Rick, being a chemical engineer myself, I will and always have, left the
details to the structural guys, but the engineer probably is increasing the
base size due to his knowledge of the local soil conditions. You'd need a
larger base if the soil can't support the lateral forces to prevent the tip
over event. Also, when an licensed engineer stamps a drawing/design, they are
taking full responsibility (and possible liability) for it's design and use. I
wouldn't argue with an extra yard of concrete; whatever it'll take to get the
permit. Gd luck, Phil KB9CRY
> 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?
>
> 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?
>
> 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.
>
> Thanks in advance,
> Rick
> W2RDS
>
>
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