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.