4.1.7.1 — Specified Wind Load

(1) Except as provided in Sentences (2) and (3), wind tunnel tests on scale models to determine wind loads on buildings shall be conducted in accordance with ASCE/SEI 49, “Wind Tunnel Testing for Buildings and Other Structures”. (2) Where an adjacent building provides substantial sheltering effect, the wind loads for the main structural system shall be no lower than 80% of the loads determined from tests described in Sentence (1) with the effect of the sheltering building removed as applied to, (a) the base shear force for buildings with ratio of height to minimum effective width, as described in Sentence 4.1.7.2. (2), less than or equal to 1.0, or (b) the base moment for buildings with a ratio of height to minimum effective width greater than 1.0. (3) For the design of cladding and secondary structural members, the exterior wind loads determined from the wind tunnel tests shall be no less onerous than those determined by analysis in accordance with Article 4.1.7.3. using the following assumptions: (a) Cp = ±0.72 and Cg = 2.5, where the height of the building is greater than 20 m or greater than its minimum effective width, and (b) CpCg = 80% of the values for zones w and r provided in Article 4.1.7.6., where the height of the building is less than or equal to 20 m and no greater than its minimum effective width.

(1) Except as provided in Sentences (2) and (3), a building is permitted to be classified as not dynamically sensitive. (2) A building shall be classified as dynamically sensitive if, (a) its lowest natural frequency is less than 1 Hz and greater than 0.25 Hz, (b) its height is greater than 60 m, or (c) its height is greater than 4 times its minimum effective width considering all wind directions, where the effective width, w, of a building shall be taken as, w = (Σ h_i w_i) / (Σ h_i) where, the summations are over the height of the building for a given wind direction, hi =the height above grade to level i, and wi =the width normal to the wind direction at height hi. (3) A building shall be classified as very dynamically sensitive if, (a) its lowest natural frequency is less than or equal to 0.25 Hz, or (b) its height is more than 6 times its minimum effective width, where the minimum effective width is determined in accordance with Clause (2)(c).

(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. (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: [[FORCE_FORMULA]] 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: C_gi = 1 + 1 / sqrt(1 + V_0 / (6950 · A)) where, V0 =internal volume in m3, and A =total area of all exterior openings of the volume in m2.

(1) For the application of the Dynamic Procedure, the provisions of Article 4.1.7.3. shall be followed, except that the exposure factor, Ce, shall be as prescribed in Sentences (2) and (3), and the gust effect factor, Cg, shall be as prescribed in Sentence (4), when determining the wind loads on the main structural system. (2) For buildings in open terrain as described in Clause 4.1.7.3. (5)(a), the value of Ce for the design of the main structural system shall be calculated as follows: Ce = (h/10)0.28, but 1.0 ≤ Ce ≤ 2.5 (3) For buildings in rough terrain as described in Clause 4.1.7.3. (5)(b), the value of Ce for the design of the main structural system shall be calculated as follows: Ce = 0.5(h/12.7)0.5, but 0.5 ≤ Ce ≤ 2.5 (4) For the design of the main structural system, Cg shall be calculated as follows: Cg = 1 + gp(δ/μ) where, gp =peak factor calculated as, √(2 ln(ν·T)) + 0.577 / √(2 ln(ν·T)) and δ/μ = √((K / C_eH) · (B + sF / β)), where, υ =average fluctuation rate calculated as f_nD · √(sF / (sF + βB)), T =3 600 s, K =0.08 for open terrain and 0.10 for rough terrain, CeH =exposure factor evaluated at reference height h = H, B =background turbulence factor, a function of w/H determined from Figure 4.1.7.8., s =size reduction factor calculated as π/3 · (1 / (1 + 8 f_n H / (3 V_H))) · (1 / (1 + 10 f_n w / V_H)), F =gust energy ratio calculated as x_0^2 / (1 + x_0^2)^{4/3}, where x0 = (1 220 fn/VH), and β =damping ratio, which shall be determined by a rational method or may be taken to be 0.01 for steel structures, 0.02 for concrete structures and 0.015 for composite structures, where, fnD =natural frequency of vibration of the building in the along-wind direction, in Hz, fn =lowest natural frequency of the building, in Hz, as described in Sentences 4.1.7.2. (2) and (3), H =height of the building, w =effective width of windward face of the building calculated as Σ h_i w_i / Σ h_i, where wi = width normal to wind direction at height hi, and VH =mean wind speed at the top of the structure, in m/s, calculated as V̄ · √(C_eH), where, =reference wind speed at a height of 10 m, in m/s, calculated as √(2 · I_w · q / ρ), where, Iw =importance factor, q =reference velocity pressure, in Pa, and ϱ =air density = 1.2929 kg/m3.