----- Original Message -----
From: "Rick Stoneking" <w2rds@arrl.net>
To: <towertalk@contesting.com>
Sent: Monday, November 18, 2002 5:20 PM
Subject: [Towertalk] General Questions about tower strength
> All,
>
> Have a couple of questions that I was hoping one of the many learned
people
> on this reflector could help me with, for my own understanding.
>
> 1. I have a ham buddy who is putting up a 72' freestanding tower and as
> part of the permit process had to have the drawings reviewed and approved
> by a New Jersey PE. One of the things the PE said was that the size of
the
> base had to be increased from the 4x4x7 (or whatever the exact dimensions
> were per the mfg) to 5x5x8 (essentially one foot larger in each
> dimension). So my question is, what does that do for the strength of the
> tower? It seems to me that all it does is make it more difficult for the
> entire tower (including the base - intact) to tip over, which seems like a
> pretty unlikely failure anyway?
Two hypothesis gain currency in Belgium, some ingeneers say 4x4x7, others
say inversely
7 x 7 x 4.
My own tower 70' has its base 7,6 x 7,6 x 3,0
>
> 2. I have heard that the size of the base is such that it would support
the
> tower and rated load, at the rated wind speed, even if the base was not in
> the ground (i.e. sitting on top of the ground). Is that true - always,
> sometimes, never?
Thats seems to be correct , same method in Belgium.
In the ground is a complementary security, which in general is considered to
be unstable as long as a specific behavier of the ground is unknown.
> 3. Does anyone know of a reference source that talks about the windload
> ratings of a crank up tower as a function of the height it is raised
> to? In other words, if my tower is rated at 50mph winds with 9 sq feet of
> load, at its full height of 67', what is the rating if it is extended to
> half that height? I believe the rating increases as the tower is lowered
> but I have not found anything authoratitive (or semi-authoritative) that
> discusses this - even at a generic level.
Theoretically, for a given construction, lower height supports higher
windload. But one shall be aware that practical conditions change from
region to region. So its a good practice to ask a weather institute of your
region what parameters should be taken into account. If a tower is very
high, say 250' it might be exposed to higher windloads at 150' then 250'.
Jos
> Thanks in advance,
> Rick
> W2RDS
>
>
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