Hi Tom,
I see your point. Adding guys will distribute
the compressive load over all 3 legs of the structure,
rather than having it load only one.
Even if the structure is pre-loaded with guy tension,
it should still have the load distributed more widely,
and therefore should withstand higher windloads.
While I agree that guys convert the overturning moment to
compressive force. In the original post, however, we were
talking about failure modes. The objective was to have the
tower fail without hitting the power line.
If there is no guy pre-load, however, then there is NO CHANGE
to the system...the guys are effectively not there until the
tower fails.
In the case of the X-braced, stamped metal, HBX series tower,
there is also a torsion problem. You'll note the severely limited
boom length rating for those towers. 12', I think. The structure
corkscrews. I saw a kt34a...a 16' boom, rotationally flex an HBDX48,
when the rotor brake was dropped in, and the rotating load was
imposed on the tower!
Which brings me back to my observation that a structural analysis
is required, to determine the impact of the guys. While I accept
the load-distribution argument, in general, I am not convinced
that there isn't a weak point in the system, which might be
aggravated by the pre-load.
For example, the smaller sections on top peak-load-shed by flexing.
What happens if those peaks are transferred to compressive load by
the guys? I don't think that can be answered without a fairly
rigorous analysis.
Which returns us to the thrust of my initial comment: FOLLOW THE
MANUFACTURER'S INSTRUCTIONS, DON'T FREE-LANCE. And what I didn't say,
but was thinking: OTHERWISE, YOUR LIABILITY INSURANCE CONTRACT WON'T
COVER YOU. If the tower came down across the power line, and they
dispatched a crew to repair the problem....you have to know they'd
want to recoup their expenses.
Why do you think every tower manufacturer offers a safety warning to
stay away from power lines? It's not because they're worried about you.
It's because their liability insurance carrier mandates it.
n2ea
jimjarvis@ieee.org
-----Original Message-----
From: Tom Rauch [mailto:w8ji@contesting.com]
Sent: Monday, April 11, 2005 09:20
To: jimjarvis@ieee.org; towertalk@contesting.com
Subject: Re: [TowerTalk] Guying free-standing towers: the initial post
> There was ONE direct answer...keep it loose and you'll
> be fine. Then the thread took on a life of its own.
Yep.
> Let's be clear, however: Tensioned guys add load to a
tower.
> Therefore, the tower will reach its material failure point
> sooner. Might be 5%, might be 50%, depending on how much
> you stress it, and where.
Jim, I seriously doubt that statement is true.
The reason is pretty basic. Without a guy line and with
wind, one or more legs of the tower go into compression
while one or more legs are under tension. Failures are
generally compression failures, where a leg or leg buckles,
and a self support has to be planned so ONE leg handles all
of the load.
Not so with a guyed tower.
When we guy a tower, we spread the load out over a much
larger base and the vertical tower legs are under
compression, and that compression is MUCH more evenly
divided between all the legs instead of perhaps falling only
on one leg. Not only that, the reduced angle between the
horizontal (sideways) force and the countering tension of
the guy means there is significantly less downward force
transfered to the tower legs, and all the stretch is
transferred to the guyline(s). I can't see anything
happening except tower stress being reduced under high wind
loads.
> The function of the guys, is NOT to add strength to the
> system. The function is to force a different failure
mode,
> and the price of doing that will be reduced load rating,
or
> earlier failure, if loaded to original specification.
The primary function of the guylines is to absorb horizontal
forces, and when the guy tension increases it converts some
of that force to downforce or compression. While I agree
everyone should check with the manufacturer, I seriously
doubt adding guys with a wide footprint is going to make a
system fail at LESS windload than it would without them. The
only exception would be if the installer did something
really stupid like using a huge guyline or guy tension a
significant portion of the compression limit of the tower
legs.
I'm sure we could MAKE the tower handle less windload by
adding guylines that were grossly oversized, but I'd bet
money that proper guying would greatly increase the windload
of any self-supporting tower that has a narrow base width
compared to its height.
73 Tom
_______________________________________________
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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