Jerry, K3BZOK writes:
>Chuck...I have to ask: what would be "counter-intuitive" about adding
> guys to a self-supporting tower? What "unintended consequences" would
> result from that? Wouldn't it just be a little insurance?
Jerry,
Self-supporting towers are designed to withstand bending loads (from wind),
but their capacity to withstand vertical loads is based on just the weight
of the tower itself and the antennas it supports (plus the usual safety
factors). When guys are added, the lateral wind forces are converted to
vertical loading on the tower (the tower doesn't deflect as it was designed
to do, and the loads are all picked up by the guys). Depending on the angle
of the guys, these forces can be huge, often exceeding the vertical strength
of the tower. On the other hand, towers intended for guying are designed for
high vertical and very low lateral/bending loads.
This has been discussed on Towertalk several times by structural engineers;
I'm just an EE, so I am only parroting what I've seen them say, but it makes
a lot of sense. I remember that one of the older self-supporting towers was
reported as allowing supplemental guys, but I sure wouldn't do it unless the
tower manufacturer built that into his design requirements! This is
particularly important (so they say) in the case of crank-ups, where the
vertical strength is provided only by the cable. But even crank-ups with
latches should not be expected to hold up to guying, unless the capability
is designed-in.
Sure, there are plenty of guyed self-supporters that are still
standing....but likely with less margin. Your "just a little insurance" is
the intuitive assumption that is 'counter'...:>).
Chuck, N4NM
|