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Re: [TowerTalk] Guying a self-supporter

To: Steve Maki <>
Subject: Re: [TowerTalk] Guying a self-supporter
From: jimlux <>
Date: Sun, 09 Nov 2008 12:22:37 -0800
List-post: <">>
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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