Chris wrote:
> I'm not an engineer but the analysis is relatively straightforward. It's
> easy to 'simplify' the structure and see what is actually happening in
> high
> winds. (As a mathematician I'm more interested in proving the existence of
> solutions rather than actually finding them!)
>
> For the experts to suggest that guying a self-supporting fixed tower will
> result in leg failure under compression (assuming sensible guying) is
> untenable.
>
> In the case of cable dependent crankups, sensible guying can still prove
> advantageous. My own, cable dependent, crank up is rated, by the
> manufacturer, (mindful of the prime directive) for a horizontal headload
> of
> 145lbs (sic) in a 100mph wind while un-guyed and 880lbs in a 100mph wind
> when guyed.
>
> Chris
I don't think we disagree. Your tower manufacturer has evidently done the
analysis; mine has not. Therefore I don't plan to guy mine. I'm an engineer,
but of the wrong flavor for this discipline, so while I can understand the
straightforward stress analysis of my tower, I would hesitate to "simplify"
the structure in a problem with a different set of boundary conditions. My
ignorance of the discipline adds the risk of ignoring things like the effect
of a single guy constraint at the top (or 80%, or 50%, etc.) on bending
stress, for example. My gut tells me it LOOKS ok, but I am not qualified as
a stress analyst, and my tower's stress calcs avoid the guy issue.
Let's try it, though. A 'simple' analysis of my Hy-Gain 52SS with 9 Sq. Ft.
at the top (no significant mast), guyed at 52' and anchors spaced 50% yields
a vertical load of just under 900 lbs under 50 MPH conditions. My cable is
rated at 4200 breaking strength, but the winch is rated at 1500 lbs. The
winch looks like the weakest link, with a safety factor of 1.66, but there's
probably a pretty good margin already applied by the winch manufacturer, so
it appears to be OK - at 50 MPH and with a short mast.
BUT, if my tower, whose manufacturer's installation instructions do not
mention guys, were to fail with liability consequences, and it had a guy
attached, I might be vulnerable to a sharp lawyer. That's reason enough!
Chuck, N4NM
_______________________________________________
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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