That scenario is consistent with the failure that I experienced several
years ago. We were visited by a sudden mini-tornado, and I'd
guesstimate that the wind peaked at around 100 mph. Tower was
supporting a four element, six band quad on a 24 foot boom and was fully
extended. The tower buckled at about the midpoint. We've never had
winds that severe since at this QTH.
73, Joe
K2XX
Bob Selbrede, K6ZZ wrote:
> 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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>
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