Note: Only goes to TT
w4lde wrote:
> I recently have moved to a new home, SW of Atlanta and have a SteppIR
> 3-element with the 30/40 add on sitting in the garage waiting on a new
> tower.
<snip>
> My Counties wind rating is 70MPH, we have already seen 50MPH gusts
> during a thunderstorm recently.
>
>
With all the storms this spring I think you'd be hard pressed to find
many areas that don't hit 70 at times and some areas may be raising
their speeds.
> The tower is rated at 9 sq/ft at 70 MPH and 61#'s weight, the specified
> wind load by SteppIR is 8.1 sq/ft. at 58#'s I would elect to use the
> rotating n base thus keeping the rotor at the bottom of the tower and
> reducing the extra weight and little load.
>
> Now the questions?
>
> 1. Since I am almost right on top of the spec's, wind load and weight,
> should I abandon this Tower selection.
>
<snip>
> Maybe I can improve on the ratings by not keeping the Tower raised to
> the maximum height unless I am chasing some DX expedition or am I dreaming?
>
Sounds like a royal pain.
I've seen a number of suggestions, but if something happens your
insurance company is likely to take into consideration the advertised
ratings of the tower over a PE unless the PE derates it in their favor.
Safety margins exist, BUT they don't exist. Take rope for example. It
has a breaking strenght (or yield) and it has a working strength which
is some percent of the yield strength. IOW they take into consideration
lots of "what ifs" including age, maintenance, and repeated stressing
when rating a tower. In the normal day to day life of a tower these
things aren't much of a consideration until you start pushing the
limits. A tower, like a piece of pipe can be sprung just so far with out
any thing permanent happening. Beyond that point it may end up with some
permanent deformation and change in strength possibly with no visible
deformation. Continuing on will eventually reach the point where the
structure will yield. Even the way an antenna is mounted can affect the
wind loading if the antenna can move. That margin is there to allow for
variables and still remain safe. Any time you start taking advantage of
that margin The risk factor goes up. Yes it's something that hams in
particular do nearly every day.
In years well past, I've worked on some smaller towers (200 to 300 foot
range) in winds that were uncomfortable, but manageable. Those towers
were properly loaded and maintained. They were rock steady. The winds
came up unexpectedly on one climb and I had to stay up there until they
slacked off. Again the tower was rock steady.
OTOH I've been up some ham towers (25G) that were well guyed with a
large tribander on top. When the winds hit about 15 and I could feel the
tower moving under me, I came down. I've been on my 45G with that big
array in winds over 20 and never felt it even quiver.
Were it me, and it's not, I'd want something with plenty of safety
margin. If I was comfortable bringing it down every time I left the
driveway or wasn't using it then so be it. That is a safe approach and
I know of several local hams with some pretty good sized antennas that
do just that. They only extend the tower when they plan on using it. I
have neither the patience, ambition, or memory to take that approach. I
may fire up the rig when I walk into my den or turn the one in the shop
on when I start working and listen to it in the background. OTOH at
my age, I'd sure like to have a couple of good crank-ups in place of the
big 45G. STILL I may just get that "fold over" constructed
"one-of-these-days" <:-)) After all, I did get that big Ash tree by the
shop trimmed today. Now I gotta get the ruts out of the North yard.
Roger (K8RI - ARRL Life Member)
www.rogerhalstead.com
N833R (World's oldest Debonair)
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