Paul Plants wrote:
>
> Gust remember even though you live in a 70 MPH rated area as I do, you will
> get winds over that. Last December we had winds to 87 MPH for an 8 hour
> period. Overrate your equipment for safety!
>
> Paul N3WMV
>
Hi Paul & All,
While it is never dangerous to make things stronger than they need to be,
it's good to know where the extra safety margin is coming from so we can
know if we are really getting it.
The "Basic Wind Speed" used in the prevalent American specifications is the
average wind speed that is associated with the passing of one mile of wind
past the structure.
That is not the only way to come up with an average wind speed. It is just
one way of measuring average wind speed, that is based on distance.
There is another way of doing it, based on time. One can calculate the
average speed during a specified time frame. Short averaging time periods
will produce higher average speeds, long time periods produce lower speeds.
One can get a headache thinking about it, but ASCE 74 provides some good
information on the subject.
Let's take your numbers and see what they might mean.
The "Basic Wind Speed" of 70 mph means that it takes 51.4 seconds for one
mile of wind to pass by an anemometer (wind speed gizmo) that is mounted 33
feet off the ground. During those 51.4 seconds, the average wind speed is
70 mph. That intimates that during the 51.4 seconds, the instantaneous
speed will sometimes be higher and sometimes lower than 70 mph.
It is unclear where the 87 mph measurement came from, but it sounds like it
could be a
peak wind speed that was measured by a device, that by definition must
apply some kind of averaging technique to come up with a number. Either, it
was using an algorithm that allowed it to compute the speed for the
"fastest mile of wind", or it was using a specific time period for the
averaging calculations to arrive at the number.
If it was Pauls own device, he can tell us what height it was mounted at
while taking this reading, and what the averaging time period was, likely
to be a few milliseconds shorter than the update frequency on the display.
Lacking the necessary information, I'll make a guess (dealers choice) and
say it might have been using something like the 3 second averaging period
used in the U.K. CP3 specification. Some users might not want to wait
a long time to have their wind speed updated by their gizmo, so the
manufacturer provided a device with updates at shorter time intervals.
Using the ASCE 74 information, we can translate the 70 mph "fastest mile
wind speed" to a "3 second average", and come up with ~85 mph. Sounds
pretty close, eh?
If we wanted to go the other way and find the "mean hourly wind speed" and
get ~55.6 mph.
If the 87 mph came from a "fastest mile" measurement at 33 feet, then by
all means reconfigure the tower for that value, the "basic wind speed" for
the site is not 70 mph, it's more like 90 mph.
If, it was the "3 second average" at 33 feet, then it means about the same
thing
as the 70 mph "fastest mile wind speed", so we don't need to do anything,
unless we are compelled to have a higher safety margin than the original
design.
Just like antenna area values, wind speed ratings and site measurements are
valuable when we know what they mean.
_________________________________________________________________________________________
On another topic in this thread:
> ----- Original Message -----
> From: <n4kg@juno.com>
> To: <TOWERTALK@contesting.com>; <scsueepe@mtcnet.net>
> Sent: Wednesday, May 16, 2001 3:56 AM
> Subject: Re: [TowerTalk] Questions on my tower
>
> > Mike,
> >
> > You really need to get a 70 MPH rating (assuming that is the
> > maximum wind speed rating for your county. K7LXC can look it
> > up for you). Note that the wind FORCE doubles from 50 to 70 MPH.
> >
I would agreee with Tom, that the tower should at least be configured for
the minimum 70 mph "basic wind speed" (fastest mile). And, that the
pressure is doubled going from 50 mph to 70 mph. That makes the adjustment
pretty straightforward. The allowable antenna area at 70 mph will be 1/2 of
the allowable area at 50 mph.
Put it up on a mast out the top of the tower or use the wrong antenna area
and all bets are off.
> > IF the tower had ZERO surface area, that would reduce your load
> > capacity by half to 15 sq ft. BUT your tower does have a finite
> > surface area so the REAL load capacity is even LESS.
> >
> > Most likely, vertical load capacity is NOT the limiting factor unless
> > you are talking about a tilt-over tower.
> >
Think I agree, and would add that the vertical load limit is a rating that
relates to the longevity of the lifting cable, so it should be important to
the crankup owner.
--
73, Kurt, K7NV
http://yagistress.freeyellow.com
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