increase stiffness of pole by pretensioning?

I am trying to make a 30' aluminum pole 1"dia, 0.098 wall thickness as stiff as possible while keeping weight as low as possible and also
allowing for breakdown into 6 5' sections. Would running a cable down the center and then tensioning the cable thus putting the pole under tension make the pole stiffer? thanks, rick
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demiurge wrote:

You would actually be putting the pole in compression. Only the cable would be in tension.
Pretensioning of reinforcing rods in concrete is commonly done, but that's because concrete has almost no strength in tension.
You could also shot-peen the surface. That would add compressive stress to the outer surface. That would make it stiffer and less prone to breaking. In bending, a crack would be most likely to initiate at the surface under greatest tension, and adding compressive stress would ensure an extra margin of stress before the pole failed.
What forces would be acting on the pole? What failure modes do you anticipate? I'm not sure I see the point of precompressing an aluminum pole. It might make failure due to crack formation less likely, but failure due to formation of a wrinkle or buckling more likely.
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Mark Thorson wrote:

yes, but it's not done to increase stiffness, it's done to give it "pseudo" tensile strength - by "borrowing" some of the material's compressive strength. [check into hooke's law - it's a straight line. for a material to get stiffer with greater load, the line would have to curve.]

which does not increase stiffness, only fatigue resistance.

from fatigue, not a function of stiffness.

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demiurge wrote:

no, although it is a popular misconception.
if you need stiffer, try a pole of:
1. larger diameter. 2. greater wall thickness 3. different material.
a more complete description of your application could yield more complete advice.
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I am attempting to make a pole for aerial photography/video that is in the 30-40' length range. It must break-down into 3-5' long sections. It must be as lightweight as possible, for field use (backpacking). I have a considerable supply of 1"dia 6063 Al tube, so was thinking of using that to save some money.

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demiurge wrote:

try bracing it with rope - like you see with high radio transmitter towers?

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>>> demiurge wrote: >>>> I am trying to make a 30' aluminum pole 1"dia, 0.098 wall thickness as >>>> stiff as possible while keeping weight as low as possible and also >>>> allowing for breakdown into 6 5' sections. Would running a cable down >>>> the center and then tensioning the cable thus putting the pole under >>>> tension make the pole stiffer? thanks, rick >>> no, although it is a popular misconception. >>> >>> if you need stiffer, try a pole of: >>> >>> 1. larger diameter. >>> 2. greater wall thickness >>> 3. different material. >>> >>> a more complete description of your application could yield more >>> complete advice.
jim beam wrote:

That's a good idea if the space is available around the base of the pole. Sounds like it needs to be stiffer to resist bending... Can you lash three 1" tubes together near the bottom of the pole, adding stiffness where you need it most?
--
Dave
dvt at psu dot edu
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Fill it with expanding polymer foam
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snipped-for-privacy@hotmail.com wrote:

That's a good idea. Treating the inner surface with a primer would make it even better.
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snipped-for-privacy@hotmail.com wrote:

1. what is the modulus of the foam? 2. how does it compare to that of the tube?
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For Americans it's about 30,000 psi for a foam of 15 pounds/ft^3
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snipped-for-privacy@hotmail.com wrote:

so how exactly is that supposed to stiffen a metal rod with a modulus of 10,000,000psi?
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