From:
Fred Hopengarten K1VR 617/2590088
Six Willarch Road
Lincoln, MA 017735105
permanent email address: fhopengarten@mba1972.hbs.edu
On Fri, 5 Sep 1997 15:31:24 0700 palomino!rabbit!darrel@uunet.uu.net
(Darrel Van Buer) writes:
>
>The conversion was from memory, I haven't looked at the code document
>in some time.
>
>The definitive source for the conversions (and factors for round vs
>flat loads) is probably
>in EIA222.
>
>In looking at manufacturers' data sheets, there seems to be two
>different sets of numbers
>used (maybe older vs newer EIA222 revisions?). In either case, force
>is proportional to
>the square of windspeed (so 70 MPH is about twice 50 MPH), but Mosley
>and Heights figure
>20 psf at 80 MPH while HyGain and US Towers use 25 psf at 80 MPH. I
>derived 88 MPH as
>sqrt(30psf/25psf) * 80 MPH.
>An additional confusing factor is flat vs round, since a round element
>has a wind load about
>2/3 of a flat element of the same projected area. Off hand, I don't
>see how to get exactly the
>Rohn rating from either set of common numbers (80 MPH as 25 psf gives
>87.64 MPH for 30 psf,
>70 MPH as 20 psf gives 85.7 MPH, Rohn's 30 psf at 83.3 MPH gives 27.7
>psf at 80 MPH) but it is certainly in the ballbark. The common
>figures
>may be enough of an approximation that the Rohn numbers are valid, or
>nearly so.
>Unless you are aiming for as close to 100% capacity as your building
>codes allow, the Rohn
>figures are likely the safest to use for a Rohn tower, permit wise.
> Darrel AK6I
Dear Darrel:
A very interesting discussion. Very interesting. The problem is
that the industry has not adopted a standard. Furthermore, there is the
tough question as to what is "safe." For example, Rohn uses a safety
factor of 3, while Cushcraft uses a safety factor of 1.25. My friends
who are mechanical engineers tell me that 1.25 is closer to
"appropriate," and comment that the real reason for Rohn to use 3 is to
sell Rohn 55 when Rohn 45 would do. No serious mechanical engineer has
ever told me to use more than 1.5. Ever.
In the meantime, hams who are opposed by neighbors on what are
really other grounds are forced to spend mucho dineros to meet some
unrealistically high construction requirement, or forego a tower and
beam. Many give up.
In my view, too many. For that reason, I am exploring the idea
of trying to create some minimum standards as "recognized." This effort
may even attempt to change national bldg codes (BOMA Code).
Fred K1VR

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