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Tower Ratings - Stupid Question

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Subject: Tower Ratings - Stupid Question
From: (Stan Griffiths)
Date: Tue, 24 Dec 1996 02:55:39 -0800 (PST)
>>The tower is heavy duty model, rated @ 30 sf / 50 mph  (I know, I know,
>>the wind blows harder than that!!!)
>This raises an interesting question. If the lowest wind in the TIA 
>standards is 70 mph, why do tower manufacturers specify tower capacity in 
>terms of 50 mph wind? 

Because if you knew how little it would take at 70 mph, you wouldn't buy it.

>Since wind pressure increases with the square of the velocity, and 50 mph 
>is roughly 1/sqrt(2) * 70 mph, this has the effect of presenting DOUBLE 
>the capacity at 70 mph. (Other things being equal -- which they rarely 
>are, since the tower face itself contributes considerable drag)

I believe most of the freestanding crankups I have seen would be lucky to
stand there extended with NOTHING on them at 70 mph.

>Also, for crank-up units, shouldn't there be different ratings for the 
>extended and nested positions?


>Sign me, Confused.
>Bill Coleman, AA4LR           Mail:

In my opinion, you have just come a LONG WAY toward getting unconfused.  You
have noticed that crankup manufactures seem to deliberately confuse the
issue of specifications.  I really believe that if you fully understood
exactly what a crankup manufacture will stand behind, you would have nothing
to do with them.  Your confusion is what they are DEPENDING UPON to make sales.

It is interesting to note that certain cities I have heard of REQUIRE
crankups to used in their city to ease the esthetics issue.  I am still
waiting for the first lawsuit against a city for REQUIRING the use of an
unsafe tower within their city.  After that occurs, they may a different
view of crankups . . .


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