No not today, but I can remember 19 March 1969 when heavy freezing rain
affected west Yorkshire after at least two months of Arctic /continental
easterlies . The rain was prolonged and heavy on this day with the
temperature at or near -2 Deg for much of the day. The weight of ice on the
old mast brought it crashing to the ground. It wasn't lomg before the UK's
largest structure of over 1200 feet was rebuilt to withstand any icing. The
mast in my recolection has not experienced icing to this degree since and
certainly not at such a late date in March!

Rob
Farsley

Rob
Farsley

"Rob Brooks" wrote in message
...
No not today, but I can remember 19 March 1969 when heavy freezing
rain affected west Yorkshire after at least two months of Arctic
/continental easterlies . The rain was prolonged and heavy on this
day with the temperature at or near -2 Deg for much of the day. The
weight of ice on the old mast brought it crashing to the ground. It
wasn't lomg before the UK's largest structure of over 1200 feet was
rebuilt to withstand any icing. The mast in my recolection has not
experienced icing to this degree since and certainly not at such a
late date in March!

These high level masts were (and are) subject to icing - the problem was
the persistence of this event, and the combination of the wind and the
uneven formation of the ice. Also, there was some suggestion (which the
following appears to confirm), that the aerodynamic design was at fault.
Here is the relevant section from a report on the 'Brewer Consulting'
site to save anyone looking it up ....


" The British Standard at the time required that the mast should be
capable of withstanding wind speeds of 80 miles per hour at 40 feet
above the ground. In the event, the mast collapsed with wind speeds of
no more than 20 miles per hour, although an enquiry into the collapse
concluded that very cold weather conditions had played a part. There had
been thick fog, snow and freezing rain which had produced a condition of
glazed frost on the mast and its cable stays. This ice loading had been
asymmetric. A steady breeze for over four days in very cold weather had
caused cable stays on one side of the mast to be coated with ice, whilst
the cables on the other side remained relatively free from ice. The
result was that the mast was being pulled over towards the cables which
were laden with ice, creating tension on the opposite side.

More critical to the collapse however, was a phenomenon called 'vortex
shedding'. Vortex shedding apparently occurs when vortices, or pockets
of negative air pressure, are formed around cylindrical objects such as
a mast or chimney, even in relatively light wind conditions. When the
frequency of vortex shedding approaches the natural frequency of the
mast, large forces may build up creating oscillation.

BICC accepted at trial that it had not taken into account ice loading in
its design of the mast, having assumed wrongly that ice would be blown
off the cables long before critical wind speeds were reached. It had
failed to recognise that the critical wind speeds for a cylindrical mast
might be very much lower than would be the case with masts of other
designs. "

Martin.


--
Martin Rowley
Bracknell

On Sun, 18 Mar 2007 20:11:06 GMT, Martin Rowley wrote in

big snip
quote
More critical to the collapse however, was a phenomenon called 'vortex
shedding'. Vortex shedding apparently occurs when vortices, or pockets
of negative air pressure, are formed around cylindrical objects such as
a mast or chimney, even in relatively light wind conditions. When the
frequency of vortex shedding approaches the natural frequency of the
mast, large forces may build up creating oscillation.
endquote

Von Karman vortex shedding is a topic I became interested in some 35 years
ago. The first set of boiler chimneys (about 100 feet high) at my uni
partly failed, banging into each other as the process went on, before
finally collapsing after a fire. Vortex shedding was clearly the major
factor in inducing the vibration. The next set were built with a helix
around the top 1/4 to reduce the effect and that worked.

The nearby concrete (10 storeys I think) tower block was also subject to
the same effect. There were frequent complaints from the occupants of the
higher floors about feelings of nausea and "sea sickness". There followed
a fairly large research project to identify the mode and reasons for the
oscillation.

The interesting thing about this sort of wind induced vibration is that
there is a critical wind speed range (not necessarily very high) within
which oscillation occurs. A well documented example of such a structural
failure is that of the Tacoma Narrows Bridge failure in 1940. More on that
he

http://www.tech.plym.ac.uk/sme/Inter...Cases/sf1.html

--
Mike Tullett - Coleraine 55.13°N 6.69°W posted 18/03/2007 21:15:24 GMT