Page 351 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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LATERAL-FORCE DESIGN

LATERAL-FORCE DESIGN 8.5

8.2 DETERMINATION OF WIND LOADS

Wind loading as described in Art. 8.1 is the basis for design wind loads specified in American Society
of Civil Engineers, “Minimum Design Loads for Buildings and Other Structures,” ASCE 7-02. (The
2005 version of this specification was not available at the time of preparation of this chapter, but only
modest changes in the wind-load provisions are expected.) The International Building Code (IBC) is
the standard model building code commonly used for buildings in the United States, and the IBC
currently adopts ASCE 7 wind provisions by reference. Thus, the wind-load discussion herein will
focus on the ASCE 7 design provisions. ASCE 7-02 provides three methods for establishing design
wind loads. Method 1 is a simplified procedure, which can be applied to relatively simple low-rise
buildings of regular shape and geometry that are not susceptible to wind-induced vibration. Method 2
may be applied to buildings which are aerodynamically stable but that may have limited vibration and
discomfort issues. This method is analytically more complex, and it requires greater consideration
of the dynamic response of the building and the local variation of wind pressures around the build-
ing. Method 3 employs wind-tunnel testing. This method must be used for any buildings that have
irregular characteristics, extreme local variation in wind velocity, or may be susceptible to aerody-
namic instability or excessive vibration. Many larger buildings may employ Method 3 for the eco-
nomic advantage that may be achieved with greater certainty regarding the local wind loading. All
three methods have similar considerations of wind pressure and local variations of the wind pressure,
but Methods 2 and 3 are more complex. The primary discussion of wind-load design provisions will
focus on Method 1 of the ASCE 7-02 provisions.

8.2.1 ASCE 7-02 Method 1 Provisions
The basic wind speeds specified in the ASCE 7-02 provisions are shown in Fig. 4.1. The contours on
the map indicate wind speeds that have a 2% probability of being exceeded in a year at a height of 10 m
(33 ft) above ground at an open site. The wind speed includes estimated local gust velocity effects.
(These wind speeds are approximately the maximum wind speed expected during an average 50-year
period.) The effects of extreme conditions, such as tornadoes, hurricanes, or local wind currents in
mountainous regions, are not covered by this map. Further, special wind regions are identified in the
map where local wind velocity may significantly exceed the indicated values for the location. The
possibility of occurrence of these local variations should be considered in design.
As noted in the background discussion, wind pressure depends on wind velocity, and the wind veloc-
ity varies with height and the characteristics of the local terrain. In addition, large variations in local wind
velocity and pressure are noted on specific locations of the building as summarized in Figs. 8.1 and 8.2.
As a result, the ASCE 7-02 Method 1 wind-design pressures are established by the equation
(8.4)
p s = λIp s30
The simplified design wind pressure, p s30 , is effectively computed by relationships such as Eqs. (8.1)
and (8.3). Also, values are tabulated for various wind velocities, locations on the building, and wind
exposure conditions in the ASCE provisions (see Fig. 4.2). The importance factor, I, is based on the
building category assignments and the characteristics of the wind velocity used in design. (See
Table 4.9.) Regions with hurricane wind velocities greater than or equal to 100 mi/h are considered
to be high-wind-velocity regions. The factor λ is an adjustment factor for building height and expo-
sure. (See inset table in Fig. 4.2.) The factor accounts for the fact that the wind velocity increases
with building height and is a function of the local terrain, as suggested by theory in Eq. (8.1). Exposure
B is obstructed terrain typical of wooded areas or suburban or urban areas. Exposure C represents
moderate terrain with scattered obstructions, and Exposure D is a smooth open terrain.
The simplified Method 1 procedure can be applied to simple, regular, modest-sized buildings. By
this method, maps such as those shown in Fig. 4.1 establish the basic wind speed for the region. The
importance factor I of the building must be established based on the function and characteristics of
the building. (See Table 4.9.) The adjustment coefficient λ is determined as a function of height and
exposure (see Fig. 4.2). Finally, the simplified design wind pressure, p s30 , is determined from the

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