K7LXC@aol.com wrote, responding to the questions from Dick Green:
Comments from Dick:
> << The new numbers could have some serious implications for some of
us. Let's
> translate this discussion into English for the non-engineers among
us.
>
> >>Obviously, the new computation standards have significantly
increased the
> published windload ratings on some Hy-gain antennas. For example, the
TH-7
> was previously rated at 9.4 sq. ft., but is now rated at 11.5 sq. ft.
That's
> better than a 22% increase. The question was asked before, but not
answered,
> so I'll ask it again: Have the new computation standards also
increased the
> windload handling capacity of popular Amateur tower configurations,
or have
> many of us overloaded our towers by relying on the old formulas?
Hi Again,
Attempting to help clarify things,
Not readily answered Dick, because it is not something that can be
covered by a
blanket statement.
As I stated in a previous post, we don't really know what everyone has
been doing
all these years. This applies to both tower and antenna suppliers. Many
P.E.'s
that have been processing the information have been starting from
scratch to
determine both tower and antenna areas and then processing them
according to the
design spec's required by the local authorities. I have a few examples
of these.
That's what got me started thinking about the problem.
Since, this discussion started with antennas, I'll comment about it
first.
I know that Hygain for many years used a .66 reduction in projected area
to arrive
at the "effective areas" published in the spec's. Roger Cox presented
papers on
the subject for conventions and club meetings. I'm sure Roger can verify
this.
This "shape factor" came from the older EIA-222-C spec. Without
appearing to pick
on Hygain, which I'm not, I have nothing to say about what Cushcraft,
KLM, Wilson,
and others were doing, because I have no evidence!Now, to the Rohn side
of the
equation. I have a Consumer Products catalogue, dated Jan 1986. This one
states
"All towers are designed according to E.I.A. specifications."The last
revision
date on drawing C-640603-R2 (25G Zone "A" - 30 Psf) is 2/19/76. Since I
found out
that EIA-222-D was a 1986 revision, one could expect that the Rohn EIA
reference
was to 222-C or earlier. This tower layout drawing states, in note 2,
that that
"Allowable load at apex of tallest tower is 180 Lbs. lateral thrust
(equivalent to
6 SqFt)."
The tallest tower in the collection is 200'. Nothing is stated about
allowable
loads for the shorter towers. The towers in this drawing all have the
upper guy
set attached to the top of the tower, unlike the newer drawings. The
newer
drawings also show different, increasing areas for shorter towers.
So, back to your question. Before we can attempt to find an answer, we
first need
to find out what U.S. Towers meant by their antenna area value, along
with the
definition of what they used as the guiding spec? I think we may find
that they
used a U.B.C. spec which changed in 1988. So, now we get to put apples
and oranges
into the barrel and bob for an answer.
A side note: Based on the work of another, I have learned that all of
the popular
recognized spec's (and their rev's) use different methods to determine
the actual
loads to be applied to a structure. Comparing them all, shows that
however they
get there, they do get very close to the same applied loads. One can
understand
the confusion this has created for the antenna builders, trying to keep
up with
it.
This is why I opened my big mouth and talked about it. We won't resolve
this deal
immediately, across our segment of the industry. But, the discussion is
good, and
I hope it will lead to widely accepted and understood standards.
Let's not panic yet about your tower, until more is understood.
Comment from Steve:
> This is an important point and my particular area of interest as
well.
> What are the implications vis-a-vis Rohn projected area antenna specs
- either
> round-member or flat member antennas? What will enable us to make
direct
> comparisons between antenna and tower figures?
The flat projected areas will.
The new Hygain values are directly applicable to the 222-E Rohn tower
layout
drawings. I just got a new catalogue from Champion Radio that contains
222-E
drawings. Please advise when the 222-F info is available. Since, the EIA
(and most
other) spec's allow a reduction in wind loading of an antenna (they call
it an
appurtenance) based on the use of "round members", we get to use the
larger areas
contained in the oval outlines of the Rohn diagrams. We get to use a
larger area
because the "round membered" antenna needs more projected area to
produce the same
load on the tower as a "flat membered" antenna (values in the
rectangles). The
area in the rectangle produces the same load on the tower as the area in
the oval.
The only difference is the shape of the members.
This is a clue not to build your antennas with square or rectangular
tubing,
because it has a higher drag coefficient and will result in higher loads
on the
tower!
Comments from Dick:
> >> A year ago, when I bought my TH-7 (then rated by Hy-Gain at 9.4 sq
ft
> windload area) for placement on my U.S. Tower MA-770MDP 72' tubular
rotating
> crankup (then "conservatively" rated by the manufacturer at 10 sq.
ft. @ 50
> MPH), I thought I could rely on the configuration being safe at wind
speeds
> up to 50 MPH (assuming no ice loading.) I knew that the configuration
was
> unlikely to survive winds much above 50 MPH. In fact, U.S. Tower's
own
> engineering calculations showed a (conservative) derating of the
fully
> extended tower to less than 2 sq. ft. at 70 MPH! I suspect that the
original
> 50 MPH rating was done long ago, before the computational standards
changed,
> whereas the 70 MPH rating was done relatively recently.
Comments from Steve:
> According to Bruce Kopitar, KB6JAH, the President of US Towers,
the
> towers are really rated at 60 MPH while the literature indicates 50
MPH
> ratings.
>
> Bruce Horn, WA7BNM, has done some calculations and written an
> interesting little paper on crank-up specs and windloading. It
illustrates how
> a crank-up can be de-rated for different windspeeds and loads. The
tower is
> one of the 89-footers but the implications are similar for their other
crank-
> ups. Also, Roger Cox, WB0DGF, of Hy-Gain wrote an article that
addressed some
> of the same topics about ten years ago. Both are interesting reading
and any
> crank-up owner or potential owner would be well advised to have copies
of
> both. They are available for a buck each from Tower Tech, Box 572,
> Woodinville, WA, 98072.
There is still more to be learned about what everyone was, and is now
thinking and
doing.
> A BIG TowerTalk Thank You!! to Kurt and Roger for their work in
working
> on this topic and present the information to us.
If this discussion gets us somewhere, it will have been worth the
effort.
Unfortunately, all of the parties are not yet involved. At the risk of
becoming
very unpopular with some folks, I'll say that, the sooner the buying
public starts
understanding and asking the right questions of the suppliers, the
sooner we'll
get the thing resolved.
73, Kurt
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