4.1.7.4.Topographic Factor

(1) Except as provided in Sentence (2), the topographic factor, Ct, shall be taken as 1.0. (2) For buildings on hills or escarpments with slope, Hh/(2Lh), greater than 0.1 (See Figure 4.1.7.4.), the topographic factor, Ct, shall be calculated as follows: ∆S Ct = (1 + ) (1 + ∆S) Cg where |x| ∆S = ∆Smax (1 − ) exp (−αz/Lh ) kLh where ΔSmax = applicable value from Table 4.1.7.4., x = horizontal distance from the peak of the hill or escarpment, Lh = horizontal distance upwind from the peak to the point where the ground surface lies at half the height of the hill or escarpment, or 2Hh (where Hh is the height of the hill or escarpment), whichever is greater, Z = height above ground, and k and α = applicable constants from Table 4.1.7.4. based on shape of hill or escarpment. Figure 4.1.7.4. Speed-up of Mean Velocity on a Hill or Escarpment Forming Part of Sentence 4.1.7.4.(2) Notes to Figure 4.1.7.4.: (1) V(z) = wind speed. Table 4.1.7.4. Parameters for Maximum Speed-up Over Hills and Escarpments Forming Part of Sentence 4.1.7.4.(2) Shape of Hill or Escarpment ΔSmax(1) α k, where x < 0 k, where x ≥ 0 2-dimensional hill 2.2 Hh/Lh 3 1.5 1.5 2-dimensional escarpment 1.3 Hh/Lh 2.5 1.5 4 3-dimensional axi-symmetrical hill 1.6 Hh/Lh 4 1.5 1.5 Notes to Table 4.1.7.4.: (1) For Hh/Lh > 0.5, assume Hh/Lh = 0.5 and substitute 2Hh for Lh in the equation for ΔS.