[TowerTalk] Rohn Info

[N8SM]

For those of you looking at the Rohn TVRO drawings, please note that
they are referenced to "A.N.S.I. A58.1 (now ASCE 7-98), exposure B,
70 MPH basic wind, 1.0 importance factor, 86' height, 13 psf". This is
different from ANSI/EIA-222-E (now ANSI/EIA/TIA-222-F?)
which Rohn uses for the 25G/45G/55G guyed towers. There are also
differences between the two Rohn configuration for feedlines, mounts,
etc.  Make sure you (or your P.E. as Stan suggests) know the wind loads
expected at your site and how they affect the tower's wind load capacity

compared to the Rohn drawings before designing and installing your
system.

The main thing to realize is the guyed 25G/45G/55G numbers versus
the TVRO numbers are an apples to oranges comparison.

George, K5TR, asked about the Rohn TVRO wind load capacity 10 or
12 months ago that prompted me to look around the web. The most
informative site I found was:

http://www.heightstowers.com/engspecs.htm

Section 5.1 combines height, exposure and gust factors for exposures B,
C
and D.  If I'm reading this data correctly, the resultant force on the
a tower with exposure D would be over twice that of exposure B for
heights less than 30 feet at the same basic wind speed! Exposure D still

shows a substantial 66% increase at the 100 foot level. A reduction in
wind load or a more sturdy tower design is required for the more
demanding exposures for a given basic wind speed.

For anyone wondering about the difference between the differnt
exposures,
there is a brief description at the following roofing site:

http://www.powersrawl.com/Roofing/59.2.htm

however, the description differs somewhat from the UBC footnote at the
heights tower site....

73,

Steve  N8SM


W4EF wrote:

> Hi Stan,
>
> I just noticed the Rohn drawings for TVRO
> applications with EPA of 49 sqft last night
> while purusing my new Rohn consumer catalog.
<snip>
>
> Mike, W4EF...................................
>
>
> W7NI wrote:
<snip>
>> 3.  There is some very interesting new information about the
>> use of 25, 45, and 55 towers in TVRO (Television Receive
>> Only) applications in the latest Rohn Consumer Catalog.  The
>> guying digrams show guys at a much steeper angle than any I
>> have seen before on any Rohn diagram and with a much larger
>> Effective Projected Area on the top than I have seen
>> before.  (48.9 square feet at 70 mph on 25G, for example).
>> This is a TVRO application, remember, and the load is
>> expected to be a 6 foot diameter dish and not a long boom
>> Yagi.  Still, the information is interesting because it is
>> so different than any I have seen before from Rohn.  Use
>> caution when applying this to ham antennas.  Better yet, use
>> a PE to apply it for you.  For 25G TVRO Guying Diagram and
>> Parts List, order documents 2538a and 2538b.
>>
>> 4.  For 45G TVRO Guying Diagram and Parts List, order
>> documents 2539a and 2539b.  (87 square feet at 70 mph, 8
>> foot dish)
>>
>> 5.  For 55G TVRO Guying Diagram and Parts List, order
>> documents 2540a and 2540b.  (94 square feet at 70 mph, 10
.> foot dish)
>>
>> Are there any other Rohn documents or drawings you would
>> find useful that I can scan and send you?  I hope I can
>> handle the volume . . .
>>
>> Stan  w7ni@teleport.com



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