[TowerTalk] A Different Take On Guy Wires?
notawc@juno.com
notawc@juno.com
Fri, 30 Apr 1999 15:39:49 -0400
All of this discussion over 60' of 45G?
Suspect Bill had the right idea yesterday about traffic on the reflector.
Norm
On Thu, 29 Apr 1999 21:15:28 -0700 Michael Tope <W4EF@pacbell.net>
writes:
>
> 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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