[TowerTalk] de-rating tower wind loading limits

[Stan Griffiths]

>To: henry gillow-wiles <henry@rio.com>
>From: w7ni@teleport.com (Stan Griffiths)
>Subject: Re: [TowerTalk] de-rating tower wind loading limits
>Cc: towertalk@contesting.com
>
>>I have a crank-up tower rated at 30 sq. ft. for a 50 mph wind. I have 20
>>ft. of chrome-molly mast with 4 ft. in the tower. I want to de-rate the
>>wind load for 70 mph. How do I do this and what effect does the mast have
>>on the wind loading limits of the tower? I want to put 8 sq.ft. near the
>>top of the mast and 8.5 sq. ft. at the top of the tower. Can I do this, or
>>am I dancing with disaster?
>>
>>tnx,    Henry   KB7RTA
>>
>>
>> 
>>henry gillow-wiles
>>henry@rio.com

Hi Henry:

I don't like my OWN answer to this and I will revise it as follows:

>Hi Henry,
>
>Let me think "outloud" about this for a minute or two.  
>
>First, when the wind goes from 50 to 70 mph, the forces on objects (towers
and beams) DOUBLES since it goes up with the square of the wind speed.
>
>Second, the tower itself has windload.  Just for discussion purposes, let's
assume the tower equals 20 square feet of load by itself.  I have never seen
a crankup tower manufacturer ever mention this point.
>
>This means that the tower plus the beams on top represent a load of  50
square feet and the tower is designed to withstand that at 50 mph.  Since at
70 mph it will only withstand a total of 12.5 square feet, we are in serious
trouble, because the tower with NOTHING on it is still 20 square feet . . .

Using my own reasoning, the tower should withstand 25 square feet at 70 mph,
not 12.5 square feet.  So it would still stand there if it had less than 5
square feet of antenna on it.

>You get the idea.  This is precisely WHY your tower was rated at 50 mph.
At 70 mph, I would expect it to fold with nothing on it . . .

Guess it is not quite that bad.  It would probably fold at 80 mph, though . . .

>Sticking a mast out the top with some of the load up there makes the
problem significantly worse . . . and yes, the mast also has windload which
has to be added to the total.
>
>Get the PE that someone else suggested.  Don't you find it strange that
derating information of this type is not published for crankups?  Certainly
you are not the first guy who asked this very question.  The reason is
simple.  Publishing that data would "unsell" a LOT of crankups . . .
>
>Stan  w7ni@teleport.com

I still stand by the last statement which is to get PE involved and I second
K6LL's comment that you really have to refigure the whole installation which
is a pretty complicated process . . . way beyond anything I would attempt
without help . . . but then, I'm not a PE either.

The only purpose in my comments was to try to raise awareness that it is NOT
a simple process and guessing will almost certainly lead to trouble . . .

Stan  w7ni@teleport.com


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