4.1.8.1. Specified Wind Loading (1) The specified external pressure or suction due to wind on part or all of a surface of a building shall be calculated from   where

p

=

the specified external pressure acting statically and in a direction normal to the surface either as a pressure directed towards the surface or as a suction directed away from the surface,


 

q

=

the reference velocity pressure as provided for in Sentence (4),


 

Ce

=

the exposure factor as provided for in Sentence (5),


 

Cg

=

the gust effect factor as provided for in Sentence (6), and


 

Cp

=

the external pressure coefficient averaged over the area of the surface considered. (2) The net wind load for the building as a whole shall be the algebraic difference of the loads on the windward and the leeward surfaces, and in some cases may be calculated as the products of the external pressures or suctions and the areas of the surfaces over which they are averaged as provided in Sentence (1). (3) The net specified pressure due to wind on part or all of a surface of a building shall be the algebraic difference of the external pressure or suction as provided for in Sentence (1) and the specified internal pressure or suction due to wind calculated from   where

pi

=

the specified internal pressure acting statically and in a direction normal to the surface either as a pressure (directed outwards) or as a suction (directed inwards),


 

q

=

the reference velocity pressure, as provided for in Sentence 4,


 

Ce

=

the exposure factor, as provided for in Sentence 5, evaluated at the building mid-height instead of the height of the element considered,


 

Cg

=

the gust effect factor as provided for in Sentence (6), and


 

Cpi

=

the internal pressure coefficient. (4) The reference velocity pressure, q, is the appropriate value determined in conformance with Subsection 2.5.1. for the following conditions: (a) the reference velocity pressure, q, for the design of cladding shall be based on a probability of being exceeded in any one year of 1 in 10, (b) the reference velocity pressure, q, for the design of structural members for deflection and vibration shall be based on a probability of being exceeded in any one year of 1 in 10, (c) for all buildings, except those listed in Clause (d), the reference velocity pressure, q, for the design of structural members for strength shall be based on a probability of being exceeded in any one year of 1 in 30, and (d) the reference velocity pressure, q, for the design of structural members for strength for post-disaster buildings shall be based on a probability of being exceeded in any one year of 1 in 100. (5) The exposure factor Ce shall be (a) the value shown in Table 4.1.8.1. for the appropriate reference height for the surface or part of the surface, (b) the value of the function (h/10)1/5 but not less than 0.9 where h is the reference height above grade in metres for the surface or part of the surface, or (c) if a dynamic approach to the action of wind gusts is used, an appropriate value depending on both height and shielding. Table 4.1.8.1. Exposure Factors, Ce Forming Part of Sentence 4.1.8.1.(5)  Height, m

Exposure Factor


>0 to ≤ 6

0.9


> 6 to ≤ 12

1.0


> 12 to ≤ 20

1.1


> 20 to ≤ 30

1.2


> 30 to ≤ 44

1.3


> 44 to ≤ 64

1.4


> 64 to ≤ 85

1.5


> 85 to ≤ 140

1.6


> 140 to ≤ 240

1.8


> 240 to ≤ 400

2.0


Column 1

2 (6) The gust effect factor Cg is one of the following values: (a) 1.0 or 2.0 for internal pressures as appropriate, (b) 2.0 for the building as a whole and main structural members, (c) 2.5 for small elements including cladding, or (d) if a dynamic approach to the action of wind gusts is used, an appropriate value depending on the turbulence of the wind and the size and natural frequency of the structure.