From:
Fred Hopengarten K1VR 781/259-0088
Six Willarch Road
Lincoln, MA 01773-5105
permanent e-mail address: fhopengarten@mba1972.hbs.edu
A week ago I inquired of this reflector about the Rohn safety
factor. K6LL came up with the best answer, and he deserves a lot of
credit. Thank you, Dave Hachadorian! Look for the sentence with the
words: "1/3 increase in allowable stress."
To let you know what I did with the information, I have removed
identifying information from the letter sent to the building inspector,
and reprint it below. If a building inspector comes chasing you at the
behest of some neighbors, I think it is always best to (1) be helpful,
not confrontational, and (2) ask if you can reply in writing. The latter
gives you time to do some research, and also allows you to think about
how to put your best foot forward. The object in the letter below was to
provide the building inspector with a letter he could show to the
complaining neighbors, a letter which would make it look like they'll get
no where with further complaints.
Note to fans of minimalism in tower construction: Steel is
cheaper than litigation. (Unless you are a lawyer.) Overbuild!
If you want to criticize my arithmetic, or to offer suggestions
as to how better to word an argument below, please send me private e-mail
-- as no one likes being embarrassed in public.
Fred K1VR
And now, the letter:
Date, 1998
Town of
Attn: John Doe
Zoning Enforcement Officer
Town Road
Town, State Zip123
Dear Mr. Doe:
I understand that you have visited Joe Ham, of Ham
Road, and raised some concerns. He wants to be certain that
you are assured on every issue you have posed, and has asked
me to provide all the information you need in that regard.
No Commercial use
Let me begin by stating clearly and unequivocally that
there is no cellular telephone, Personal Communications
Service (PCS), paging or any other commercial use of the
antenna support structure at Ham Road. None. No way. No
how.
Two Yagi Antennas
One question that has arisen is whether or not the
application specified only one Yagi antenna. Please refer
to Exhibit 2 [which was the bid from the erector], which
reads: "The tower will be fully erected and ready to accept
two Yagi antennas with 9 feet of separation." It is
inappropriate to apply for a building permit specifying the
antennas by make and model number, as by its very nature,
amateur radio is an experimental service, posing the
possibility that the radio amateur may change antennas at
any time. For this reason, the application was careful to
include specifications for antennas at the top expressed in
square feet of windload.
It does not matter what sails a sailor hangs from his
mast, so long as the mast is built to tolerate the load.
Similarly, antenna support structures are specified in wind
load, expressed in square feet.
Safety
With respect to questions of safety, I am also pleased
to reassure you on this issue. If you will refer to Exhibit
4 of Mr. Ham's Building Permit Application, you will see
that Mr. Ham's tower, Rohn Model 55G, is so heavy duty that
the manufacturer does not expect it to be used, nor specify
its use, at heights under 100 feet (this is an 80 foot
structure, seriously overengineered). However, at 100 feet,
for round members, the specification you will see in the
upper left hand corner of the page (the 100 feet box), in an
oval, is 16.0 square feet (s.f.).
In addition to the manufacturer's specification of 16
s.f., Rohn (the manufacturer) allows an additional 8.0 s.f.
for "three side arms, symmetrically placed, having a total
effective projected area equal to 8.0 square feet."
(General Notes #4). That would be 16 + 8 = 24 s.f.
Furthermore, the manufacturer assumes (see General
Notes #5) "two 7/8 dia. lines on each tower face." That
would be a total of six. This structure does not have this
load, increasing the permissible 24 s.f. to some number even
larger.
One should add about one more s.f. of windload for the
difference between a 100 foot and an 80 foot structure.
On top of that, Rohn notes the conservative nature of
its design for its Model 55G tower in drawing C630655 r9
("Nomenclature"), in which compression capacity, moment
capacity, and tension capacity are all specified with "1/3
increase in allowable stress". Guy wire specifications are
at a safety factor of 2:1.
By comparison, the structure presently has a
[manufacturer's name] Model ABC (4.36 s.f.) and a
[manufacturer's name] Model DEF (7.4 s.f.), for a total of
11.76 s.f., leaving an obvious margin of safety equal to (24
- 11.76=) 12.24 s.f., plus an accommodation for missing
lines and missing 20 extra feet of height. And I haven't
even added the safety factor of the design.
I hope this analysis will promote an understanding of
the margins of safety built in to this system. As we said
in the original building permit application, "(t)he system
has been designed to dramatically exceed the Massachusetts
state building code requirement for windloading of 30 lbs.
per square foot." (Building Permit application, page 4).
Certificate of Completion
I hope I have provided you with everything that you
need to conclude that this construction has been completed
in consonance with the Building Permit. If you have not
already done so, would you be kind enough to provide a
Certificate of Completion to Mr. Ham?
Sincerely,
Joe Ham
by his attorney,
/s/
Fred Hopengarten
P.S. A personal note: John, it has been nice to work with
you, and I wish you well in your new position in Town
2. Never can tell, we might do this dance there one of
these days! -- Fred
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