C

where

K = a factor related to the surface roughness coefficient of the terrain K = 0.08 for exposure A K = 0.10 for exposure B K = 0.14 for exposure C

Figure 1.16. Background turbulence factor as a function of width and height of structure. (From NBCC 1995.)

CeH = exposure factor at the top of the building, H, evaluated using Fig. 1.15 B = background turbulence factor obtained from Fig. 1.16 as a function of building width-to-height ratio W/H H = height of the building W = width of windward face of the building

5 = size reduction factor obtained from Fig. 1.17 as a function of W/H and reduced frequency n0H/VH n0 = natural frequency of vibration, Hz VH = mean wind speed (m/s) at the top of structure, H

Figure 1.17. Size reduction factor as a function of width, height, and reduced frequency of structure. (From NBCC 1995.)
Figure 1.18. Gust energy ratio as a function of wave number. (From NBCC 1995).

F = gust energy ratio at the natural frequency of the structure obtained from

Fig. 1.18 as a function of wave number n0/VH 5 = critical damping ratio, with commonly used values of 0.01 for steel, 0.015 for composite, and 0.02 for cast-in place concrete buildings

Design Example: Calculations for Gust Effect Factor Cg. Given.

Fundamental frequency n0 = 0.125 Hz (period = 8 sec)

Critical damping ratio ยก = 0.010

Average density of the building = 195 kg/m3 (12.2 pcf)

Terrain for site = exposure B

Reference wind speed at 10 m, open terrain (exposure A) = 26.4 m/s (60 mph) Required. Gust factor Cg

Solution. From Fig. 1.15, for H = 240 m and exposure category B, exposure factor Cch = 2.17

Mean wind speed VH at top

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