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August 17th 03, 06:42 PM

Please see comments below.
--
J. Mc Laughlin - Michigan USA
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"Brian Kelly" wrote in message
snip

That's nonsense. If you want a properly-designed pipe mast you do it
by the numbers, not with amateur "eyeball structural engineering"
and/or broad-brush statements like "have to use some fairly thick
steel pipe . . ". You won't know a thing about any pipe sizes until
and unless you run the numbers. A 40 foot pipe mast analysis is as
simple as it gets in the field of applied mechanics, any sophomore
student in any engineering discipline including the EEs who brought
you your HT can handle the job. A pipe mast is just a classic
cantilever beam.

I agree with the essence of Brian's observation. However, I offer two
additional observations:
1. The engineering understanding of tall columns using steel has gone
through quite an evolution in my lifetime. Buckling of a vertical
cantilever beam (A.K.A. a column) is a different issue from the breaking
of an actual cantilever beam. Need to study the latest steel handbook
and the tower standard and use an appropriate safety factor that is
larger than 1. [Yes, the latter is a snide remark. People do sell
stuff with specifications that are accurate for a safety factor of one!]
2 The engineering understanding of the effects of wind has also
undergone quite an evolution. See the tower standard (222F, I think is
the last one published). Any simple relationship between "miles per
hour" and pressure is almost certain to be inappropriate. [Do not
believe most antenna manufacturer's survival ratings in terms of MPH.]

However, for the heights being discussed, the "roughness" of a typical
urban neighborhood will significantly reduce wind loading from what one
would expect using weather wind info. That fact provides an additional
safety factor that just does not exist where I have placed antennas and
towers.

I give a big Amen to Brian's observation that even EEs should be able to
run the numbers AND a big Amen to the need to run the numbers. This is
an area where one wants to check twice - or more.
73 Mac N8TT

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