform be structurally sound if hung by all four corners, especially if the strain is not 100% vertical, but more along the lines of 45 deg.? All the platform will hold is a mattress and myself with a simple A-frame covered in shrink which I don't anticipate will add to the weight significantly. I'm planning on using four 1-ton come-alongs in each corner with 3/16 galvanized cable.

My fear is that the horizontal component of the wire strain will prove problematic, but I may be mistaken. Any specific building additions/precautions I need to include?

Any advice would be greatly appreciated.

 

[Guastavino]

Not trying to be smart here, but my advice is to consult with a local engineer in your area. With that said, I don't think I know a single engineer that would take on such a project. I say that only to say don't be surprised if you get referred around.

 

[racookpe1978]

It sounds like you're mimicing a "deck" (2x8 stringers and edges at 15 inch spacing with a plywood cover board) but then want to use lightweight plywood - to save weight?

If just yourself and a cot/pad/sleeping bag it appears way over-specified. If just yourself (no liability for others!) then you become responsible for your own safety - like a person building a treehouse for himself. Read a few treehouse guidebooks at the the ;ocal Home Depot or Lowes store.

Usual spacing for underfloor reinforcement is 16 inch on-center. Lighter weight will be an outside rim board, then regularly spaced 2x4's across the center. For more rigidity, glue the plywood to the 2x4's, use outdoor-rated plywood - insect proof!

Build it, suspend it a fw inches above the ground from the 4 corners, test it.

 

[BUGGAR]

You're right, the angled cables will pull things apart. Put tension cables between the hooks where they connect to the platform. There will still be lateral forces in the cable which the trees will have to support. The tree will tell you if it is not happy with your cable arrangement. When I hung things from trees as a kid, we used old seat belt webbing to wrap around the branches and hang things from and protect the tree. We even bolted it to hanging beams - amazing stuff.

 

[treeclimberr]

Thanks for the help!
Buggar-- if, instead of tension cables, I ran the cable through both pieces of wood (and steel corner brackets connecting rim joists) at the corners, would this hold it together? Instead of the tension cables relieving stress would the wood just take on a tensile stress from both ends? As long as the rim joists were properly secured and the wire ran through the bracket as well, the pulling forces on the wood wouldn't be a problem right?

 

[HouseBoy]

Sounds like the 45 degree angle is in the vertical plane. Seems to me that the connection of steel (cable) to wood and then back from wood to steel again will be the "Achilles' heel" and either a steel bracket or somehow running two cables all the way across the thing would be better. What about the horizontal plane? What will the orientation of the cables be? That will have an effect on the details (and the ease) with which you can resist the tension forces.
Seems like the weight of the deck will be a significant component. I don't recall ever hearing about "lightweight plywood". Maybe it's a thing but I would expect the joist framing will be as big or a bigger factor and I don't see the need for 2x8's. I would think mostly 2x4 framing would work. Not sure of why you are using 15" spacing either. Seems like 24" should work fine. Guess there is some "balance" between the plywood contribution and the joist contribution to the weight Maybe by "lightweight" you really mean "thin". Perhaps you have done a comparison of all the combinations. I have not so... that should help.

 

[BUGGAR]

If you need to, put some cable clamps on your cables to keep beams and things from sliding on them. We used veneer plywood, probably about 1/8" thick because that was the only material light enough to lift up into the tree. With lots of nails, it will support you through membrane (tensile) loading. No cocktail parties and no high heels though! Sounds like its time to build a model.

 

[earthsuntech]

How high up? Railings/ladder? How to adjust for level if trees/tree mount points shift? What kind of trees? Diameters? How to attach to trees? No dangerous limbs above platform? Lightening rod in adjacent tree? In spring/fall we see two/three ft diameter trees lean dramatically over in high winds.

 

[Guastavino]

I'll try to be more helpful than my original post, because, I really don't think you can find anyone local that would help, I know I would run from a project like this due to the liability and uncertainty.

Anyways, whatever you do, after you build it, maybe give it a good "load test", by throwing something heavy up there and leave it for a few days and see what happens. Maybe do that during a wind storm too just to see what it does. Last thing you want to do is be the guinea pig yourself!

 

[treeclimberr]

HouseBoy: Ahh 15" spacing was a slip of the hand-- meant 16" 2x8 might be a little beefy; but wouldn't 2x4 be skimping a little for a platform that big? Maybe I'm mistaken about that but from what I understand 2x6 would be recommended for a "normal" deck of this size. Not 100% sure of cable orientation yet. But ideally 4 cables running straight out from all four corners as high up as I can safely get them while still having enough stability in the trees.

earthsuntech: hoping to get it 10' up. Simple hanging rope ladder to get in. Hopefully six railing posts total with fishnet or some sort of lightweight rope netting in between. I'm still nailing down the exact spot to put it but as of now it looks like 4 pine trees about 1.5' diameter. I figured I'd live with a little tilting in the wind but hadn't considered extreme or potentially dangerous tilting. No branches above. Planning on tacking some wood strapping vertically around the tree, sawing a small notch into the straps then running the cable around them. Tradeoff as to how high I attach to trees. Higher up means more of a vertical force where cables attach to platform but also means more tilting, less tree strength, etc. will have to look into lightning rod as well.

njlutzwe: definitely will be letting it hang with a heavy load for a day or two before I move up.

 

[HouseBoy]

I would be very surprised is 2x4's don't "work". At 10x10, that's 100 SF and at 40 psf that's 4000 lbs.
Not sure what you are planning to do in that tent but I wouldn't guess it will involve 4000 lbs of anything.

A lot of times the "normal" framing size is needed so the floor doesn't feel bouncy. Being suspended like you will, I don't think that is a concern.

About that, (stable "feel") - If the cables are "straight out" (in plan view) I assume you mean they are basically 2 parallel cables. Seems like that will allow some side sway (like a swing). Nothing wrong with that. Just making sure I understand. In that case, I DEF would expect the cables to be continuous between two trees. Seems like the "connection" would be much simpler and the stresses placed on the wood framing would be much less complicated.

 

[BUGGAR]

Any main support cable gets a redundant double to a different branch - no questions on this.
If you haven't ridden a skimpy platform up in the trees in a strong wind, you haven't lived.

 

[Screwman1]

Why don't you just purchase a Portaledge that is designed for big wall climbing use? It is already engineered and constructed for just this use.

http://www.metoliusclimbing.com/portaledges.html

Dick

 

[earthsuntech]

One nice feature of a tree platform is that if you're out camping on your land and some stray dog walks by you won't get your tent urinated on... in addition to the fun factor.

Maybe this would be close -

http://earthsunfarms.com/Tree_Platform/

If it were me a fundamental element of my design would be that the wires going into the trees would go under the platform and "cradle" it. Seems like that would reduce stress on the platform and help prevent twisting and tearing. If it were me I'd design an extra solid square 6x6 frame, have the wires go under that and be positioned onto the 6x6 frame with heavy "U" bolts at the corners. Once they were all together you could tension or relax the wires to level the platform. Then designing the top wouldn't be any more complex of a problem than designing a free-standing deck sitting inches off the ground with concrete blocks under the corners.

For railings I'd use some left over sheep and goat fence, I've seen that on glamping setups and it actually looks acceptable and would be more than strong.

 

[KootK]

I do think that the horizontal component of the cables may cause problems. I've proposed a beefier thing below. It's quite similar to earthsuntech's concept I think.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[treeclimberr]

Earthsuntech & KootK-- ahhh both of these designs are going to help me sleep at night.. While I'm 10' in the air !! Earthsuntech, that design was exactly what I had in mind but hadn't thought to run the cables under the platform to cradle it-- perfect. I'll follow one of these concepts depending if I can find the proper tree orientation to allow for the cables to cross close enough to 90deg at the center of the platform.

Screwman1: buying a portaledge would be too easy! But I do like the idea!

Thanks for all of the help!

 

[earthsuntech]

Tree platform frame 1 -> 9 -

http://earthsunfarms.com/Tree_Platform/

Wouldn't rely on a single bolt to hold the 6x6 frame together, throw some some addt'l hardware in there. Also, I'd use fender washers and double up the nuts with the 2nd one being a nyloc and tightening them against each other. If you want to get picky for press treated lumber you should use galvanized rather zinc coated hardware. You'll easily cut yourself fooling with cut braided metal cable/rope and I'd use something substantial like 1/2 or 5/8. Once the cable is threaded through the eyebolts you can level the platform and seize the cable with a pinching device at the eyebolts. Construction of any kind of platform desired with whatever lumber you like would be easy on a pre-hung frame.

Anyway.........

 

[HouseBoy]

earthsuntech -
Not comfortable with the end condition for those eyebolts.
Lots of eccentricity too (relatively speaking).
Seems like tear-out would be a problem.

treeclimber -
I have three words of advise: free body diagram

 

[earthsuntech]

HouseBoy - thoroughly agree. Yeah, I was just coming back in to post that 8x8s are nice but more expensive and much heavier, and looking at KootK's drawing and I like that also. Absolutely those eyebolts could tear out but I believe it could be made to work with more reinforcing. 8x6s?

Also thought that running the cable through the beams might be ok but would rule that out due to the same problem with the eyebolts. Hm, I could see any variation of what anybody has mentioned being fine and holding three people for years and then the whole thing being crushed like it was made of toothpicks in heavy winds...

 

[HouseBoy]

I say "cut out the middle man". I would not even want to introduce the steel eyebolt-to-wood connection. Keep the tension in the steel and trim the wood materials down to minimum.
The real question boils down to understanding what the actual forces are.
That's why I say free body diagram will be essential here.

Totally agree with treeclimber's reply, "store bought" would be too easy!

Looking forward to pics of this!