CROSS Safety Report
Application of wind load code BS EN 1991 - wind actions
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A reporter's concern is about the application of the wind load code BS EN 1991-1-4: 2005 Eurocode 1: Actions on structures - Part 1-4.
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Another reporter is concerned about the application of the wind load code BS EN 1991-1-4: 2005. This requires the engineer to select a basic wind speed from a map, and then by means of a series of factors derive a value for wind pressure by considering various exposure factors.
In most cases, they say, this gives a wind pressure greatly exceeding that which would result from the basic wind speed. However, having checked many calculations the reporter has noted that some design engineers clearly do not understand this very difficult to apply code, and just calculate a wind pressure that is the result of the basic wind speed.
In their opinion there is clearly a danger of accidents occurring as a result of this. They feel that the code should be amended to have a similar format to the old, superseded code CP3 Chapter V, which gave higher basic wind speeds which could then be reduced if the site was in a sheltered position. They hope that their concern receives due consideration.
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This report highlights the difficulties of dealing with new codes couched in different ways. The point about the format of code guidance is well made but is not always practical when working across disciplines or indeed internationally. Switching from wind speed definition between mean hourly and gust is one potential cause of confusion.
It must also be borne in mind that pressure is proportional to wind speed squared, whatever the code, so minor changes in the speed can have a significant effect on the predicted design loads. CP3: Chapter V was superseded in 1995 by BS 6399-2 and incorporated considerable advances in wind engineering. In fact, CP 3: Chapter V used factors greater than 1, which rarely caused a problem.
The process used in BS6399 is not greatly dissimilar from EN1991-2 in that a mean periodic wind speed is factored up based on the size of the gust, the topography, elevation and various other factors to give a design wind speed. Furthermore, a purely gust driven approach ignores the issue of continuous structures where relieving effects can make a huge difference to load effects.
Another concern is that simple structures such as hoardings may not be properly designed because of the perceived complexity of the approach. The temptation is to use a nominal pressure which may or may not adequate. Also, to be resisted is the temptation to mix Codes.
Designers must be aware of the impact of Eurocodes and modify their approach accordingly. Two useful publications from the Temporary Works Forum are: The use of European standards for temporary works design and Site Hoardings (April 14).