I would also add that you need to be aware of the fact that local qth
conditions (microclimate) in sudden, violent storms can vary ENORMOUSLY from
your general area and what the NWS or FAA thinks is representative of your
area. In fact the max wind speeds in your qth “zone” may only bear a passing
resemblance to the likelihood of what you will actually experience.
Ridge crest/Slope are some of the exacerbating factors, but so is the direction
of the winds at a given qth. At my qth the most violent storms by far are when
they approach the NY/CT border (12 miles West of me) at a particular angle and
funnel between the hills on both sides of I-84. They are terrifying and have
on three occasions become tornadic (wind speed plus twisting winds – far more
damaging that just wind speed alone). During hurricanes the same thing is
true – except the location of the storm and its path have a lot to do with what
to expect. Even a 10 mile shift or wobble in direction can have enormous
differences in impact – and the “tighter” and more violent the storm, the
greater the different in impact a few miles can make. And then there are the
effects of elevation. In hurricanes, your height above sea level can subject
you to a higher sustained level of extreme wind, over and above whether or not
you are on a ridge line or on a slope. In major hurricanes, elevations as low
as 800’ – 1500’ ASL can subject you to winds 20 – 40 mph stronger than if you
were at 300’ ASL, which is what happened at my qth during Hurricane Sandy. And
the difference in force between 70 mph and 110 mph is equal to the square of
the ratio of velocities, so the force of 110 mph is not 1.57 times 70 mph, it
is 2.47 x!! Which can easily induce a failure. If you are elevated enough AND
you are on a ridge crest / slope, (as I am) then you can be in even more
trouble!
Simply put, it is during the particularly vicious storms that you are most
likely to experience failure and the wind velocities experienced during those
storms are least likely to be predicted by your location wind zone that is used
for calculation purposes. Please remember that when planning for your qth.
73
Bob KQ2M
From: Rob N1KEZ
Sent: Saturday, June 10, 2017 11:20 AM
To: Richard Thorne ; Towertalk
Subject: Re: [TowerTalk] US TOWER HDX-89 / Tash DX86 Wind Rating Differences
Make sure you know what the wind ratings are for your area before getting
excited about a particular tower. Manufacturers assume you live in the most
favorable place. Wind rating for my tower location turned out not to be 90 Mph
but 130Mph! (Ridge crest/Slope etc).
73!
N1KEZ de Rob
Sent from my mobile device.
Pse excuse brevity and any errorz.
On Jun 10, 2017, at 7:00 AM, Richard Thorne <rthorne@rthorne.net> wrote:
This may not be comparing apples to apples but here's my question after
studying the two towers.
US Tower HDX-89 claims 9.2 sq ft of max allowable antenna wind area 90 mph 3
second gust
Tash DX86 claims 21 sq ft of max allowable antenna wind area 85 mph 3 sec gust
After looking at both designs they both start with a lower section that is
28.75" wide and a top section that is 13 7/8" (Tash) and 13 15/16" (US Tower)
So the designs appear to be the same, why are the allow able antenna wind loads
so different? I realize that one is based on 90 mph the other on 85 mph. Does
the 5 mph delta add up to a 11.8 sq ft difference?
Thanks in advance.
Rich - N5ZC
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