----- Original Message -----
From: "Steve Maki" <steve@oakcom.com>
To: "towertalk" <towertalk@contesting.com>
Sent: Saturday, April 09, 2005 7:54 AM
Subject: Re: [TowerTalk] Guying a self-supporting tower - NO
> Jim Lux wrote:
>
> > Off hand, here's the problem I would contemplate: Increased down force
from
> > the guy tension will increase the compressive load at the base.
Whether
> > it's a problem is another issue. A self supporting tower will be
generally
> > stronger than a guyed tower of the same height (because it has to
support
> > the bending loads, which manifest themselves as compressive loads at the
> > base). When you guy, you're turning some of the bending moment into a
> > tension in the guy.
>
> Yes - with guys you convert almost ALL of the bending moment into
> guy tension and straight downward force. The numbers (on a simplified
> model) don't give any support to the "don't guy" mantra. IOW, the
> increased straight downward force is minor, compared to what the
> tower MUST be capable of, if it can survive the extreme compression
> in the down-wind leg(s), unguyed.
>
> Here's the way I look at it:
>
> Consider a 100' properly guyed Rohn 45 tower. Now, while holding the
> top face width at 18", slowly increase the size of the bottom of the
> tower - to 24", then 48", then 10', then 100' - all the while, making
> appropriate changes to the diagonals & horizontals of course.
>
> I think it's obvious that when taken to the extreme (a pyramid), that
> having 1/4" guys does not compromise the structure. So at what point
> in the widening of the tower do you reach an unsafe condition? All
> you are doing is *strengthening* the tower over the stock Rohn 45
> configuration.
>
I think that the problems, if any, would come from changing the distribution
of stresses, not from the increased stress at the base, for instance.
There's a classic exercise in structures textbooks where adding a gusset to
a "T" junction makes it much weaker, because the increased stiffness from
the gusset increases the loads just beyond the gusset. There's another
interesting example with parallel piano wire and steel cables, where the
fact that one is much stiffer than the other makes the load transfer not as
intutively expected. In theatrical rigging, there's always concerns about
"overguying" (particularly when used to "stiffen" a horizontal member
holding up a lot of lighting instruments) making that piece of the structure
too stiff, relative to the rest of the structure.
I think the take home message is that when it comes to structural analysis,
simple analyses of a modification might miss some key factor, especially if
you don't understand what went the design of the original structure. A
casual inspection (unless you happen to be in the tower designing business)
might not reveal where the "load limiting" joints or members are.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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