Tom:
Unless I am mistaken an "80 mph wind zone" will have 3 sec gusts of about
100 mph as a part of the 80 mph wind. To be totally comfortable with your
installation I would think you would want to do the stress analysis of the
tower [including the torque analysis] and assure yourself that that it will
stand up to the 100 mph gusts. You might even want to have a safety factor
-- perhaps 1.25 .
I am curious about the ability of your mast to withstand the 80 mph
average/100 mph gusts. When I calculated the wind stress on my system, which
has about 18 sq feet with 6 up 10 ft and 12 up 1 ft from the thrust bearing
at the top, I found that the "weak" link in the system was the mast -- even
with a 70,000 psi yield strength. I happen to have a motorized telescoping
tower so I could reduce the stress at the base by lowering the tower. But at
any height the mast stress at 100 mph [actually even less than that] could
be expected to bend the mast given that the wind at each height was the
same. Since you have a guyed tower, there is only one height and that is the
one with the 80/100 wind velocity.
Frank Tranvanty, W9JCC, has written a very nice EXCEL spreadsheet that
allows one to calculate the stress on a mast with multiple antennas on it
and for various wind speeds. You enter the average wind speed [80 mph in
your case] and it will calculate the gust speed. It will solve the stress on
the mast for antennas at various heights above the thrust bearing and with
different areas for the antennas. The program is written to allow one to
calculate tip over moment at the base of a telescoping tower but it will
function for non telescoping towers too.
The program is available from the ARRL web site and has the name =>
Generalized Tower & Mast.xls which is located in a zip file named
Travanty.zip.
The note on the program is the following:
Travanty.zip contains files pertaining to "Tower and Antenna Wind Loading as
a Function of Height," by Frank Travanty, W9JCC (QEX, July/August 2001, pp
23-33).
Generalized Tower & Mast.xls is a Microsoft Excel spreadsheet workbook that
can be used to assess the "survivability" of Amateur Radio antenna/tower
installations.
Tod, K0TO
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Tommy
> Sent: Tuesday, November 10, 2009 10:29 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Basic tower installation question
>
> I live in an 80-mph wind zone and am going to put up a 70
> foot Rohn 45G tower, using 3/16 EHS for the lower guys and
> HPTG6700 for the top guys. I will have a 22-foot DOM 1026
> mast with a 6.8 sq. ft. wind load at the top of the mast; 10
> foot below that yagi and 2 foot above the top of the tower, I
> will have a 12.8 sq. ft. wind load yagi. The mast will extend
> 8-foot down into the tower, through a thrust bearing and then
> resting on a Msquare 2800PX rotor.
>
>
>
> My basic question is about the base. Should the base for this
> system be a pier-pin type base or just a Rohn concrete base
> section? At this point I could do either, but would like to
> install the best base for this system.
> The other question would be would it really make any
> significant difference which type base to use for this
> application, considering the load I will have at the top of the tower?
>
>
>
> Thanks for any information/suggestions.
>
>
>
> Tom - W4BQF
>
>
>
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