Click here to view this article in the Structural Engineer e-zine

According to Section 1609.1.1 of the 2006 International Building Code (IBC), wind loads on buildings and structures are to be determined by the provisions of Chapter 6 of the 2005 edition of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05). Exceptions in Section 1609.1.1 permit wind forces to be determined on specific types of residential buildings, flag poles, and telecommunication towers by industry standards other than ASCE/SEI 7.

The provisions of Section 1609 of the IBC or ASCE/SEI 7 may be used to determine the basic wind speed, the exposure category, and the type of opening protection that is required.

Figure 1609 in the IBC and Figure 6-1 in ASCE/SEI 7 are identical and provide basic wind speeds based on 3-second gusts at 33 feet above ground for Exposure C. It is important to note that design wind speeds on these maps do not include effects of tornadoes. In earlier editions of ASCE/SEI 7 and the legacy codes, wind pressures were based on fastest-mile wind speeds, which were collected at weather stations over an averaging time greater than that for 3-second-gust wind speeds. Section 1609.3.1 of the IBC provides an equation and a table that can be used to convert 3-second-gust wind speeds to fastest-mile wind speeds, since some standards that are referenced in the IBC contain criteria or applications based on fastest-mile wind speed.

Chapter 6 of ASCE/SEI 7 provides the following three methods to determine design wind pressures or loads:

• Method 1—Simplified Procedure
• Method 2—Analytical Procedure
• Method 3—Wind Tunnel Procedure

Method 1—Simplified Procedure

The design requirements of Method 1 can be found in Section 6.4. This method, which is based on the low-rise buildings procedure in Method 2, can be used to determine wind pressures on the main wind-force-resisting system (MWFRS) of a building provided the conditions of Section 6.4.1.1 are met. Although there are eight conditions that must be satisfied, a large number of low-rise buildings meet these conditions. Method 1 can also be used to determine design wind pressures on components and cladding (C&C) provided the five conditions of Section 6.4.1.2 are satisfied.
Wind pressures are tabulated in Figures 6-2 and 6-3 as a function of the basic wind speed for a specific set of conditions:

• Occupancy Category II (I = 1.0), enclosed buildings;
• mean roof height = 30 feet;
• primarily flat ground (K_zt = 1.0); and
• Exposure B.

The horizontal pressures in Figure 6-2 are net horizontal wind pressures, meaning that they are the summation of the windward and leeward pressures on the MWFRS (the internal pressures on the walls cancel out). The vertical pressures on the horizontal projection of the roof surface include the internal pressures for enclosed buildings (GC_pi = ±0.18).

For C&C, values are provided for enclosed buildings and are net pressures (sum of external and internal pressures) applied normal to the surface.

Tabulated pressures are multiplied by the appropriate adjustment factor given in Figures 6-2 and 6-3 based on actual building height, exposure, occupancy, and topography at the site. These adjusted pressures are applied normal to projected areas of the building in accordance with Figures 6-2 and 6-3. The load patterns illustrated in Figure 6-2 for the MWFRS must be applied to each corner of the building in turn as the reference corner for wind in the transverse and longitudinal directions.

Flowchart 1 can be used to determine net design wind pressures on the MWFRS in accordance with Method 1. Referenced section, figure, table, and equation numbers are from ASCE/SEI 7-05 unless noted otherwise.

Method 2—Analytical Method

Method 2 provides wind pressures and forces for the design of MWFRSs and C&C of enclosed and partially enclosed rigid and flexible buildings, open buildings, and other structures including freestanding walls, signs, rooftop equipment, and other structures (Section 6.5). Definitions of the different types of buildings can be found in Section 6.2.

Equations are provided in Commentary Section C6.5.8 that can be used to determine the approximate fundamental frequency of a variety of structures. These equations are especially useful in the preliminary design stage to help in ascertaining whether a building or structure is rigid (fundamental natural frequency greater than or equal to 1 Hz) or flexible (fundamental natural frequency less than 1 Hz).

In general, Method 2 entails the determination of the following:

• velocity pressures (which are a function of exposure, height, topographic effects, wind directionality, wind velocity, and building occupancy),
• gust effect factors,
• external and internal pressure coefficients, and
• force coefficients.

Pressures and forces can be determined for many types of buildings and structures provided the following conditions are satisfied:

• The building or structure is regularly shaped, such that the building has no unusual geometrical irregularity in spatial form (both vertical and horizontal).
• The building or structure responds to wind primarily in the same direction as that of the wind (there are no response characteristics that make it subject to across-wind loading, vortex shedding, or other dynamic load effects).
• The building or structure is located on a site where there are no channeling effects or buffeting in the wake of upwind obstructions.

Method 3—Wind Tunnel Procedure

The Wind Tunnel Procedure in Section 6.6 can be used for any building or structure in lieu of Methods 1 or 2, and must be used where the conditions of Methods 1 and 2 are not satisfied. In general, more accurate wind pressures on the MWFRS and the C&C will be obtained from a properly conducted wind tunnel test.

Wind tunnel tests should be performed for any building or structure that meets one or more of the following conditions:

• The building or structure has a shape that differs significantly from a uniform rectangular prism or box-like shape.
• The building or structure is flexible.
• The building or structure is subject to buffeting by the wake of upwind buildings or structures.
• The building or structure is subject to accelerated flow caused by channeling or local topographic features.

Such tests should also be performed in cases where it is anticipated that the building or structure will be subject to across-wind and/or torsional loads, periodic loads caused by vortex shedding, or loads resulting from instabilities such as flutter or galloping.

ASCE/SEI Section 6.6.2 contains the requirements for proper wind tunnel testing.

Minimum wind pressure

Provisions for minimum design wind loading are given in Section 6.1.4. Figure C6-1 illustrates the minimum wind pressure that must be applied horizontally on the entire vertical projection of a building or structure for the design of the MWFRS. This 10-pounds-per-square-foot (psf) pressure is to be applied as a separate load case in addition to the other load cases specified in Chapter 6.

For C&C, a minimum net pressure of 10 psf acting in either direction normal to the surface is required.

David A. Fanella, Ph.D., S.E., P.E., is associate principal and director of New Structures in the Chicago office of Klein and Hoffman, Inc. He can be reached at dfanella@kleinandhoffman.com.