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acceptable angle of deflection for temporary support Answered

Hi,

I am in no way an engineer.  I'm trying to figure out what is an acceptable angle of deflection for a combination frame and hammock support.  I'm thinking of using the the 80/20 t-slot beams.  I want something that is portable, easily assembled/disassembled for moving, and lightweight.  I'm go to make a box in our garden to protect our plants and when the corn is big enough, move the box around to make a hammock support(chicken wire around the bottom part to keep the critters out).  It's basically a box 6 feet wide, 8 feet long and about 6 1/2 feet high. 4 beams to make a rectangle on the bottom, 4 uprights(1 in each corner), and another 4 beams on the top to make it more rigid.  Chicken wire will keep the pests out but let me in to relax until the corn is too high and I move the frame.  :)  I'll put the hammock diagonally from pole to pole; total weight with me and the hammock is about 250 pounds.  It will be anchored to the 2 uprights with 300lb chain at each end.

Using the angle of deflection calculator from 8020.net, I found some beams that had 5mm deflection with 150lbs of dead weight in the center of the 6 foot span.  I thought that even if I anchored the hammock to the center of the 6 foot span, that would be 300 pounds, more than the 250 max the hammock and I will be.  And anchored in the center of the beam is the worst case scenario since I'll be anchored to the corners.

But is that an acceptable amount of give in the beam?  It's not meant to be permanent.  I won't be in it more than 2 or 3 hours tops.

Thanks!

Discussions

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iceng

2 years ago

The straighter the angle of deflection the greater the force due to tension.

When fully straight the tension is infinite according to electrical engineering for high tension power lines ( That's Hydro-lines for you Canadians :-) ...

So it will never be straight, try 30%..

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Toga_Daniceng

Reply 2 years ago

The OP was talking about angle of deflection, meaning how much the posts flex or bend.

There may be another term for how straight a tightrope (or hammock) is.

That gets into vector analysis.

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Toga_Dan

2 years ago

when I want somethin lightweight, I use cable (rope, cord, etc) for the cross bracing. Look at how they did cross bracing on biplanes.

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BobR90

2 years ago

Ok, here is a really bad drawing of me in a hammock above some plants. :)

Untitled-1.gif
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Toga_DanBobR90

Reply 2 years ago

good pic. Worth 1200 words. Put a cable or rope across the 2 top corners which the hammock does Not touch. Tighten that cable. It will oppose the force of the hammock trying to flex the frame around.

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Moem

2 years ago

I can't answer the question as asked, but I can tell you that adding one or two beams in diaginal positions, to make triangles, will improve the stiffness of your contruction by a metric fluckton. Especially if placed somewhat strategically.

For example, I'd want to add one corner to corner in the top plane, parallel to the hammock.

My advice is to make a simple model out of drinking straws, add a string to simulate the hammock, put pressure on that and see what the failure mode would be. Then you'll know where to add a diagonal.

PS I like your bad drawing!

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MoemMoem

Reply 2 years ago

Diagonal. The word is diagonal. I know that.

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Toga_Dan

2 years ago

can u send a sketch of your design? Keep in mind that a tightrope with a walker on it is a force multiplier.

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BobR90Toga_Dan

Reply 2 years ago

Ok, there is a really bad drawing of me in a hammock above some plants. :)