I want to install two posts to hang a hammock. For aesthetic reasons I dont
want to camber them away from one another as typically
suggested. What size steel tube would I need such that deflection is
basically non-existant with a typical 1-person weight on the hammock? (lets
say 200 lbs) The posts would be 6' above ground, 4' below ground. The
hammock would hang from the top of the posts.
I was thinking 3" diameter but I'm not sure if the minimum sidewall (1/8")
would be acceptable or if I should go thicker.
What size would be equivalent to a 6x6 wood post for this application? (ie.
primarily bending strength)
It's also going to depend a lot on whether you like a good solid sag in
the hammock or whether you like it tight as a fiddle string so there is
very little sag when the kids do a running jump on it. There are
several websites that deal with catenary sag and the tension applied at
the tie points for a given weight and sag. From there you can calculate
basic beam deflection using formulas in Machineries handbook or similar
based on your preference about how far the tops of the posts can deflect
under the weight.
I'd also probably fill the pipe with concrete. It will add a little
more strength against deflection and will make the posts seem less
"springy" if you choose a size on the low end of the strength scale.
Personally, I'd go fairly large and thick walled on the pipe as I think
it looks better (people perceive it as SOLID) and even when the rust
gremlins start taking over (rust never sleeps) there will be pleanty of
material there for a long time to keep things standing.
Thanks. I dont want any sag as a result of the posts, ideally the posts
would be completely deflection-free.
I will probably fill the pipe with concrete, and I will definitely be
setting them in the ground with concrete.
The posts will ALWAYS deflect under the load. The question is, how much
deflection is not visible to you (or you may not care if they move 1/2"
With regards to he hammock itself, as you approach zero deflection
(straight line hammock) the amount of tension on the posts will approach
infinity....there will always be some sag. The reason I bring this up
is somethwere between straight and hitting the ground in the middle is
the place you like it. If you like it to be taught, this will apply a
huge force on the posts (ignoring post deflection at this point). If
you can tolerate a foot or two of center bow, the forces will be better.
Everyone's different on this and cutting the bow in half applies 4
times the force on the end of the post so it does matter a lot.
*note**note**Calculations below are quick and dirty and I may
goof...this is not trying to be perfect, just show the process*
So, doing it the easy way and assuming the load on the hammock is evenly
distributed from post to post you can use the formula below to
approximately calculate the tension on the end of the post:
T= (3(L^2)W)/2S where T = tension, L = span in feet, W = weight and S sag in inches
So, let's assume posts 12 feet apart, a 200 pound guy, and that you want
the center to sag only 6 inches under the weight you have a tension on
the posts of : T = (3*144*200)/(2*6) T= 7200 pounds. Huge, aint it?
Small amounts of catenary sag cause huge amounts of tension on the
posts. Also remember that what's holding the posts in the ground needs
to counteract this force..4' above ground and 2' below can act as a
lever and pry itself out.
For safety, you have to assume that the full 7200 pounds is on one end
(it's actually jumping back and forth as you move).
So, assuming the pipe is fixed under the ground and the hammock attaches
4 feet above the ground, the formula for end point deflection from
machinery's handbook can be used: D=(WL^3)/(8EI) where D = deflection
(inches), I = Moment of Inertia of the beam, W = load on beam end
(pounds), E = modulus of elasticity of the material, L = length of the
beam (inches) . (engineers are gunna kill me for not using SI units
here but who cares?)
Assuming that 1/4" deflection is about the maximum allowed we can
re-arrange the equation and start plugging in numbers:
I = (7200*48^3)/(8*E*.25) From the table for E, steel is about
30,000,000. so, I = about 13
Now we're getting somewhere....looking at a table of I for pipe, you're
looking at a 5" sch 20 (or 4" sch 80) hollow pipe or greater to meet
(can calculate on any pipe using (.7854OR^4-.7854IR^4) where OR outside radius and IR = inside radius
Ok, so that's worst case. Most people will allow more sag than 6 inches
in the middle of the hammock and the loads on the posts will be lower.
Run some numbers yourself to see what comes out right in the real world.
Koz (who obviously wanted to avoid work today)
True, which is why I mentioned 'ideally' :-) A 1/2" sag would be
acceptable, 1/4" would be better.
The sag comes from the hammock itself stretching, not the posts. I hope this
is what you
mean. For example, if the hammock was tied to two large trees the trees
would not bend
but the hammock would not be stiff as a board. I want to simulate that
No problem. I greatly appreciate the walkthrough
Considering E = 1,800,000 for wood, the following numbers crank out:
steel 6" .25" I = 13
steel 6" .5" I = 6.5
steel 12" .25" I = 6.5
steel 12" .5" I = 3.25
wood 6" .25" I = 221
wood 6" .5" I = 110
wood 12" .25" I = 110
wood 12" .5" I = 55
I for a 6x6 beam = bh^3/12 = 76, equivalent I for a steel pipe = about 5.
Note that the
typically recommended 6x6 wood beam seems to fall somewhere in the middle of
numbers, a good compromise.
To meet an I=5 you need 3" schedule 160, or 3.5" schedule 40 (or a little
more than 4.5"
diameter hollow structural steel with a 1/8" minimum sidewall).
The middle option sounds promising.
I hope I helped you accomplish your goal :-) Thanks again!
Heck no -- I'm an engineer and I think English units are perfect for
this kind of calc! SI is a real pain when the inputs and outputs are
English anyway (weight, dimensions, especially of the pipe/tube/beam
and the tables of I and S in the books). With all the infrastructure
in the US built around English units, it'll be a looong time before we
can get SI-dim'd raw materials, hardware, etc. as easily as English
ones. Long live the King!
SI is great for dynamics, though--them darn slugs and lb-masses...
I'm far from an engineer, so I'm unable to provide specifics, but by your
description, with 4' of pipe underground, I'd be totally surprised if you
weren't satisfied with a plain piece of 3" schedule 40 pipe. It should be
the least expensive material you can buy, and is very close to your
specifications. Wall thickness is greater than the 1/8" you mentioned
(it's almost double @ .216"), and the OD would be 3.500" instead of 3".
I fully agree with the idea of filling the pipe with concrete, but I'd also
cap it when finished, well enough to prevent water from entering. If you
could keep it dry inside, you'd have no rusting except from the outside in,
and that would take a long time to destroy the pipe----perhaps not in your
lifetime. If you could tolerate the looks of galvanized pipe, even that
wouldn't be an issue.
One thing------I'd set these pipes in concrete, not just dirt. I've
watched guys install chain link fence---simply pouring bags of concrete
premix into the holes, adding water after the fact. No need to premix the
concrete before pouring. Makes the job a lot easier, and seems to work just
Sorry, I'm not familiar with the term HSS where pipe is concerned----care to
That could still let in some water. I'd be careful to preclude any water
entering the top, which will certainly not be to your advantage. You could
do something as simple as cover the top with black visqueen, draping it over
the sides, before covering it with wood. Anything to prevent water from
entering the top.
I think that would be marginal depending on how tight the hammock is strung.
Harold's suggestion of 3" sched 40 pipe should be ok but is not quite as
strong as a 6x6 post of good wood.
Note that E is a property of a material and is approx. 30E6 for any
steel. Formulae for I for various geometries are given in Machinery's
Handbook. For 3" sched 40 pipe, I=3 in^4 approx.
Note also that you don't have a simple cantilever beam. You have an
eccentricly loaded column carying both compression and bending.
A few years ago I helped my son put some posts in for a hammock. The
hammock came with a recomendation to use 6 by 6 wood posts. We used 4
by 6 posts with the 6 inch being the depth of the beam. To provide the
soil resistance of a six inch post, we put a foot or so of 2 by 6 at
the bottom of the 4 by 6 so as to have the 6 inch diamension resisting
the movement of the post at the bottom of the hole. We then dug a
trench about two feet long and maybe 8 inches deep intersecting the
hole. And nailed some 2 by 6 across the 4 by 6 on the hammock side of
the 4 by 6.
I doubt if anyone can follow this without a picture. But the result
was a really solid post.
I think I get what you're saying. Thats an interesting suggestion, thanks.
In my case
the two posts are also forming two footers for an 18" high deck. So, the
deck will actually be
holding the posts apart too, and I think the strength of this will be
similar to what you have done.
I'm too big and too old to desire a hammock any longer. But if it was
me, I'd at least string a top spreader beam in there too, at least the
size of the uprights. That way, you could convert it use as kids
swings, or a soccer style goal, or a clothes line, or some other good
use, once you get too old for the hammock too.
And be sure to remember the enormous "side loads" any hammock swinging
puts on any of this.
Our company designs shoring for excavations. Based on typical cantilever
shoring designs I'd say you need W21x122 grade 50 steel beams, 18'
embedment, centered in 30'' dia drillholes filled with structural concrete.
That should take care of any pesky deflection in the beams.
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