Date: Thu, 02 Aug 2012 18:23:52 -0700
From: Steve Dyer W1SRD <firstname.lastname@example.org>
Subject: [TowerTalk] Wind Load Spec Differences Between Tower and
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,
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 (
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
is circa 1976 ! They should get with the program, it”s now 2012. These
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
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
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
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
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