David and Steve,
If I recall my statics from college correctly, the deflection for a given
applied load will
be different if the Young's modulus of the two materials is different. A tower
can be thought
of as being analagous to a cantilever beam suspended between two springs that
are
pre-tensioned. When a side load comes along from the direction of one of the
guys, the
tension in that guy increases while the tension in the opposing guy decreases.
If the
stress/strain relationship in the two guys is
linear, then the increase in force on the windward guy will equal then decrease
in
force on the leeward guy. There will be an accompanying deflection proportional
to
the Young's modulus of the guy material. In practice, as long guy deflections
are small
in comparison to the allowable tower deflections, then as Steve points out they
are of
little practical consequence. All of the guyed towers that I have climbed (both
EHS and
Philly) are stable as a rock. I would guess allowable deflections are at least
an order of
magnitude below safe limits, perhaps much more (Rohn 25 is rate at 6700 ft*lbs
of
overturning moment). Interesting mental exercise though.
Does anyone know if preload specs are driven mainly by side load deflection
limits
or torsional deflection limits?
Mike, W4EF.................
-----Original Message-----
From: K7LXC@aol.com [SMTP:K7LXC@aol.com]
Sent: Friday, April 30, 1999 8:11 AM
To: davidc@bit-net.com; towertalk@contesting.com
Subject: Re: [TowerTalk] A Different Take On Guy Wires?
In a message dated 99-04-30 10:34:42 EDT, you write:
> > What tower flex? You mean torsionally? That'll happen no matter what
> guy material you use.
>
> But is not Phillystran more flexible at the same tension than EHS? Thus
> would
> it not allow a little more sway under high wind gust conditions?
>
Flexible? That's not a term used in tower/guy specs. The momentary
'stretch' (if any - I'm not an engineer either) of these materials is of no
practical consequence. I'm assuming of course that you follow the
manufacturer's specs in all cases.
> > Vibration? Haven't run across that one before.
>
> Are not the strands of EHS more stiff (and-to-end) than Phillystran, thus
> vibrations
> initiated at any point would be transmitted to anything connected? Or do
> the insulators effectively deaden that?
Even *if* measureable, it's of no practical consequence.
>
> > Joint stress? Leg compression is the biggest factor in tower
forces.
> > The leg joints are stronger than the rest of the leg.
>
> I was imaging the tower swaying a bit in 125-150mph gusts and rocking
around
> the connecting bolts. (Sounds like something from a song, "rocking around
> the tower bolts, have a happy hurricane ..." :-) )
>
The whole tower structure is designed for different windload/windspeed
scenarios and they DON'T rock.
> > While I don't have the technical data in front of me, if you're
> asking
> > about possible elongation of EHS and Phillystran - for our ham purposes
> they
> > are both small enough that you don't have to worry about it.
>
> Not permanent elongation but momentary under high stress moments?
Even *if* measureable, it's of no practical consequence.
>
> > Here's a suggestion - use the EHS-with-insulators on the bottom set
> and
> > 50% Phillystran/EHS on the top set. I think this'll reduce your risk
> exposure
> > while giving you some Phillystran benefits.
>
> 50%? I would use a 4 guy top rather than three? Will have to explore
that.
>
What's a "4 guy top"? 50% refers to the top half of the guy being
Phillystran and the bottom half being EHS w/insulators.
Most of these questions that you are asking are things that you are
perceiving that don't have any practical considerations. You're over-thinking
the whole structure. Follow the manufacturer's instructions. If you're
worried about Phillystran being more likely to be cut by things flying
through the air - then use EHS w/insulators. And get an engineer involved.
Everything else is background noise.
Cheers, Steve K7LXC
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