4.1.6.6 — Horizontal Gap between a Roof and a Higher Roof

(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., where the accumulation factor, Ca, is, [[FORCE_FORMULA]] Ca = Ca0 – (Ca0 – 1)(x/xd), for 0 ≤ x ≤ xd or [[FORCE_FORMULA]] 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. (2) The length of the drift, xd, shall be calculated as follows: x_d = 5 · (C_b S_s / γ) · (C_ao − 1) where, γ =specific weight of snow as specified in Article 4.1.6.13. (3) The value of Ca0 for each of Cases I, II and III shall be the lesser of, C_aθ = β · γ h / (C_b S_s) and C_a0 = F / C_b 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 F = 0.35 · β · √(γ (l_cs − 5 h_0) / S_s) + C_b; but F ≤ 5 when C_ws = 1.0 where, Cws =value for Cw applicable to the source of drifting, lcs =the characteristic length of the source area for drifting, defined as l_cs = 2·w_s − (w_s^2 / l_s), where ws and ls are 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 h_p' = h_p − (0.8 · S_s / γ), with 0 ≤ h_p' ≤ (l_cs / 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) 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 Sentence (3) and Figure 4.1.6.5.B.