Bill Aycock wrote:
> then- I still have a quibble with acceptability of the combined usage of
> the Aramids and EHS tegether. See below.
I'm not so sure we have a quibble about this one Bill
> >
> >> John Langdon ( N5CQ ) pointed out a usage (very common, in fact) of both
> >> Phillystran and EHS, together, that is proper, and OK. This is the use of
> >> the two in series to break up conductivity, using Phillystran, with EHS at
> >> the ends to give mechanical and fire protection.
> >
> >This is a common, recommended practice.
> >
> >> He is certainly right-
We agree that series (end to end) use is ok, per your comments at the bottom.
Both cables carry exactly the same load. One section of the guy just stretches
more than the other.
> I was meaning only parallel usage.
> >
> >"Don't do this!" Cables with different elogations will not load share
> >properly.
> >
> True, but this is only a problem when the load is split, as in the use of
> different guys with different materials. This (load sharing) is a disaster
> looking for a time to happen.
I think you are referring to the use of EHS for some guys and aramid for others,
on one tower. It is not necessarily a disaster in the offing. Tower guys at
different elevations do not always "truly" load share, due to the placement of
the antennas and verticlal guy angles, so they really can't be considered to act
in parallel. The varying loads and elongations in the different guys control the
deflections in the tower at the guy points. The tower deflections determine the
bending moments in the tower sections.I agree that it it can be done rather
poorly. A rule of thumb, for a top loaded tower, would be to use the stiffer
steel cable for the top set of guys. This would reduce the bending loads in the
top sections, above the next lower guy point. If no lower antennas exist, you
are
correct in saying the guys should be the same material. They share the
horizontal
loads equally, but the vertical angle always makes the upper guy, in any
vertical
pair work harder to support the load.
My recollection of my observations of my models is that, generally all top
loaded
towers like having the top guys one size larger than the lower sets. The
concentrated top load creates higher loads in the top guy, and usually more
tower
deflection (wire stretch). Then the lower loads on the lower, smaller size sets,
provides approximately enough deflection to keep the tower straight, Albeit,
leaning over a bit. This is great until we get to the footing, where all of the
tower lean causes the highest tower bending stresses to occur.
As pointed out, by Steve Sawyers, in a previous post, for all steel guys,
usually
the limiting load in the system is the upper guy load. I haven't run enough
models to see what an all aramid guyed tower does, but expect to see the safety
factors for stress at the base of the tower approach the safety factors for the
top guys.This is not a bad thing!
A really good design tries to achieve the same safety factor for all system
components. This is rarely possible in the real world.
> However, in many cases, such as the commonly recommended practice of
> combining the two materials in one guy, the load is the same in both
> materials (series hook-up) , and the load is not 'shared', as in 'split'.
> Many people use a guy arrangement that uses one of the Aramids for most of
> the guy, and use EHS at the base. This is done to provide greater
> resistance to abrasion, physical and fire damage, while having an RF inert
> guy, for the most part.
Ditto!
This winter I'll be working on the ?? variations of what I might do with all the
junk in the back lot. When I find something valuable I'll put it up on the
reflector. Maybe It'll snow a bunch and I'll get around to running a bunch of
Rohn layouts. Again, I fully expect to find that the Rohn designs are just fine.
A reminder! To make it perfectly clear to all, what I have been analyzing is
essentially the same as everyone else. The one subtle difference is that I have
concentrated on including the effects of the guy stretch. Nobody should expect
that I'll find some earth shattering behavior, I don't. This whole thing started
for me during the '92 Americas Cup, when all my engineers were spec'ing rigging
based on elongation, not just the live loads. I thought I ought to check it out
on towers.
BTW, we use tons of aramid rigging to support a yacht mast. The difference is
all
of the aramid rigging is not fixed, but adjustable via a winch or hydraulic
cylinder. This allows the effects of stretch to taken out manually during each
load scenario.
73, Kurt
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