Pete:
As far as I can determine there is nothing else involved. The rule of
thumb is just that a rule of thumb. It is probably a value erectors used
and it seems to work. There is not hard and fast specification for the
initial tensioning value just a recommendation (TIA/EIA-222-F) as to the
amount of pretension. Is 7% ok or how about 15%. I would judge both are
allowable. The creation of tension in the guys to minimize slap or
dynamic jumping and to facilitate the smooth transfer of the horizontal
load as it is applied to the guy system is what is intended. The
absolute value of the tension to be used is not critical. It is a
practical matter more than a hard scientific theory.
So if Mike wants to use say 5 or 6% for the larger phillystrand and it
behaves adequately then it is ok. Try it and if the tower jumps around
when the wind loads it up, tighten up the guys a little more to minimize
the erratic movement.
Hank / KR7X
Pete Smith wrote:
>
> At 12:54 PM 7/17/01 -0700, Hank Lonberg wrote:
> >Actually the number of guy points required is a function of both the
> >axial load, wind moment in tower section and distance between guy
> >points.
> >
> >There is no need to beef up the lower guys because you are using a
> >larger guy higher up. In practical terms the added vertical load does
> >not increase and load required to keep the lower guy points in plumb.
>
> But Mike raises an interesting question. I'd always understood that the
> purpose of pretensioning is to take enough of the sag out of a guy so that
> it is operating primarily in elongation when wind forces are applied. The
> 10% rule of thumb is usually cited for EHS steel, and I know it is also the
> manufacturer's recommendation for Phillystran. but even the biggest
> Phillystran is much lighter, per foot, than even small EHS. Doesn't it
> follow that it really requires far less pre-tensioning to take a given
> amount of sag out of a run of Phillystran guy? Or is there something else
> at work here that I don't understand?
>
> 73, Pete N4ZR
> No, no ... that's WEST Virginia
>
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