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LATERAL-FORCE DESIGN

8.6 CHAPTER EIGHT

wind velocity. Different p s30 values are computed for a wide range of structural geometries, and for
different locations on the structure and wind velocities as shown in Fig. 4.2. This determination is
based on the variation in the direction and influence in wind velocity as illustrated in Figs. 8.1 and 8.2.
The Method 1 procedure requires that the wind-force design be accomplished in each of the pri-
mary structural directions as depicted in Fig. 4.2. In addition, two separate load cases may be
required in each of the two directions based on the geometry of the structure. Figure 4.2 shows
the direction of wind pressure on the structure for the longitudinal and transverse building axes, and
it defines the pressure at different locations such as the windward and leeward areas and the walls
and roof of the structure. Values of p s30 are established for each of these regions. Some locations may
have wind pressures acting in either direction. For building geometries which may develop wind
pressures in either direction, two separate values are specified for these regions of the building.
Separate load cases are then required to accommodate the two different pressure conditions. The
pressure distributions defined in Eq. (9.4) and Fig. 4.2 are used to establish the design wind loads on
the walls, roof, and structural framing.
The Method 1 procedure then requires an additional local pressure, p net , which is used to design
local components and cladding. This pressure is defined by the equation
(8.5)
p net =λIp net30
where the values of λ and I are established as described earlier. The pressure, p net30 , is defined in
terms of interior zones, end zones and corner zones in ASCE 7-02. (See Fig. 4.3.) The three zones
of behavior capture the extreme local variation in wind pressure caused by building geometry as
noted in Fig. 8.1.

8.2.2 ASCE 7-02 Methods 2 and 3
As noted earlier, the engineer has the option of using Methods 2 and 3 for defining the design wind
loads on the building. These methods will frequently offer some economic advantage through
reduced wind forces, but they require significantly more complex analysis and evaluation of the wind
load. Method 2 is also an analytical method. It uses the same basic wind speed, pressure variations,
and locations or zones of the structure. However, the local wind pressure variation is computed based
on a combination of pressure coefficients, which are computed based on the specific design para-
meters. These coefficients require considerable time and evaluation, but they are expected to lead to
a more realistic estimate of the design wind pressures on the various components of the building
structure. This method is required for some buildings with greater complexity and greater dynamic
sensitivity to wind load. They may be used for very simple buildings, but they require additional time
and effort for evaluation of the design wind loads.
Method 3 is based on wind-tunnel testing. Wind-tunnel testing is essential for buildings with
great dynamic sensitivity to wind-induced vibration and for major buildings in urban areas where the
local wind pressures are strongly influenced by surrounding structures. The method clearly gives a
far better indication of the response of the building due to wind loading, but requires the time and
cost of the wind-tunnel test.

8.3 SEISMIC LOADS IN MODEL CODES

The Uniform Building Code (UBC) of the International Conference of Building Officials has been the
primary source of seismic design provisions for the United States in past years. The UBC historically
adopted provisions based on recommendations of the Structural Engineers Association of California
(SEAOC). The UBC and SEAOC defined design forces and established detailed requirements for seis-
mic design of many structural types. However, another document, “National Earthquake Hazard
Reduction Program (NEHRP) Recommended Provisions for the Development of Seismic Regulations
for New Buildings,” of the Building Seismic Safety Council (BSSC), Federal Emergency
Management Agency (FEMA), Washington, D.C., has maintained a parallel set of seismic load provisions

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