Page 348 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

8.2 CHAPTER EIGHT

and wind velocity increases more slowly with height. Wind velocity increases with height in all cases
but does not increase appreciably above the critical heights of about 950 ft for open terrain to 1500 ft for
rough terrain. This variation of wind speed over height has been modeled as a power law:
 z  n
V = V   (8.1)
2
 z 
g
where V is the basic wind velocity, or velocity measured at a height z g above ground and V z is the
velocity at height z above ground. The coefficient n varies with the surface roughness. It generally
ranges from 0.33 for open terrain to 0.14 for rough terrain. The wind speed used in this evaluation
procedure has varied over time. Early wind-load predictions were based on fastest-mile wind speeds,
which are effectively the maximum average wind speed measured over a distance of 1 mile, at a
given height above ground. (ASCE Task Committee on Wind Forces, Committee on Loads and
Stresses, “Wind Forces on Structures,” Transactions, ASCE, vol. 126 part 2, pp. 1124–1198, 1961.)
Current structural design codes use a 3-s-gust wind speed for their wind design requirements.
(“Minimum Design Loads for Buildings and Other Structures,” SEI/ASCE Standard 7-02,
American Society of Civil Engineers, Reston, Va., 2002.) While there is variation in the definition of
the basic wind speeds that have been used in wind-load estimation, there is clearly a relationship
between the various definitions. However, it is important to note that wind speeds established by dif-
ferent definitions should not be arbitrarily combined or compared. Design loads are based on a sta-
tistical analysis of the basic wind speed, and maps such as the one shown in Fig. 4.1 have been
developed. Typical design limits may be based on maximum wind speeds with 2% annual probability of
exceedance or approximately a 50-year event, but winds associated with higher- or lower-probability
occurrences may be appropriate for some structures. The statistical wind-speed design maps nor-
mally exclude the occurrence of tornadoes and hurricanes. Further, extreme local variations in wind
speed are possible in some regions because of climatic and geographic variations. As a result, wind-
speed design maps typically require additional consideration of these events. The wind-speed design
data are normally maintained for open sites, and the wind speeds must be corrected for other site
conditions.
Wind speeds V w are translated into pressure q by the equation
ρ
q = C D V w 2 (8.2)
2
where C D is a drag coefficient and ρ is the density of air at standard atmospheric pressure. The drag
coefficient C D depends on the shape of the body or structure. It is less than 1 if the wind flows around
the body, but it may be significantly greater than 1.0 if the wind is forced to reverse its direction. The
pressure q is the stagnation pressure q s if C D = 1.0, since the structure effectively stops the forward
movement of the wind. Thus, on substitution in Eq. (8.2) of C D = 1.0 and air density at standard
atmospheric pressure,

q = 0 00256 V w 2 (8.3)
.
s
2
where the wind speed is in mi/h and the pressure is in lb/ft .
The drag coefficient and the shape and geometry of the structure have substantial effects on
wind pressure, because the shape of the body may merely divert the direction of the wind, stop the
wind, or reverse its direction. These characteristics are illustrated in Fig. 8.1. Large inward pres-
sures develop on the windward walls of enclosed buildings as illustrated in Fig. 8.1a. Negative pres-
sures may develop on the leeward side of these enclosed buildings, and this may result in an
additional outward pressure on the leeward walls of the structure. Buildings with openings on the
windward side will allow air flow into the building, and internal pressures may develop as depicted
in Fig. 8.1b. These internal pressures cause loads on the overall structure and structural frame.
More important, these internal pressures place great demands on the attachment of roofing,
cladding, and other nonstructural elements. Openings in a side wall or leeward wall may cause an

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