Page 186 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

Brockenbrough_Ch04.qxd 9/29/05 5:09 PM Page 4.14

BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.14 CHAPTER FOUR

SEI/ASCE 7-02 permits the use of either Method 1 or Method 2 to define the design wind loads.
Method 1 is a simplified procedure and may be used for enclosed or partially enclosed buildings
meeting the following conditions:
1. Wind loads are transmitted through floor and roof diaphragms to the vertical main wind force-
resisting system in each direction with either a flat roof or a gable or hip roof with angle ≤ 45°.
2. Approximately symmetrical cross section of building with roof slopes less than 10°.
3. Low-rise building, i.e., mean roof height less than or equal to 60 ft and height does not exceed the
least horizontal dimension.
4. Building having no unusual geometric irregularity in spatial form.
5. Building whose fundamental frequency is greater than 1 Hz.
6. Building structure having no expansion joints or separations.
7. Building is not subject to topographical effects.
The design procedure for Method 1 involves the following considerations:

1. Determine the basic design speed, V, from Fig. 4.1.
2. Select the wind importance factor, I w , using Table 4.9.
3. Define the exposure category, i.e., A, B, C, or D, using Table 4.6.
4. Define the building enclosure classification, i.e., enclosed or partially enclosed.
5. Using Fig. 4.2, determine the design wind load for the main wind force-resisting system.
6. Using Fig. 4.3, determine the design wind load for the component and cladding elements.
Method 2 is a rigorous computation procedure that accounts for the external and internal pres-
sure variation as well as gust effects. The following is the general equation for computing the design
wind pressure p:
p = qGC p − q i (GC pi ) (4.6)

where q and q i = velocity pressure as given by SEI/ASCE 7-02
G = gust effect factor as given by SEI/ASCE 7-02
C p = external pressure coefficient as given by SEI/ASCE 7-02
GC pi = internal pressure coefficient as given by SEI/ASCE 7-02

Building codes and standards may present the gust factors and pressure coefficients in different
formats. Coefficients from different codes and standards should not be mixed.
Designers must exercise judgment in selecting wind loads for a building with unusual shape,
response-to-load characteristics, or site exposure where channeling of wind currents or buffeting in

TABLE 4.9 Importance Factor I for Wind Loads
Non-hurricane-prone regions
and hurricane-prone regions Hurricane-prone regions
Category with V = 85–100 mi/h and Alaska with V > 100 mi/h
I 0.87 0.77
II 1.00 1.00
III 1.15 1.15
IV 1.15 1.15
Note: The building and structure classification categories are listed in Table 4.9.
Source: From Minimum Design Loads for Buildings and Other Structures, SEI/ASCE 7-02,
American Society of Civil Engineers, Reston, Va., with permission.

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