This Part of this British Standard has been prepared by Subcommittee B/525/1, Actions (loadings) and basis of design, and supersedes CP3 : Chapter V : Part 2 : 1972.
This Part of BS 6399 is a technical revision of CP3 : Chapter V : Part 2 and incorporates the considerable advances made and experience gained in wind engineering since that time. CP3 : Chapter V : Part 2 will not be withdrawn immediately so as to allow an overlap period with this Part of BS 6399.
The basic wind speed in this British Standard is given as an hourly mean value; this differs from CP3 : Chapter V : Part 2 in which it was based on a 3 s gust value. However, the hourly mean basic wind speed is subsequently converted into a gust wind speed for use in design (by a gust peak factor which takes account of gust duration time, height of structure above ground and the size of the structure). The adoption of the hourly mean value for the basic wind speed is for technical reasons. Primarily it allows a more accurate treatment of topography, but it also provides the starting point for serviceability calculations involving fatigue or dynamic response of the structure. Its use is also a move towards harmonization as mean values (sometimes 10 min means) are often the basis for wind loading calculations in European and international standards.
Structure factors are used to check whether the response of the structure can be considered to be static, in which case the use of the calculation methods in this standard is appropriate. If the response is found to be mildly dynamic the methods can still be used but the resulting loads will need to be augmented. Structures which are dynamic will also be identified but their assessment is outside the scope of the standard. Two alternative methods are given:
a) a standard method, which uses a simplified procedure;
b) a directional method, from which the simplified method was derived.
The standard method gives a conservative result within its range of applicability. Calibration has shown that loads on typical buildings obtained by the standard method are around 14 % larger than obtained from the directional method. The degree of conservatism can be much larger close to the ground and in towns, but decreases to zero around 100 m above the ground. In addition to reduced conservatism, the directional method assesses the loading in more detail, but with the penalty of increased complexity and computational effort. Because of this it is anticipated that the standard method will be used for most hand-based calculations and that the directional method will be implemented principally by computer.
Procedures are also given to enable the standard effective wind speed to be used with the directional pressure coefficients and for the directional effective wind speeds to be used with the standard pressure coefficients. CP3 : Chapter V : Part 2 allowed for the effect of ground roughness, building size and height above ground by a single factor. This required the calculation of separate wind speeds for every combination of reference height above ground and the size of the loaded area. However, a simplification has been introduced in the standard method which involves the calculation of only a single wind speed for each reference height. The effect of size is allowed for by a separate factor, Ca.
BS 6399 : Part 2 also gives values for external pressure coefficients for a greater range of building configurations than did CP3 : Chapter V : Part 2.
This Part of BS 6399 gives methods for determining the gust peak wind loads on buildings and components thereof that should be taken into account in design using equivalent static procedures.
Two alternative methods are given:
a) a standard method which 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.
b) a directional method in which effective wind speeds and pressure coefficients are determined to derive the wind loads for each wind direction.
Other methods may be used in place of the two methods given in this standard, provided that they can be shown to be equivalent. Such methods include wind tunnel tests which should be taken as equivalent only if they meet the conditions defined in annex A.
Specialist advice should be sought for building shapes and site locations that are not covered by this standard.
The methods given in this Part of BS 6399 do not apply to buildings which, by virtue of the structural properties, e.g. mass, stiffness, natural frequency or damping, are particularly susceptible to dynamic excitation. These should be assessed using established dynamic methods or wind tunnel tests.
