4.1.6.5.Multi-Level Roofs

(1) The drifting load of snow on a roof adjacent to a higher roof shall be taken as trapezoidal, as shown in Figure 4.1.6.5.-A, and the accumulation factor, Ca, shall be determined as follows: Ca = Ca0 – (Ca0 – 1)(x/xd), for 0 ≤ x ≤ xd or Ca = 1.0, for x > xd where Ca0 = peak value of Ca at x = 0 as specified in Sentences (3) and (4) and as shown in Figure 4.1.6.5.A., x = distance from roof step as shown in Figure 4.1.6.5.-A, and xd = length of drift as specified in Sentence (2) and as shown in Figure 4.1.6.5.-A. Figure 4.1.6.5.-A Snow Load Factors for Lower Level Roofs Forming Part of Sentences 4.1.6.5.(1) and (3), Table 4.1.6.5.-A and Sentence 4.1.6.6.(1) Notes to Figure 4.1.6.5.-A: (1) If a > 5 m or h ≤ 0.8Ss/, drifting from the higher roof need not be considered. (2) If h ≥ 5 m, the value of Ca0 for Case I is permitted to be determined in accordance with Sentence 4.1.6.5.(4). Table 4.1.6.5.-A Wind Exposure, Slope and Accumulation Factors in Figure 4.1.6.5.-A Factors Distance from Roof Step, x Cw Cs(1) Ca 0 1.0 f() Ca0 0 < x ≤ xd 1.0 f() Ca0 − (Ca0 − 1)(x/xd) xd < x ≤ 10h’ 1.0 f() 1.0 1.0 for unexposed roof areas 0.75 for exposed roof areas x > 10h’ f() 1.0 0.5 for exposed roof areas north of tree line Notes to Table 4.1.6.5.-A: (1) For lower roofs with parapets, Cs = 1.0; otherwise, Cs varies as a function of slope, , as defined in Sentences 4.1.6.2.(5) and (6). (2) The length of the drift, xd, shall be calculated as follows: Cb Ss xd = 5 (Ca0 − 1) γ where γ = specific weight of snow as specified in Article 4.1.6.13. (3) Except as provided in Sentence (4), the value of Ca0 for each of Cases I, II and III shall be the lesser of γh Ca0 = β and Cb Ss F Ca0 = Cb where β = 1.0 for Case I and 0.67 for Cases II and III, h = difference in elevation between the lower roof surface and the top of the parapet on the upper roof as shown in Figure 4.1.6.5.-A, and γ(lcs − 5h′p ) F = 0.35β√ + Cb , but F ≤ 5 for Cws = 1.0 Ss where Cws = value for Cw applicable to the source of drifting, w2s lcs = characteristic length of the source area for drifting, defined as, lcs = 2ws − , where ws and ls are ls respectively the shorter and longer dimensions of the relevant source areas for snow drifting shown in Figure 4.1.6.5.-B for Cases I, II and III, and 0.8Ss lcs h′p = hp − ( ) , but 0 ≤ h′p ≤ ( ) γ 5 where hp = height of the roof perimeter parapet of the source area, to be taken as zero unless all the roof edges of the source area have parapets. (4) Where h ≥ 5 m, the value of Ca0 for Case I is permitted to be taken as 25 − h F Ca0 = ( ) ( − 1) + 1 for 5 m ≤ h ≤ 25 m, and 20 Cb Ca0 = 1 for h > 25 m (5) The value of Ca0 shall be the highest of Cases I, II and III, considering the different roof source areas for drifting snow, as specified in Sentences (3) and (4) and Figure 4.1.6.5.-B. Figure 4.1.6.5.-B Snow Load Cases I, II and III for Lower Level Roofs Forming Part of Sentences 4.1.6.5.(1), (3) and (5), and Table 4.1.6.5.-B Table 4.1.6.5.-B Parameters for Snow Load Cases in Figure 4.1.6.5.-B Parameter Case I Case II Case III  1.0 0.67 0.67 parapet height of upper-roof source hp parapet height of lower-roof source area parapet height of lower-roof source area area with ws and ls being the shorter with ws and ls being the shorter w2 s and longer dimensions of the and longer dimensions of the lcs = 2ws − with ws and ls being the shorter and ls longer dimensions of the upper roof source area on the lower roof for source area on the lower roof for upwind-facing step downwind-facing step