jimlux wrote:
> 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.
> 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.
For some reason I never think of BX tower when I think of self
supporters, but it's designed as one, so I guess we must. I started out
with that style tower in the 60s, eventually extended it to 74' and
guyed it. It held a 4 el triband quad for quite a few years. I like to
think of "normal" self supporter as one that is a fairly balanced (no
glaring weak spots for normal loading). OTOH, my seat of the pants
theory is that a reasonable guy system won't increase peak stress on any
of the components, so if BX tower can serve to illustrate the fallacy in
that thinking, it's useful.
A number of folks have mentioned twisting torque as the reason that
guying a self supporter may not increase load rating. That may be true.
I've been thinking though that "reasonable guying" should include star
brackets and double guys in the cases where boom lengths need to be
increased.
We get hired all the time to beef up towers. On self supporters, about
half of the time it involves strengthening certain diagonals and
horizontals, and half of the time the legs. My gut feeling is that in
some of these cases, if guy wires were not off the table (for reasons
other than engineering), they would be a cheaper fix.
I wish I could justify the purchase of one of the finite element
analysis programs, but I know they're not cheap...
Steve K8LX
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