Notes to Figure 4.1.6.12.:
4.1.7.3. Static Procedure (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 = IwqCeCtCgCp where, p = specified external pressure acting statically and in a direction normal to the surface, considered positive when the pressure acts towards the surface and negative when it acts away from the surface, Iw = importance factor for wind load, as provided in Table 4.1.7.3., q = reference velocity pressure, as provided in Sentence (4), Ce = exposure factor, as provided in Sentences (5) and (7), Ct =   topographic factor, as provided in Article 4.1.7.4., Cg = gust effect factor, as provided in Sentence (8), and Cp = external pressure coefficient, as provided in Articles 4.1.7.5. and 4.1.7.6. Table 4.1.7.3.   Importance Factor for Wind Load, IW Forming Part of Sentence 4.1.7.3.(1) Item   Column 1 Importance Category   Column 2 Importance Factor, IW ULS   Column 3 Importance Factor, IW SLS 

  1.   Low   0.8   0.75 

  2.   Normal   1.0   0.75 

  3.   High   1.15   0.75 

  4.   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, such as to produce the most critical effect, of the external pressure or suction calculated in accordance with Sentence (1) and the specified internal pressure or suction due to wind calculated as follows: pi = IwqCeiCtCgiCpi 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 defined in Sentence (1) q = reference velocity pressure, as defined in Sentence (1) Cei = exposure factor for internal pressure, as provided in Sentence (7), Ct = topographic factor, as defined in Sentence (1), Cgi = internal gust effect factor, as provided in Sentence (10), and Cpi = internal pressure coefficient, as provided in Article 4.1.7.7. (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 based on the reference height, h, determined in accordance with Sentence (6) for the surface or part of the surface under consideration and 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 thereof, (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 20 times the height of the building, whichever is greater, or (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 20 times the height of the building
from a change in terrain conditions, whichever is greater, provided an appropriate interpolation method is used. (6) The reference height, h, shall be determined as follows: (a) for buildings
with height less than or equal to 20 m and less than the smaller plan dimension, h shall be the mid-height of the roof above grade, but shall not be less than 6 m, (b) for other buildings, h shall be, (i) the actual height above grade of the point on the windward wall for which external pressures are being calculated, (ii) the mid-height of the roof for pressures on surfaces parallel to the wind direction, and (iii) the mid-height of the building for pressures on the leeward wall, and (c) for any structural element exposed to wind, h shall be the mid-height of the element above the ground. (7) The exposure factor for internal pressure, Cei, shall be determined as follows: (a) for buildings
whose height is greater than 20 m and that have a dominant opening, Cei
shall be equal to the exposure factor for external pressures, Ce, calculated at the mid-height of the dominant opening, and (b) for other buildings, Cei shall be the same as the exposure factor for external pressures, Ce, calculated for a reference height, h, equal to the mid-height of the building or 6 m, whichever is greater. (8) Except as provided in Sentences (9) and 4.1.7.6.(1), the gust effect factor, Cg, shall be one of the following values: (a) 2.0 for the building
as a whole and main structural members, or (b) 2.5 for external pressures and suctions on secondary structural members including cladding. (9) For cases where Cg and Cp are combined into a single product, CpCg, as provided in Article 4.1.7.6., the values Cp and Cg
need not be independently specified. (10) The internal gust effect factor, Cgi, shall be 2.0, except it is permitted to be calculated using the following equation for large structures enclosing a single large unpartitioned volume that does not have numerous overhead doors or openings:  where, V0 = internal volume in m3, and A = total area of all exterior openings of the volume in m2.