bob smith wrote:

"Koz" wrote in message
...


bob smith wrote:



"Anthony" wrote in message
.18...




"bob smith" wrote in :





Hi,

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)




www.efunda.com
look for cantilever calculations




Thanks, but that site costs money to signup (it seems they let you do 2


free


calculations first).

Also, I cant quite figure out how E and I are affected by sidewall
thickness?





--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

http://www.machines-cnc.net:81/








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.

Koz




Thanks. I dont want any sag as a result of the posts, ideally the posts
would be completely deflection-free.


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"
inward).

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
that deflection.

(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)





I will probably fill the pipe with concrete, and I will definitely be
setting them in the ground with concrete.