[TowerTalk] Wind Load Spec Differences Between Tower and

[Jim Thomson]

Date: Thu, 02 Aug 2012 18:23:52 -0700
From: Steve Dyer W1SRD <w1srd at yahoo.com>
Subject: [TowerTalk] Wind Load Spec Differences Between Tower and
Antenna

I tried to get an answer from SteppIR to no avail so I thought I would 
reach out to the TT community. Searched TT archives and googled for 
answers. I've read K7NV's and W6NL's writeup as well about this subject.

Would like to be confident my LM-470 can handle a DB36 (17.5 sq ft using 
100 MPH EIA-222-C) .
I believe the 4 El SteppIR (9.7 sq ft) is fine on the WT51

Usual caveats about crank it down in the wind already noted. Also, the 
Tashjian specs are for a *new* tower. I don't know the vintage or 
history of the LM-470 I have. It came with the property.

Thanks es 73,
Steve
W1SRD

Here is my original email to SteppIR:

I am sure I am not the first to ask to understand the difference between 
wind load ratings.

SteppIR calculates wind load using EIA-222-C at 100 mph (wind zone B) 
which is equivalent to 120 mph for a 3 second average used by 
EIA-222-G. See Leeson, W6NL ( 
http://www.kkn.net/dayton2007/w6nl_ant.pdf )!

Tashjian uses TIA-222-G 85 mph 3 sec average which maps to a 70 mph 
fastest mile (222-A wind zone A).

This leads me to to the questions:

1. Is a tower rated at 13 sq ft/85 mph using 222-G is OK for the 4 El? 
Tower is a WT-51.
2. Is a tower rated at 18 sq ft/85 mph using 222-G is OK for the DB36? 
Tower is a LM-470D.

I understand you cannot commit to the safety of the tower, but I need to 
reconcile and understand the differences between the two measurement 
systems before I go forward.

##  why is  stepir  even using the ancient  EIA-222-C  in the 1st place.   That old spec
is circa  1976 !    They should get with the program, it”s  now  2012.   These revisions
have changed for a good reason. 

##  tower’s  rated for...  exposure B  are semi flaky at best..esp if the tower is high, like
70-100 feet , including a mast.   UST  does the same thing.  When you work out the maths,
with exposure B,  it assumes that a 70 mph  wind speed at the top of the tower will be  much
much less,  further down the tower.   Like 56 mph 20 feet above the lawn.   It might be....
and it might not be.  A wireless wind speed indicator at both the top of the tower and also
way down the tower will give you a real eye opener.   Sure,the wind may be partially blocked
from one or two directions, but not in others, and the tower is fully exposed. 

##  The actual pressure on the ant is a function of  wind speed + surface area of the yagi.   The surface
area is fixed.  Your DB-36  at  17.5 sq feet is a big mother.   Are you sure the LN-470 is rated 
for 18 sq feet at 85 mph.  Most crank up towers are only rated for 70 mph. 
It’s the GUSTS that will break stuff apart, and the gusts only have to last
2-3 secs  to have their destructive effect. 

##  at the very  least, key an eye on the WX at all times.  When they predict 50 mph winds for your
area,  it would be prudent to lower it way down in advance.   And if they predict 70 mph gusts, drop
it all the way down.   IMO, it’s  not worth taking a chance,considering the duration of the windstorm
is miniscule compared to the rest of the year.  IE:  2-3 hrs of total high wind speeds  per year. 

##  another thing to consider is it only takes 35-50 mph winds  to start  bring down trees  across 
power lines.   Then you have no power,  and no way to bring the tower down.  It’s  now stuck in the
UP position, meanwhile the wind velocity may well start increasing.   UST offer’s  a small hand crank assy,
to manually winch the tower down in the event of a commercial AC  power failure.   It simply clamps 
onto the 14 inch diam pulley.  Takes a while toi bring the tower down that way, but at least its  doable.

later... Jim   VE7RF