Building Design and Construction Handbook - Merritt - Ventech!

5.10 SECTION FIVE
Exposure A applies to centers of large cities, where for at least one-half mile
upwind from the building the majority of structures are over 70 ft high and lower
buildings extend at least one more mile upwind.
Exposure B applies to wooded or suburban terrain or to urban areas with closely
spaced buildings mostly less than 70 ft high, where such conditions prevail upwind
for a distance from the building of at least 1500 ft or 10 times the building height.
Exposure C exists for flat, open country or exposed terrain with obstructions
less than 30 ft high.
Exposure D applies to flat unobstructed areas exposed to wind blowing over a
large expanse of water with a shoreline at a distance from the building or not more
than 1500 ft or 10 times the building height.
For design purposes also, the following formulas may be used to determine, for
heights z (in feet) greater than 15 ft above ground, a pressure coefficient K for
converting wind speeds to pressures.
For Exposure A, for heights up to 1500 ft above ground level,
2/3
z
K 0.000517 (5.3)
32.8
For z less than 15 ft, K 0.00031.
For Exposure B, for heights up to 1200 ft above ground level,
4/9
z
K 0.00133 (5.4)
32.8
For z less than 15 ft, K 0.00095.
For Exposure C, for heights up to 900 ft above ground level,
2/7
z
K 0.00256 (5.5)
32.8
For z less than 15 ft, K 0.0020.
For Exposure D, for heights up to 700 ft above ground level,
1/5
z
K 0.00357 (5.6)
32.8
For z less than 15 ft, K 0.0031.
For ordinary buildings not subject to hurricanes, the velocity pressure q z, psf, at
height z may be calculated from
2 qz KV (5.7)
where V basic wind speed, mi/hr, but not less than 70 mi/hr.
For important buildings, such as hospitals and communication buildings, for
buildings sensitive to wind, such as slender skyscrapers, and for buildings presenting
a high degree of hazard to life and property, such as auditoriums, q z computed
from Eq. (5.7) should be increased 15%.
To allow for hurricanes, q z should be increased 10% for ordinary buildings and
20% for important, wind-sensitive or high-risk buildings along coastlines. These
increases may be assumed to reduce uniformly with distance from the shore to zero
for ordinary buildings and 15% for the more important or sensitive buildings at
points 100 mi inland.