Wind loads are produced by the flow of wind around the structure. The magnitudes of wind loads that may act on a structure depend on the geographical location of the structure, obstructions in its surrounding terrain, such as nearby buildings, and the geometry and the vibrational characteristics of the structure itself. Although the procedures described in the various codes for the estimation of wind loads usually vary in detail, most of them are based on the same basic relationship between the wind speed V and the dynamic pressure q induced on a flat surface normal to the wind flow, which can be obtained by applying Bernoullis principle and is expressed as
in which r is the mass density of the air. Using the unit weight of air of 0.0765 lb/ft3 for the standard atmosphere (at sea level, with a temperature of 59F), and expressing the wind speed V in miles per hour, the dynamic pressure q in pounds per square foot is given by
The wind speed V to be used in the determination of the design loads on a structure depends on its geographical location and can be obtained from meteorological data for the region. The ASCE 7 Standard provides a contour map of the basic wind speeds for the United States (Fig. 2.4). This map, which is based on data collected at 485 weather stations, gives the 3-second gust speeds in miles per hour (m/s). These speeds are for open terrain at the heights of 33 ft (10 m) above ground level. To account for the variation in wind speed with the height and the surroundings in which a structure is located and to account for the consequences of the failure of structures, the ASCE 7 Standard modifies Eq. (2.2) as
in which qz is the velocity pressure at height z in pounds per square foot; V is the basic wind speed in miles per hour (Fig. 2.4); I is the importance factor; Kz is the velocity pressure exposure coe°cient; Kzt is the topographic factor; and Kd is the wind directionality factor. When converted to SI units, Eq. (2.3) becomes
with qz and V now expressed in units of N/m2 and m/s, respectively. The importance factor I accounts for hazard to human life and damage to property in the event of failure of the structure. The values of I to be used for estimating wind loads for the various categories of buildings are listed in Table 2.3. The velocity pressure exposure coe°cient, Kz, is given by
in which z ¼ height above ground in feet (or meters); zg ¼ gradient height in feet (or meters); and a ¼ power law coe°cient. The constants zg and a depend on the obstructions on the terrain immediately surrounding the structure. The ASCE 7 Standard classifies the terrains to which the structures may be exposed into three categories. These three categories are briefly described in Table 2.4, which also provides the in which r is the mass density of the air. Using the unit weight of air of 0.0765 lb/ft3 for the standard atmosphere (at sea level, with a temperature of 59F), and expressing the wind speed V in miles per hour, the dynamic pressure q in pounds per square foot is given by
