----- Original Message -----
From: "Steve Maki" <steve@oakcom.com>
To: "towertalk" <towertalk@contesting.com>
Sent: Saturday, April 09, 2005 5:35 PM
Subject: Re: [TowerTalk] Guying a self-supporting tower
> 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?
PVC pipe is probably a bad example, but you could probably do a calculation
for something like steel tubing fairly easily. The problem is that antenna
towers (and theatrical truss) is like a tube that is quite strong and stiff
for the weight (which, after all, is why we put antennas on lattice work
towers and not 12" diameter steel pipe). Somewhere I have calculations for
1.92" aluminum pipe (Speedrail), but that's pretty thick walled.
And, because of the liability issues, you're not likely to find "material
properties" for an equivalent structural member from the manufacturer.
Stiffness numbers for truss, you can get, but yield or ultimate load
capability, nope. Maybe you could infer it from the data in a tower
engineering package, but that's a fairly involved engineering exercise (as
in, more than an hour's work).
If someone has the properties, I can take a shot at the calcs. I'd need the
mass, the radius of gyration, the modulus, and the yield strength.. treating
it as a single column and ignoring the cross mode coupling (no
bending/torsion/tension coupling). Maybe we could model it as a thin steel
cylinder with the same diameter as the tower and the thickness set so that
the mass/unit length is the same as a tower.
There are some structural engineers on the list, and they may pitch in with
some more realistic approach.
It might turn out that the difference is negligble, but you wouldn't know
without doing the analysis.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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