Notes to Table 4.1.5.12.:
4.1.7.1. Specified Wind Load (1) The specified external pressure or suction due to wind on part or all of a surface of a building shall be calculated using the following formula: p = IwqCeCgCp 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, Iw = importance factor for wind load, as provided in Table 4.1.7.1. 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. Table 4.1.7.1.   Importance Factor for Wind Load, LW Forming Part of Sentence 4.1.7.1.(1) and (3)  Column 1

Column 2

Column 3


Importance Category

Importance Factor, IW


ULS

SLS


Low

0.8

0.75


Normal

1.0

0.75


High

1.15

0.75


Post-disaster

1.25

0.75 (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 sum of 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, pi = IwqCeCgiCpi where, pi = specified internal pressure acting statically and in a direction normal to the surface, either as a pressure directed toward the surface or as a suction directed away from the surface, Iw = importance factor for wind load, as provided in Table 4.1.7.1. q = the reference velocity pressure, as provided for in Sentence (4), Ce = the exposure factor, as provided for in Sentence (5), Cgi = internal gust effect factor, as provided for in Sentence (6), and Cpi = the internal pressure coefficient. (4) The reference velocity pressure, q, shall be the appropriate value determined in conformance with Subsection 1.1.2. based on a probability of being exceeded in any one year of 1-in-50. (5) The exposure factor Ce, shall be, (a) (h/10)0.2 but not less than 0.9 for open terrain, where open terrain is level terrain with only scattered buildings, trees or other obstructions, open water or shorelines, h being the reference height above grade in metres for the surface or part of the surface, (b) 0.7(h/12)0.3 but not less than 0.7 for rough terrain, where rough terrain is suburban, urban or wooded terrain extending upwind from the building uninterrupted for at least 1 km or 10 times the building height, whichever is greater, h being the reference height above grade in metres for the surface or part of the surface, (c) an intermediate value between the two exposures defined in Clauses (a) and (b) in cases where the site is less than 1 km or 10 times the building height from a change in terrain conditions, whichever is greater, provided an appropriate interpolation method is used, or (d) if a dynamic approach to the action of wind gusts is used, an appropriate value depending on both height and shielding. (6) The gust effect factor, Cg, shall be one of the following values: (a) for the building as a whole and main structural members, Cg = 2.0, (b) for external pressures and suctions on small elements including cladding, Cg = 2.5, (c) for internal pressures, Cgi = 2.0 or a value determined by detailed calculation that takes into account the sizes of the openings in the building envelope, the internal volume and the flexibility of the building envelope, or (d) if a dynamic approach to wind action is used, Cg is a value that is appropriate for the turbulence of the wind and the size and natural frequency of the structure.