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Steel Designers' Manual - 6th Edition (2003)
flow charts. In all instances, the drift is idealized as a varying, triangularly
distributed load.
The drift condition must be allowed for not only in the design of the frame
itself, but also in the design of the purlins that support the roof cladding, since the
intensity of loading at the position of maximum drift is often far in excess of the
minimum basic uniform snow load.
In practice, the designer will invariably design the purlins for the uniform load
case, thereby arriving at a specific section depth and gauge. In the areas subject to
drift, the designer will maintain that section and gauge by reducing the purlin
spacing local to the greater loading in the area of maximum drift. (In some instances,
however, it may be possible to maintain purlin depth but increase purlin gauge in
the area of the drift. An increase in purlin gauge implies a stronger purlin, which in
turn implies that the spacing of the purlins may be increased over that of a thinner
gauge. However, there is the possibility on site that purlins which appear identical
to the eye, but are of different gauge, may not be positioned in the location that the
designer envisaged. As such, the practicality of the site operations should also be
considered, thereby minimising the risk of construction errors.)
Over the years, the calculation of drift loading and associated purlin design
has been made relatively straightforward by the major purlin manufacturers, a
majority of whom offer state of the art software to facilitate rapid design, invariably
free of charge.
1.3.2 Wind loads
A further significant change that must be accounted for in the design (in the UK)
of structures in general (including the single-storey structures to which this chapter
alludes) has been the inception of BS 6399: Part 2 – Code of practice for wind loads
in lieu of CP3: Chapter V: Part 2, which has been declared obsolescent.
A cursory inspection of the former will show that BS6399: Part 2 addresses the
calculation of the wind loading in a far more rigorous way than CP3: Chapter V:
Part 2, and offers two alternative methods for determining the loads that the
structure must withstand:
• Standard method – this method uses a simplified procedure to obtain a standard
effective wind speed, which is used with standard pressure coefficients to determine
the wind loads for orthogonal design cases
• Directional method – this method derives wind speeds and pressure coefficients
for each wind direction, either orthogonal or oblique.
In both methods, the dynamic wind pressure, qs, is calculated as follows:
qs = 0.613Ve 2
Ve = Vs ¥ Sb
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