Steve Maki wrote:
> To be honest, I'm not sure what the question is either :-) Folks seem to
> be equating self support towers with crankups, and who knows what else...
> Just to clarify the question in my OWN addled brain - I'd like to see an
> example of a normal tapered self support tower that has to be de-rated
> after a reasonable guy system is added.
> That's all.
> Steve K8LX
You're talking about something like a Rohn BX with added guys?
Not trying to get any additional wind load capacity? (because the BX
isn't rated for very much in the first place)
I don't remember the exact numbers, but I seem to recall that there's
not a lot of margin in the design(i.e. the verticals and diagonals are
pretty close to their buckle limit, when the tower is loaded at rated
wind speed). In these sorts of designs, the limit is usually in the
short segments, which are "slender" and fail by buckling, not by simple
compression. Part of the challenge is deciding whether the column ends
are fixed, free, or pivots, because that changes the buckling behavior.
Putting guys on will inevitably increase the compression loads on the
downwind side a bit (from the vertical component of the tension in the
guy, even if slack when unloaded, they will develop tension under wind
load). I'd be more interested in the change of the distribution of
forces among all the bracing members. On some theatrical trusses, the
main tubes are pretty fat and strong (because that's what stuff gets
clamped to) so the overall strength limit is in the fairly small
diagonal braces, particularly when there's a combination of bending and
torsional loads (imagine a truss hung by cables from each end, with a
bunch of lights hung on one pipe).
You might be depending on having components and assemblies that are on
the "good end" of the expected range of material properties.
One advantage that might accrue is that you could claim the guys will
make sure the crumpled tower debris is contained within a smaller area,
rather than laying out flat.
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