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Tower Ratings - Stupid Question

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Subject: Tower Ratings - Stupid Question
From: (Chris Pedder)
Date: Sun, 22 Dec 1996 10:55:48 GMT
At 19:04 21/12/96 +0000, you wrote:
>Just had a local amateur here have his "guyed self suporting" tower com 
>down.  The recent heavy wet snow we had brought down a large (12+" 
>diameter) branch that landed on one of his guy wires.  The tower began 
>to retract into itself, and then bent at the top of the bottom section 
>at a 45 degree angle directly over the top of his house.  His tower was 
>not overloaded, he just didn't want to have to crank it down in 
>icing/high wind conditions.  
>Once again, a potentially deadly situation avoided by luck.
>Al, KE1FO, ex. KE6BER or
>Check out my web page, for summaries 
>from the contest reflector and a growing list of amateur radio links.
Comments about crank-up towers are always of interest because it is probably
fair to say that here in England they are the type of tower most used. There
are many reasons for this, not least of which our discriminatory 'planning'
laws and the failure of our National Radio Society to realise how big a
problem this is. Getting planning consent (building permit) for a modest, 65
feet, tower is a nightmare. The presumption is that you should NOT be
allowed to have it.

My own tower comes with graphs which show the maximum permitted head load at
various wind speeds while un-guyed. Theese vary from 135Kg at 100km/h to
25kg at 185km/h and 0kg at 200km/h. I imagine thta the latter figure means
that at 200km/h the tower, if cranked up, would come over with nothing on
the top! The same tower if cranked down to say 45 feet would have a maximum
permitted headload in excess of 125kg at 200km/h.

If guyed the maximum permitted headload at full height is 400kg in 160km/h
winds. Clearly if you want to 'leave it up' it pays to guy it, though I
choose to motorise it and crank it down if the wind gets really bad. When
the tower is cranked up there are mechanical locks which take some or all
the strain off the cables used to raise and lower the tower. If you choose
to guy the tower and do so correctly the effect is to increase downward
force on the tower, which these locks absorb. It is interesting to note that
our rigging of the tower raising ropes differs from that which I have seen
applied to towers manufactured in the States. I am thinking of a
Telex/Hy-Gain that I have seen most recently. We rig the cables to maximise
the velocity ratio and thus minimise the tensions in the cables. (It may
well be that other US manufacturers do the same, my recent experience is
limited to the previously mentioned tower.)

As for a large branch fallling on the guy wires, this would surely have
brought down one of your standard Rohn guyed towers anyway. For those of us
who have had bad experiences on towers ( I had and vowed never to climb
again) a crank up has some advantages. It needs to be well designed and
constructed and some over here are not. The common failure mode is for the
wind to get up, gravity is then insufficient to lower the tower, sections of
which have 'locked' the wind increases and the tower folds in the centre
section. This can be avoided by ensuring that there is positive 'pull-down'
on the sections.

One form of 'crankup' tower is the Sky Needle. Don't hear too many people
complaining about their performance. Anyone know of one going cheap?


Chris Pedder    G3VBL/8P9EM
DX-Cluster      g3vbl > gb7dxd

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