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, C_a, shall be determined as follows: C_a = C_a0 – (C_a0 – 1)(x/x_d), for 0 ≤ x ≤ x_d or C_a = 1.0, for x > x_d where C_a0 = peak value of C_a 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 x_d = 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 [b-4.1.6.6.#^table-4166hplca|4.1.6.6.]

Notes to Figure 4.1.6.5.-A:

(1) If a > 5 m or h ≤ 0.8S_s / , drifting from the higher roof need not be considered.

(2) If h ≥ 5 m, the value of C_a0 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

Notes to Table 4.1.6.5.-A:

(1) For lower roofs with parapets, C_s = 1.0; otherwise, C_s varies as a function of slope, α, as defined in Sentences 4.1.6.2.(5) and (6).

(2)

The length of the drift, x_d, shall be calculated as follows:

where γ = specific weight of snow as specified in Article [[b-4.1.6.13.|4.1.6.13.]]

(3)

Except as provided in Sentence (4), the value of C_a0 for each of Cases I, II and III shall be the lesser of

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 [[b-4.1.6.5.#^figure-4165-ahplca|Figure 4.1.6.5.-A]], and

where C = value for C applicable to the source of drifting,

l_cs = characteristic length of the source area for drifting, defined as,

, where w and l are respectively the shorter and longer dimensions of the relevant source areas for snow drifting shown in [[b-4.1.6.5.#^figure-4165-bhplca|Figure 4.1.6.5.-B]] for Cases I, II and III, and

where h_p = 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 C_a0 for Case I is permitted to be taken as

(5)

The value of C_a0 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