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[TowerTalk] Cable Stretch is Important!

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Subject: [TowerTalk] Cable Stretch is Important!
From: ni6w@yagistress.minden.nv.us (Kurt Andress)
Date: Wed, 05 Aug 1998 00:43:18 -0700
Following up on the discussion about the Loos wire rope tension gage:

The Loos gage is designed to function over a range of nominal sizes and
wire rope constructions. The accuracy of the gage readings are dependant
on the transverse bending characteriscs. The transverse bending
characteristics are dependent on the cable strand construction. The
construction is determined by the strand count and the heilical twist
ratio of the construction. The wire rope construction determines the
flexibility and stretch coefficient of the cable. There is a valid
reason why manufacturers tell us to use a certain cable for a certain
application.
There has been some discussion on this reflector about guyed towers and
what is happening in them. I have seen arguements about why (or why not)
a Mfgr, or other somebody, tells us to use (or not use) a pier pin
foundation, some fellow put a bunch of towers up on concrete block
footings because "there are not lateral loads on a tower base", guy
wires don't stretch, and a host of other fairytales about the subject.
Unfortunately, I've run about 100 linear FEA models of the tower
configurations I might put up here and found that the guy line
properties are very important to the structural integrity of the
installation. The Linear FEA reference is only important to subscribers
who have "nut and bolts" between their ears, insted of electrons! The
laymans explanation is that it is software that models mechanical
things, much like your beloved electrical design software.

If you have lots of electrons between your ears, and don't need to get
confused about guywire stretch, press the "Delete Message Button" NOW!

Because of the Loos thread mention of cable stiffness, I thought this
may be an opportune time to ask the gang to think about this!

Guying cable stretch is a significant factor in deternining the loads in
a tower! Over half of the time, when designing a guyed sailboat mast
(part of my real job), the guy sizes are determined by their stretch
characteristics, rather than strength. The problem occurs when the
supporting cable stretch is too large and causes the guyed structure to
fail due to bending. The guying material was just fine (not even near
failure), but it stretched so much that the vertical structure it was
supporting (the mast) falls down and ruins everybody's day!
Selecting guy sizes on the tensile load alone, will only tell you if the
guy is going to break. How much it stretches determines how much the
tower (or mast)  will bend, and hence, how much stress is in the
vertical column, and hence, when the tower sections (or mast) will fail.
This is why there are lateral and bending loads along and at the base of
a guyed tower.  It is not all purely vertical compression. This is why
Rohn puts a little box in their tower section drawing that states the
"Safe Moment of Restraint" for the section. This value, in Ft-Lbs of
bending moment, tells us how much bending the tower sectiion can
withstand. If we use infinitely stiff guy cable, that doesn't stretch
(like everybody wishes for) the tower sections become purely vertical
compression members, and we only need to look at the compression loads.
This makes everybody's life easier. Anybody with the desire, the right
book, and a $12.95 Radio Shack calculator can do the math!

My, unpopular job is to say that the guy cables, no matter what they are
( even un-obtanium, which you can't afford) will stretch and cause your
guyed tower to develop lateral   bending loads, contrary to popular
folklore! This is why every fellow who has tried to verify the
manufacturers load case constraints has ended up telling us that the
manufacturers are "covering their ...?" and making us build something
"way to strong" (I.E. They won't let me put the pile of stuff on my
tower I want to!).

Now we should all understand that all of the best analytical techniques
and tools are not a replacement for good emperical evidence (I.E. My
buddy or I did this, and it worked ok). We need the results of emperical
experience to help validate the analytical tools we use! This is just
like the antenna electrial design software we spend countless hours
exercising. It's really neat to find out that somebody put up an antenna
that played like the softeware predicted.
If you know of someone who has had a tower survive wind loads that you
expect to have, then by all means copy his configuration. Remember, that
the extra 5 SqFt that you will put on it, will be your own
responsibility.

Back to the guywire deal:
The short version of this one is: If the wire has lots of strands in it,
it has been designed to roll around a sheave (or thimble) easily, don't
use it to guy your tower. The EHS cable, cable recommended for tower
guying, isn't designed to roll around a sheave, but to just layout
straight and carry an axial load. The cable with lots of strands
stretches alot, the stuff with few strands stretches less.
The difference between aramid cable and steel cable is primarily in the
stiffness difference between the steel and the Kevlar, not the cable
construction. The aramid stretches more than the steel!
Towers with stretchy cable really like a rotating, pier pin or ball
socket articulating type foundations, because the bending loads in the
tower are least. Additionally, the Rohn type of construction needs a
pier pin base to allow the tower to twist. This is due to the diagonal
bracing configuration. All of the diagonals slope in the same direction,
When the tower gets compressed, it rotates in only one direction. The
Triex stacked tower, with "W" bracing, does not rotate when compressed.
I'm relatively new to this reflector, so I may have missed a previous
discussion of it.
If someone knows that this has been previously handled, please send me a
note so I don't waste my time (I don't have enough to search the
archives to find out).
73,Kurt

--
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress




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