From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
if sending attachments: k1vr@gis.net
From: Fred Hopengarten
To: All Massachusetts Antenna Support Structure Clients
To: <towertalk@contesting.com>
Date: August 3, 1999
Subject: The Massachusetts Building Code
The Massachusetts Building Code requirements for windload on
antenna support structures are found at 780 CMR 1611 et seq. In the
Sixth Edition, there are two typos and one thing which is difficult to
read. If you are thinking about a situation in another state, which also
uses BOCA code figures, read on anyway.
The Typos.
Page 275, table 1611.4 has a note which references table 1611.1a
for empirical wind pressure formulas. The reference should be to
1611.4a.
Page 276, table 1611.4a has a note which references table 1611.1
for reference wind pressure requirements. The reference should be to
1611.4.
Difficult to Read
Table 1611.4a is entitled "Empirical Wind Pressure Formulas",
which may be used in lieu of the chart at table 1611.4. It is difficult
to read the formulas, however, because it is not on one line. Zone 3,
Exposure B should read:
p=60(h/800) (to the 0.45 power)
Sorry, but in ASCII, there is no neat way to type this.
Meaning
Why should you care? The answer is that if you have a really
close situation, where you need that last square foot and you have a
fussy Building Inspector, the formula may give it to you. Here's just
one example:
If you live in Zone 3 Exposure B, your Reference Wind Pressure is
21 p.s.f. (NOT the 30 psf or 86.6 or 90 mph for which Rohn tower is
specified). So you can put up a bigger beam than you first thought you
could when you ordered the Rohn 25 or 45 you may now own.
However, if you are erecting 75 feet and do the formula, which is
permitted by the Building Code, you need only build to a specification of
20.67 p.s.f. This gives you an additional one-third of a pound of wind
pressure to spare, which may permit one more element on your Yagi.
Rohn 45 Specifications -- Read Smaller, Think Bigger
Now that we've discussed why the Building Code permits you to
erect a bigger beam than you first thought, refer to the Rohn catalog,
Drawing No. C870478, General Notes.
Note 4 reads: "Tower designs include three side arms,
symmetrically placed, having a total effective projected area equal to
8.0 square feet."
Thus, if you've read this far, and you are further willing to
read the really small font wording pointed out, you may now think and
additional 8.0 s.f. bigger!
Here's wishing you good DX!
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