Hammock Hang post strength

[zooshooter]

I've read a few threads about which size of post to use when creating a "permanent" hammock hang site and it's got me wondering. Lots of folks say to use at least a 4x4 square post with a 4x6 or 6x6 being preferred or a similarly large round pole. I've got a very small background in theatre scene building and learned that an uncharacteristic configuration of weaker lumber can yield something much stronger than either component alone. My question then is, is a square post strongest when loaded squarely, or would loading it at the corners yield better strength? More explicitly, if we screw an eye bolt into the flat face of a square post will that be better than putting the same eyebolt into the corner of the post in a diamond configuration?

[gnar]

i would think the angle way would be stronger as far as the post, but would the thinner area thats holding the bold be an issue?? i am thinking out loud........

[nothermark]

Interesting question. I will find other answers interesting.

If you look at the hang angle discussions you can get a sense of the forces involved. It can be quite high in the direction of the hammock.

Sideways deflection is another matter. I suppose if you used a bottom entry and kept exactly centered there would be no significant forces. Since we swing or side load that has to be accommodated.

Now lets look at the other end of the post. If you assume it is a lever with the fulcrum at the bottom of the hole how solid is the material you are anchored in? For dirt I would think the flat side would provide more resistance to enlarging the hole. That is a significant issue. If you set the pole in concrete that would change the shape in the soil and increase the surface area. In that case the diamond would give you the diagonal web thickness to stiffen your pole so it would be better. There is room for a lot of fudging there like how much concrete but I think you get the picture. ;-)

[fallkniven]

of course, that's how building works. Someone on another recent similar thread mentioned he used 4x's as uprights, which will bend and warp, but he nailed another piece across the two to keep them in place. You could do the same thing by staking out the uprights from behind.
If you screw in the eyebolt of the flat side of a 4x4, it will start to bend forward over time, but if you turned that post to the corner, it will still bend, spinning it won't stop it. It can still lean on the angle, but it might cause it to turn a bit and lean to one side or the other, but that's just a guess.
Use what works for you, 2 big pilings cemented into the ground would be simplest and easiest, but you can acomplish the same thing with just a few 2x4's. It's really up to you, what you have/can afford...

[oldgringo]

Round posts with the eyes placed on the corners work for me.

[OneEye]

Loading on the diagonal does not change the stiffness. The moment of inertia is (d^4)/12 for each.

Loading on the diagonal increases the maximum stress by sqrt2 at the corners. Squarely loaded, the section modulus is (d^4)/12, vs. diagonally loaded section modulus of (d^4)/(12*sqrt2).

That's about a 30% loss of strength by loading diagonally.

Shear will be distributed differently, but is less important than bending strength and stiffness for most applications.

That's just for the post itself, not the post-soil interaction for an embedded pole.

[zooshooter]

Loading on the diagonal does not change the stiffness. The moment of inertia is (d^4)/12 for each.

Loading on the diagonal increases the maximum stress by sqrt2 at the corners. Squarely loaded, the section modulus is (d^4)/12, vs. diagonally loaded section modulus of (d^4)/(12*sqrt2).

That's about a 30% loss of strength by loading diagonally.

Shear will be distributed differently, but is less important than bending strength and stiffness for most applications.

That's just for the post itself, not the post-soil interaction for an embedded pole.

See, this is why I knew this was the place to ask this question and is exactly the type of answer I hoped for. You guys are all awesome. Cookies for everyone!

[OneEye]

Loading on the diagonal does not change the stiffness. The moment of inertia is (d^4)/12 for each.

Loading on the diagonal increases the maximum stress by sqrt2 at the corners. Squarely loaded, the section modulus is (d^4)/12, vs. diagonally loaded section modulus of (d^4)/(12*sqrt2).

That's about a 30% loss of strength by loading diagonally.

Shear will be distributed differently, but is less important than bending strength and stiffness for most applications.

That's just for the post itself, not the post-soil interaction for an embedded pole.

Oops. Ha ha, my % difference was correct, but I fumbled copying the equation for the section modulus. The d is cubed, not to the 4th power, and the divisors are 6 or 6*sqrt2, not 12 and 12*sqrt2.

The conclusions don't change, because the ratio is still the same, but I didn't want anyone taking the section modulus equation and suddenly thinking their 4x4 post had 1.75x its actual capacity.

[nothermark]

A word on post selection. Wood warps due to different shrink rates of the layers in the growth rings. Try to get one center cut cut out of the log. Chance of warpage is significantly lowered. If you cannot do that look at pictures of how wood warps and set the posts so the warpage counteracts the stress from hanging. ;-)

[OutandBack]

The problem I have with 4x4s is the amount they flex from your weigh.
If you want to use a nice tight tarp it will get really saggy when you climb in the hammock. 6x6's don't seem to flex much at all with my 200#.
Just something to consider.