On 10/16/14, 5:30 AM, john@kk9a.com wrote:
I think that the concern (on towertalk) is that guying adds compression to
the tower legs. All of the self supporting towers that I have seen have
stronger legs than equivalent guyed towers, at least towards the bottom.
While I would also recommend following the manufacture's design and I am
not an engineer, I do not see how adding guys especially toward the bottom
of a tower can make it weaker.
A lot depends on how the tower was designed. Let's say you have a self
supporter and the load increases smoothly from the top to the bottom in
arbitrary units 0-100. The designer might choose to make the tower
tapered, so that the failure load is always a fixed amount more than the
expected load, say going from 10-110 units.
So, adding some guys might decrease the bending forces, but will
increase the down forces. More importantly, it changes the distribution
of the loads. The original designer anticipated a particular
distribution of loads, and designed with that in mind. So instead of a
smooth distribution of strength that nicely matches the smooth
distribution of load, you might have a load concentration that exceeds
the design strength at some point.
A good example might be a thin, flexible mast that is designed to bend
in the wind. If you add some guys at the middle, the bottom part is now
loaded more highly in compression, and the bending is concentrated at
the guy attachment point.
Most designers put margin into their designs (which is why 50 year old
unmaintained towers with too big antennas usually don't fall down), but
the margin isn't the same everywhere.
I've been struck, when looking at the detailed analyses on some towers,
at how clever the engineer was in distributing the loads and not over
designing. For instance, on a lattice tower like the familiar Rohn 25
or 45, there's a load on the vertical tubes, on the horizontal tubes,
and on the diagonal braces. In some of the members, column buckle
loading is a bigger factor, in some it straight compression or tension.
In general, they all have about the same percentage design margin (e.g.
the load on the diagonal members are low in most guyed towers, and
they're also long and skinny, so buckling is an issue, but you can do
that, even with a fairly small diameter member).
And then, you have to consider the combination of forces (Is the tower
bending, compression overall? is there a torsional load?). Kurt
Andress's (sp?) page has some of this analysis.
A standard problem in early engineering classes is analyzing a welded T
(one member with another sticking out the side), and then analyzing it
with triangular gussets added to "strengthen" it, which actually makes
it fail sooner, because it changes the distribution of the stress.
The overall story is that without a fair amount of analysis (or
practical experience with lots of different installations, not anecdote)
it's pretty hard to predict what the effect of doing something unusual
will be.
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