Doug Renwick wrote:
> Where is it written that a self-supporting tower has to be guyed with
> the same tension as a guyed tower? Why can't the guys on a
> self-supporting tower have no to little tension? Think about it. Those
> guys can be there only as a secondary method of supporting the tower.
>
>
Guying is not a simple construct. The guys become part of the structure
and add their own set of limitations along with the help they give.
Guys at no tension will add practically nothing to the installation
except weight and when using EHS that can be substantial. Actually with
EHS it would be impossible to add them without a fair amount of tension
just due to that weight and pulling them out to the anchors. They would
also have a huge catenary which would make them subject to low frequency
oscillations exerting substantial force even in low winds. In that case
the tower would probably be far better off without them. Also the tower
reinforcement must be in a manner to withstand side pull on the legs. A
number of towers are not constructed with structural allowances in mind
for a side pull.
Guys have two modes of oscillation. A high frequency and low frequency
mode called "galloping". "I think the high frequency mode is called
singing or ringing. The high frequency mode is handled by "dampers"
attached to each line. Again, IIRC the makers of Phillystran recommend
dampers on all guy lines over 100' in length. The low frequency mode is
IIRC handled by guy size and and tension. There is a LOT of give to
lines with deep catenaries. The deeper the catenary the more give.
Should lines start oscillating in phase this can exert thousands of
pounds of side force. Enough to do a lot of damage. Out here in the
flat lands I've seen large wood power poles snapped off by steady,
sustained winds in the 30 MPH range when the lines hit the resonant
point long enough to build in amplitude. Gusty winds didn't stay at the
required speed long enough for the amplitude of the oscillation to build
to the point of doing damage.
Hopefully we have a structural engineer on here who can explain this
properly.
Guy tension is set at 10% of the working strength, so going to the 1200#
test Phillystran would give 120# per guy or 360# per level.
Basically I'd bet that attaching guys to most self supporting towers
would negate the insurance. They'd see it as trying to beef up an
overloaded tower. Otherwise, why add the guys to an already sturdy and
well designed structure.
As an added note, most ham towers I see do not have the guys attached
properly and some towers do not even make provisions for doing so. ROHN
towers tackle this by putting a very sturdy bracket _around_ the tower.
The guys attach to the bracket which absorbs ALL of the pull rather than
the legs of the tower. So with the ROHN installation. The only force
exerted on the tower from the guys is the downward component if all
three guys are tensioned the same. (of if the guys do not get into the
low frequency mode of oscillation)
IF I were going to add guys, which I wouldn't, I'd use guys of low mass
such as Phillystran, but this still puts the builder into uncharted and
unengineered territory. In many areas you'd need those engineering
specs and I doubt they'd be available. It'd sure be interesting and I'd
like to see a set showing just how the addition of guys would affect the
characteristics of the tower.
73
Roger (K8RI)
> Doug
>
>
> -----Original Message-----
>
> Subject: Re: [TowerTalk] Guying a self-supporter
>
> That means they guys on my tower exert an additional 3300# on the base.
>
> That is well more than double the total tower and antenna system
> weight. Free standing, or Self Supporting towers are build
> considerably stronger than guyed towers as they have to serve the
> function of those guy wires internally. Some are designed to allow
> guying as well. The question them becomes, If permitted how does guying
>
> the self supporting tower affect the wind load and weight support
> ratings. Guying a tower with properly tensioned guys can easily add
> more than twice the load to the base. So a 1500# tower could end up
> with 4500# on the base structure.
>
> IF the tower manufacturer says their self supporting tower may be guyed,
>
> stick strictly to their recommendations as to guy strength, tension,
> wind load, and antenna weight.
>
> I'm surprised a reputable manufacturer would recommend guying a self
> supporting tower.
>
>
>
>
>
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