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Tower Loading

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Subject: Tower Loading
From: (Bill Cotter)
Date: Mon, 10 Feb 1997 11:01:58 -0500

I agree with Bryan's ideas on discussing tower installation failures. We
can build our systems to specifications as best we can and feel somewhat
secure because we did what the manufacturer suggested. Examining the
unfortunate events that have happened to others can benefit each of us.
Listening to discussions of failure analysis develops our common sense and
practical wisdom.

I once saw a 100ft Rohn-25 tower with a TH6-DXX drop in 50mph winds for
what amounts to the dumbest of reasons (now that I know better :-) The
gentleman installed guy anchors consisting of 3" diameter 4ft high
concrete-filled water pipe, concreted 6ft into the earth. He had bolted one
3/8" formed-wire screw eye on each anchor and connected his 3/16' wire and
turnbuckles to this single eye. The eye snapped like a pretzel and the
tower dropped....... By the next time I visited his qth he had installed a
new tower and had replaced his anchors with 6" steel I-beams and proper
termination to each anchor! 

Simple lesson:    OVERDESIGN and OVERBUILD with whatever tools, specs, etc.
we have on hand. Compare
                        lifetime cost-of-ownership to installation cost and 
find a balance.


Bill N4ALG

At 04:27 PM 2/7/97 -0800, Bryan Sparrowhawk wrote:
>I'd like to pump this thread for more information ... but first a comment on
>what's transpired.
>Calculations certainly help to take the guesswork out of tower installation
>design.  However, the 'comfort numbers' you get when you are done
>with the 'calculations' are no better than the model used.  The specs we
>get from Rohn support a very simple model.  No doubt  they fudge with
>lots of safety factor to account for other things such as resonances,
>imperfectly placed guys etc..   This leaves us second guessing them
>which i think is the underlying issue of this thread.
>The other extreme in 'calculations' would be a finite element analysis, a
>kind of mechanical ELNEC but more messy since it would have to
>combine aerodynamics, soils engineering, and classical physics... and
>other stuff i haven't thought of.  The result would be wonderful.   We
>probably aren't going to see any practical info. from something like that
>for awhile!
>So... what i want to hear are the stories of tower failures.  Not bent
>masts and crinkled yagi elements... but the real juicy ones where, for
>example, a well guyed 120'  25G tower with two or three big beams on
>top came down as a result of the tower sections themselves crumpling. 
>I want to know what kind of storm and loading abuses were put on the
>tower to make it fall.  Now that's practical information!
>I bet there arent' too many failures that fit into this category.  Guy and
>anchor failures.. or skimping and not guying every 30' or nearby trees
>falling over a guywire are the causes i would expect to see.  I also
>wouldn't think many towers fail because of a bad tower base.
>Load us up with your tower post mortems.  I hear there's 1000 people on
>this reflector, assuming each one knows about 5 tower installations,
>that's 5000 towers... certainly a couple of them fell under interesting and
>NON-OBVIOUS  circumstances that we could all learn from.
>de KD7LS, bryan
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