Exposure B—
Exposure B—An urban or suburban location
A downtown location
Exposure D—A flat unobstructed site facing a
Exposure C—An open country or grassland
Wind speed measurements are made in Exposure C.
large water body
FIGURE 3.15 Site-exposure categories.
Exposure C refers to open terrain with scattered obstructions involving heights generally
less than 30 ft. An airport is a typical example of Exposure C. Exposure D refers to flat
unobstructed ground facing a large body of water (a large lake- or seafront).
TOPOGRAPHY
OF
S ITE
If the building is located on an escarpment, as shown in Figure 3.16(a), or an isolated hill,
it is subjected to greater wind pressures than if the same building is located on a flat ground.
EXPAND
YOUR KNOWLEDGE
Important Facts About Wind Loads
Wind Load Is Expressed in Terms of Wind Pressure
Wind loads on buildings are generally expressed as wind pressure, that is, in pounds per square foot (psf).
Wind Pressure Is Proportional to the Square
of Wind Speed
Everything else being the same, the wind pressure on a building
component is proportional to the square of wind speed. More
precisely,
p = 0.00256 V2
where: p = wind pressure in pounds per square foot (psf)
V = wind speed in miles per hour (mph).
Thus, if V = 100 mph, p = 25.6 psf. Because 0.00256 is approxi1
, a commonly used simplification
mately equal to 0.0025 or 400
for the preceding equation is
p = c
V 2
d
20
According to this simplified equation, if V = 100 mph, p = 25 psf.
If V = 90 mph, p = 20 psf. For V = 10 mph, p = 0.25 psf.
62
Magnitude of Wind Pressures
on Low-Rise Buildings
Because wind speed refers to that measured at a height of 33 ft
(10 m) above ground, the approximate value of wind pressure on
the components of most low-rise buildings (one to three stories
tall) can be obtained by the simplified equation just given. This is
an important result, which we will refer to in Chapter 6 in comparing the commonly prevailing (water) vapor pressures with air
pressures in buildings.
Wind Pressure on a Building Component
Equals Difference of Inside and Outside
Air Pressures
The wind pressure on a building component, such as a wall,
window, or roof is, in fact, the difference between inside and
outside air pressures. If the component is subjected to suction
under wind loads, it implies that inside pressure is greater than
the outside pressure. Conversely, if the outside pressure is greater
than the inside pressure, the component will be subjected to
push-in pressure. The inside pressure in a building is generally
the atmospheric pressure. The outside pressure changes with
wind speed.