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uk.sci.weather (UK Weather) (uk.sci.weather) For the discussion of daily weather events, chiefly affecting the UK and adjacent parts of Europe, both past and predicted. The discussion is open to all, but contributions on a practical scientific level are encouraged. |
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#1
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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 |
#2
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"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 |
#3
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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 |
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