I am a new hanger, and thought others might appreciate the problems and adventures I have had getting started. Am sure others have backyard stories or ideas.
I Love hiking and camping and live in Portland Oregon, but have never had an enjoyable nights sleep outdoors in a tent.
Found Shug's video's and others who convinced me to start hanging around outdoors.
My wife got hurt falling from a spreader-bar net hammock so had to convince her that a camping, fabric hammock would be safe and comfortable.
To get the process correct and convince her about it's safety and comfort. I would simply hang our hammocks in the backyard
Then my wife reminded me. We rent and have no trees, or swing sets in our backward. Our neighbors would complain if we attach the hammocks to their new fence.
Simple (I thought), maybe a couple or four 1-1/2 inch steel pipes in concrete.
But If we put up a couple of posts, to get the landlord's approval, they would have to be temporary, easily removable and not be in the way of the mowing.
Can't use concrete, that's not temporary. Can't use gravel, as it would interfere with the mowing. Also poles must be out of the way, on the edge of the 16' wide backyard.
I said that I would use A foot of PVC pipe for a sleeve, an inch below ground level, and 6 to 8 foot steel pipes slid in and out as needed.
What can go wrong?
Attachment 127592
Math Alert!
Now the problems. First what type and size of pole?
What is the lateral force on the poles? Start with a 2" steel pipe.
Did some figuring before I implemented my simple idea. Using; two 10.5 ft hammocks, hanging at 30 to 34 degrees, each holding 200 pounds(just for figuring, not my wife's weight), with the hammock bottom 20" above ground.
If the poles were 16 ft, 192 in, apart, to hang the hammocks, they had to be,
((sin (30[or 34} deg)) X (180/2)) +20 = ~6 ft tall, above ground.
The Hammock sag has more to do with the length of the ridge line than tree distance. If the ratio of the ridge line length to hammock length is 0.83, (cos 34 deg) it will hang at 34 deg. But if suspension hangs down from the trees, or posts at greater than 34 degrees the ridge line will go loose, and the hammock will sag at more than 34 deg. angle.
the Pressure per Square Inch, psi, stress on the base is;
Force X Height / Section Modulus.
Section Modulus is a number for the cross section shape. It can be looked up or figured on various web sites. SM = ~0.5 for a 2" hollow steel pipe, actually 2.3" and 0.145" wall thickness.
Using, 200 lbs for two hammocks, and doubled for safety and when sitting down or jerking ; 400 lbs
(400 X 72 )/(0.5) = 57600 psi stress on a 2" steel pipe.
Steel fails at 36,000 psi so a 2" pipe 6 ft long would bend. So much for that Idea.
Trying a wood fence post
A solid Douglas Fur wood post 4" x 4" (actually 3.5" x 3.5"), has a SM =7.15, 6 ft long above ground, with the same 400 pounds of lateral load,
(400x 72) / (7.15) = 4030 psi
the post has 4,000 psi of force on it, and wood can withstand 10,000 to 12,000 psi, depending on grain orientation, so it is plenty strong.
How deep to bury the posts, and can the soil handed the lateral stress?
Post depth rules of thumb;
*1)the hole should be 1/3-1/2 the post height above ground; if 6' above then 2' below.
2)The post needs to be sunk below the frost line. PERIOD. Local building codes state that the frost depth is 18".
3)The lateral force in conjunction with the soil type determines depth. checking with fence companies, they gave different depths most said 16 to 20", but that was for a fence.
The City of Portland Oregon's building code states that our soil (when dry, if ever) can support 1500 psi. To find how deep to bury the poles I went to The International Building Code, section 1807.3.2.1, (Equation 18-1)*
post bury depth, D =.5*A[1+(1+(4.36*(H/A))^(1/2)]
B is diagonal post dimension, P is lateral force, H is height of lateral force, A = 2.34 x (P/S), S is the soil load bearing ability .
the equation says that the post must be buried ~2 ft to handle the lateral stress. great a cheep, relatively light 8' post will give me 6' above ground and 2' below.
I have to make them removable.
To make the 4" square fence post removable I found Vinyl 4" x 4" Post wraps, (vinyl sleeve) 36" long at Home Depot that I could cut down and put in the holes around the posts.
Attachment 127593
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