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[TowerTalk] Building Code Units of Measure

 To: [TowerTalk] Building Code Units of Measure k1vr@juno.com (Fred Hopengarten) Mon, 08 Sep 1997 08:24:17 EDT
 ```From: Fred Hopengarten K1VR 617/259-0088 Six Willarch Road Lincoln, MA 01773-5105 permanent e-mail 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 EIA-222. > >In looking at manufacturers' data sheets, there seems to be two >different sets of numbers >used (maybe older vs newer EIA-222 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 -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search ```
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