[TowerTalk] Wind Damage Reports from West?

[Jim Thomson]

Date: Sat, 3 Dec 2011 14:56:09 -0700
From: "EZ" <EZRhino at fastmovers.biz>
Subject: Re: [TowerTalk] Wind Damage Reports from West?

Here's a cut n paste from a ham in our club.  We had some real wicked wind
here a couple days ago; made a real mess north of Salt Lake with tractor
trailers blowing over, fences gone, shingles gone, motorhomes upside down
across the street, trampolines in trees, etc.

snip

I have a SteppIR DB18E that I just installed a couple of months ago on my US
Tower HD555. The DB 18 is (now was) sitting at the top of a 20 foot UST
Heavy Duty Mast extension that used to be about 15 feet above the thrust
bearing at the top of the tower. While I was out of town this week on
business, and the tower was fully extended, the high winds here in Bountiful
bent the mast over the top of the thrust bearing so that the mast and DB18
are now 15 feet below the thrust bearing and about 5 feet away from the side
of the top tower section. This puts the DB18 upside-down. I have lowered the
tower so that the Antenna is about 25 feet above ground level. I can't see a
way that I can fully lower the tower to safely get at the ant and mast
without doing some serious damage to the antenna. As luck would have it, the
antenna and mast are hanging directly over my garage roof.

##OK,  WHICH UST  20' mast did he use ??    If he had the 20' long x 2" OD
mast, with the 3/8" wall thickness,  and made from 4130 heat  treated chromolly
steel, he may have fared better.  UST's version is 107 ksi  yield strength. 

## UST tell's me  the biggest mast/bearing set up they can put on a tower
is 2.875" OD x .25" thick.   Now the best you can do with that version is 
1026 DOM, with a 87 ksi yield strength.  Both versions  just described  weigh
7 lbs per foot. The 2.875" x .25" wall stuff, even with it's lower yield strength
is one helluva lot stronger than the 2" x .375" wall  chromolly.  Both versions
cost the same, and both weight the same, so it's a no brainer imo. 

###  Now if you use NON heat treated chromolly, it's only 70 ksi yield strength. 
Some version's of 1026 DOM  are only 60-75 ksi.   Then you also have the "junk"
stuff, and 'structural grade stuff',  which is typ 35 ksi yield.    Then the worse case
is mounting a humongous array  15'  above the top of the tower, at the top of 
a weak mast = disaster  waiting to happen.   Or adding array's  between top of tower and
top of mast...at a mid point on the mast, etc.   Then toss in all the coax  going up the side of
the mast, which now makes the mast appear to be a bigger OD  to the wind.  

## The amount of bent mast pix and stories I have heard since 1970  will spin your head. 
Hams just don't  get it.  The MOST important thing on top of that tower is the mast itself. 
That's the last item that should break.  If the tower is fully nested,  you still don't want a
weak mast.   All these calcs are easily done via the spreadsheet, or on a calculator. 
What really drives me nuts is after the bent mast is removed via the crane, another cheap
mast get's  installed.   Those muffler clamps that are used to mount the yagi to the mast,
toss em, they won't work anyway, and replace em with the DX eng super mast clamps.

##  If the wind speed  exceeds a certain threshold, XXX, the tower  should  fully retract
itself.  If you lose commercial AC power along the way, buy the optional hand crank
from UST,  which mounts to the  big 14" diam pulley, and crank it down by hand, or use
a  cordless  drill, or UPS power, etc.  Leaving the tower fully extended while out
of town is not too smart either. 

later... Jim  VE7RF