4.1.7.5.External Pressure Coefficients

(1) Applicable values of external pressure coefficients, Cp, are provided in (a) Sentences (2) to (9), and (b) Article 4.1.7.6. for certain shapes of low buildings. (2) For the design of the main structural system, the value of Cp shall be established as follows, where H is the height of the building and D is the width of the building parallel to the wind direction: (a) on the windward face, Cp = 0.6 for H/D < 0.25 = 0.27(H/D + 2) for 0.25 ≤ H/D < 1.0 = 0.8 for H/D ≥ 1.0, (b) on the leeward face, Cp = –0.3 for H/D < 0.25 = –0.27(H/D + 0.88) for 0.25 ≤ H/D < 1.0, and = –0.5 for H/D ≥ 1.0, and (c) on the walls parallel to the wind, Cp = –0.7. (See Note A-4.1.7.5.(2) and (3)) (3) For the design of roofs, the value of Cp shall be established as follows, where x is the distance from the upwind edge of the roof: (a) for H/D ≥ 1.0, Cp = –1.0, and (b) for H/D <1.0, Cp = –1.0 for x ≤ H = –0.5 for x > H (See Note A-4.1.7.5.(2) and (3)) (4) For the design of the cladding and of secondary structural elements supporting the cladding, the value of Cp shall be established as follows, where W and D are the widths of the building: (a) on walls, Cp shall be taken as ±0.9, except that within a distance equal to the larger of 0.1D and 0.1W from a building corner, the negative value of Cp shall be taken as –1.2, (b) on walls where vertical ribs deeper than 1 m are placed on the facade, C p shall be taken as ±0.9, except that, within a distance equal to the larger of 0.2D and 0.2W from a building corner, the negative value of Cp shall be taken as –1.4, and (c) on roofs, Cp shall be taken as –1.0, except that (i) within a distance equal to the larger of 0.1D and 0.1W from a roof edge, C p shall be taken as –1.5, (ii) in a zone that is within a distance equal to the larger of 0.2D and 0.2W from a roof corner, Cp shall be taken as –2.3 but is permitted to be taken as –2.0 for roofs with perimeter parapets that are higher than 1 m, and (iii) on lower levels of flat stepped roofs, positive pressure coefficients established for the walls of the steps apply for a distance b. (See Figure 4.1.7.6.-D for the definition of b) (See Note A-4.1.7.5.(4)) (5) Except as provided in Sentence (6), for the design of balcony guards, the internal pressure coefficient, Cpi, shall be taken as zero and the value of Cp shall be taken as ±0.9, except that, within a distance equal to the larger of 0.1D and 0.1W from a building corner, Cp shall be taken as ±1.2. (6) Where the top of the balcony guard is 2.0 m or less below the roof surface, the values of Cp shall be taken as equal to those determined for parapets in Sentences (7) and (8). (7) To determine the contribution from parapets to the wind loads on the main structural system, the values of Cp shall be taken as (a) on the outer faces, equal to those on the walls below, (b) on the inner face of the windward parapet, equal to that on the upwind edge of a roof surface at the level of the top of the parapet, and (c) on the inner faces of the other parapets, zero. (8) For the structural design of parapets themselves, the values of C p shall be taken as equal to those specified in Sentence (7), except that the value of Cp on the inner face of the leeward parapet shall be taken as equal to that on the outer face of the windward parapet. (9) For the design of cladding on parapets, the values of Cp shall be taken as (a) on the outer vertical surfaces, equal to those on the cladding on the walls below, and (b) on the inner and top surfaces, equal to those on the cladding of a roof surface at the level of the top of the parapet.