A few weeks back I posed the question outlined below. I
decided to pay Tashjian Towers to do an 80 MPH analysis
based on the EIA-222 Rev F standard, which uses the "fastest
mile" wind load definition, and the latest version Rev G
which uses the "3 second gust" wind load definition. As an
example of how the two standards compare, a 70 MPH basic
wind speed under Rev F equates to a 85 MPH basic wind speed
under Rev G.
The analysis was done using a comercial software package
called RISATower which can be used to analyze both guyed and
free-standing towers and allows any version of EIA-222 to be
used. The demo version on the RISA web site is interesting
to experiment with if you are so inclined.
As for the results, interestingly enough, both analysis came
up with a max antenna loading of 8 Sq Ft. The other
interesting result is that the weakest point in a fully
loaded and extended LM-470 isn't the base connection as one
might expect. The weak spot is the middle joint. If you
were to connect a cable to the top of the tower and attempt
to pull it over horizontally, it should buckle in the middle
first. I'm not sure if that was intentional or not. But, I
would rather have it fold in half rather than break off at
the bottom (if I had to choose one or the other).
I thought this info might be useful to some folks so I
decided to post the results. Obviously I would never
recommend leaving the tower fully loaded and extended in
winds of any significant magnitude. If you need
calculations run and/or a PE stamp on your tower plans for a
building permit, Karl at Tashjian Towers does a good job for
a very reasonable price.
73, Bob K6ZZ
----- Original Message Follows -----
From: "Bob Selbrede, K6ZZ" <k6zz@ccis.com>
To: towertalk@contesting.com
Subject: [TowerTalk] LM-470 at 100 MPH
Date: Sat, 29 Nov 2008 17:32:55 -0800
> Hello,
>
> Has anyone had the Triex LM-470 professionally evaluated
> for antenna load capacity in a 100 MPH Basic Wind Speed
> zone? Turns out our local building code will be changing
> on 1 Jan 09 to require new structures to meet the 100 MPH
> requirement. I suspect most free standing crankup towers,
> fully extended, may not handle these wind conditions
> unloaded let alone with a good size tribander on them.
> How does an 85 MPH 3 second gust rating of 18 Sq Ft look
> at 100 MPH? I would estimate around 4 Sq Ft.
>
> 73, Bob K6ZZ
>
> _______________________________________________
>
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