Jim Lux wrote:
> Consider this as an example. You have a piece of 1" PVC pipe as an
> unsupported vertical mast. It's pretty flexible, but will actually take a
> fair amount of side load before failure (flexing a bunch in the process).
> If it fails, it will probably fail at the base (because that's where the
> loads are highest). This is the standard canteliever beam thing.
>
> Now guy it. You've increased the total down force on the pipe, and
> constrained the position of the top of the column, changing it from a
> fixed/free configuration to a fixed/pin (or maybe a pin/pin) configuration.
> Now the failure might be more likely in the center (the buckling failure).
> You might want a much stiffer tower in this case, to reduce the amount of
> bend from the axial load. When you put a side load on the tower, you do two
> things.. one is that you increase the total down force because of the
> increase in the guy tension (unlike in the free standing, where the total
> down force remains the same), the other is that the wind load tends to bend
> the middle of the tower away from the wind (the top can't move, because it's
> constrained by the guys). Both of these increase the chance of buckling.
Good example - just the type of thing I was looking for,
except that it has so much flex that it's wind area is
reduced (through extreme bending) way before failure.
Heck, the tip may hit the ground before failure. So it's
not quite fair.
But, the question remains: does reasonable guying of this 1" PVC,
at any height which is self supporting, even with just one set of
guys at the top, compromise it's capabilities in any way?
Steve K8LX
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
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and ask for Sherman, W2FLA.
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