4.1.6.2.Specified Snow Load (See Note A-4.1.6.2.)

(1) The specified load, S, due to snow and associated rain accumulation on a roof or any other building surface subject to snow accumulation shall be calculated from the formula, S = Is [Ss (CbCwCsCa) + Sr] where Is = importance factor for snow load as provided in Table 4.1.6.2.-A, Ss = 1-in-50-year ground snow load, in kPa, determined in accordance with Subsection 1.1.3., Cb = basic roof snow load factor in Sentence (2), Cw = wind exposure factor in Sentences (3) and (4), Cs = slope factor in Sentences (5) to (7), Ca = accumulation factor in Sentence (8), and Sr = 1-in-50-year associated rain load, in kPa, determined in accordance with Subsection 1.1.3., but not greater than Ss(CbCwCsCa). Table 4.1.6.2.-A Importance Factor for Snow Load, IS Forming Part of Sentence 4.1.6.2.(1) Importance Factor, Is Importance Category ULS SLS Low 0.8 0.9 Normal 1 0.9 High 1.15 0.9 Post-disaster 1.25 0.9 (2) The basic roof snow load factor, Cb, shall (a) be determined as follows: (i) Cb = 0.8 for 𝑙c ≤ ( 2 ), and Cw (ii) 1 𝑙c Cw − 70 70 Cb = [1 − (1 − 0.8Cw ) exp (− ) ] for 𝑙c > ( 2 ) Cw 100 Cw where lc = characteristic length of the upper or lower roof, defined as 2w-w²/l, in m, w = smaller plan dimension of the roof, in m, and l = larger plan dimension of the roof, in m, or (b) conform to Table 4.1.6.2.-B, using linear interpolation for intermediate values of lc Cw2. (c) be taken as equal to 1 for any roof structure with a mean height of less than 1 + Ss/γ, in m, above grade, where γ is the specific weight of snow determined in accordance with Article 4.1.6.13. (See Note A-4.1.6.2.(2)) (3) Except as provided for in Sentence (4), the wind exposure factor, C w, shall be 1.0. (4) For buildings in the Low and Normal Importance Categories as set out in Table 4.1.2.1., the wind exposure factor, Cw, given in Sentence (3) may be reduced to 0.75 for rural areas only, or to 0.5 for exposed areas north of the treeline, where (a) the building is exposed on all sides to wind over open terrain as defined in Clause 4.1.7.3.(5)(a), and is expected to remain so during its life, (b) the area of roof under consideration is exposed to the wind on all sides with no significant obstructions on the roof, such as parapet walls, within a distance of at least 10 times the difference between the height of the obstruction and CbCwSs/γ in m, where γ is the unit weight of snow on roofs as specified in Article 4.1.6.13., and (c) the loading does not involve the accumulation of snow due to drifting from adjacent surfaces. (5) Except as provided for in Sentences (6) and (7), the slope factor, Cs, shall be, (a) 1.0 where the roof slope, α, is equal to or less than 30°, (b) (70° - α)/40° where α is greater than 30° but not greater than 70°, and (c) 0 where α exceeds 70°. (6) The slope factor, Cs, for unobstructed slippery roofs where snow and ice can slide completely off the roof shall be (a) 1.0 where the roof slope, α, is equal to or less than 15°, (b) (60° − α)/45° where α is greater than 15° but not greater than 60°, and (c) 0 where α exceeds 60°. (7) Unless otherwise stated in this Subsection, the slope factor, Cs, shall be 1.0 when used in conjunction with accumulation factors for increased snow loads. Table 4.1.6.2.-B Basic Roof Snow Load Factor for lc > (70/Cw2) Forming Part of Sentence 4.1.6.2.(2) Value of Cw Value of lc Cw 2 1.0 0.75 0.5 Value of Cb 70 0.80 0.80 0.80 80 0.82 0.85 0.91 100 0.85 0.94 1.11 120 0.88 1.01 1.27 140 0.90 1.07 1.40 160 0.92 1.12 1.51 180 0.93 1.16 1.60 200 0.95 1.19 1.67 220 0.96 1.21 1.73 240 0.96 1.24 1.78 260 0.97 1.25 1.82 280 0.98 1.27 1.85 300 0.98 1.28 1.88 320 0.98 1.29 1.90 340 0.99 1.30 1.92 360 0.99 1.30 1.93 380 0.99 1.31 1.95 400 0.99 1.31 1.96 420 0.99 1.32 1.96 440 1.00 1.32 1.97 460 1.00 1.32 1.98 480 1.00 1.32 1.98 500 1.00 1.33 1.98 520 1.00 1.33 1.99 540 1.00 1.33 1.99 560 1.00 1.33 1.99 580 1.00 1.33 1.99 600 1.00 1.33 1.99 620 1.00 1.33 2.00 (8) The accumulation factor, Ca, shall be 1.0, which corresponds to the uniform snow load case, except that where appropriate for the shape of the roof, it shall be assigned other values that account for, (a) increased non-uniform snow loads due to snow drifting onto a roof that is at a level lower than other parts of the same building or at a level lower than another building within 5 m of it horizontally, as prescribed in Articles 4.1.6.5., 4.1.6.6. and 4.1.6.8., (b) increased non-uniform snow loads on areas adjacent to roof projections, such as penthouses, large chimneys and equipment, as prescribed in Articles 4.1.6.7. and 4.1.6.8., (c) non-uniform snow loads on gable, arch or curved roofs and domes, as prescribed in Articles 4.1.6.9. and 4.1.6.10., (d) increased snow or ice loads due to snow sliding as prescribed in Article 4.1.6.11., (e) increased snow loads in roof valleys, as prescribed in Article 4.1.6.12., and (f) increased snow or ice loads due to meltwater draining from adjacent building elements and roof projections. (9) For shapes not addressed in Sentence (8), Ca corresponding to the non-uniform snow load case shall be established based on applicable field observations, special analyses including local climatic effects, appropriate model tests or a combination of these methods.