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Notes: V= 90 mph, exposure category D. Importance factor Iw = 1.0.

Wind pressure qs at 33 ft for 90 mph basic wind speed = 20.8 psf.

Notes: V= 90 mph, exposure category D. Importance factor Iw = 1.0.

Wind pressure qs at 33 ft for 90 mph basic wind speed = 20.8 psf.

Because the building height is more than 200 ft according to UBC 1997, use of method 2 is not permitted. Therefore method 1, with different values of Cq for the windward and leeward walls, is used.

Cq = 0.8 inward pressure for windward wall (Table 1.3)

Cq = 0.5 outward suction for leeward wall (Table 1.3)

Windward pressures are calculated using the tabulated values of Ce for various heights. Leeward suction is calculated only at the roof level. Therefore the suction on the leeward wall remains constant for the entire building height (Table 1.5, column 6). The combined design pressures and floor-by-floor wind loads for lateral design are tabulated in Fig. 1.8a.

It should be noted that the height of 394 ft chosen for the example problem is just under the 400-ft limit, the maximum permitted by the simple procedure of the UBC. If

Figure 1.8a. Thirty-story building example, UBC 1997 method.

the building were taller than 400 ft, we would be required by the UBC to use other notionally accepted standards for determining the wind loads. ASCE 7-02 is one such standard discussed later in this chapter

1.4.2. ASCE 7-02: Wind Load Provisions

The full title of this ASCE standard is American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures. In one of its 10 sections, ASCE 7-02 provides three procedures for calculating wind loads for buildings and other structures, including the main wind-force-resisting systems and all components thereof. The designer can use Method 1, the simplified procedure, to select wind pressures directly without calculation when the building is less than 60 ft in height and meets all requirements given in Section 6.4 of the standard. Method 2 can be used for buildings and structures of any height that are regular in shape, provided the buildings are not sensitive to across-wind loading, vortex shedding, or instability due to galloping or flutter; or do not have a site for which channeling effects warrant special consideration. Method 3 is a wind-tunnel test procedure that can be used in lieu of methods 1 and 2 for any building or structure. Method 3 is recommended for buildings that possess any of the following characteristics:

• Have nonuniform shapes.

• Are flexible with natural frequencies less than 1 Hz.

• Are subject to significant buffeting by the wake of upwind buildings or other structures.

• Are subject to accelerated flow of wind by channeling or local topographic features.

Basic wind speeds for any location in the continental United States and Alaska are shown on a map having isotachs representing a 3-sec gust speed at 33 ft (10 m) above the ground (see Fig. 1.9). For certain locations, such as Hawaii and Puerto Rico, basic wind speeds are given in a table as 105 and 145 mph (47 and 65 m/s), respectively. The map is standardized to represent a 50-year recurrence interval for exposure C topography (flat, open, country and grasslands with open terrain and scattered obstructions generally less than 30 ft (9 m) in height). The minimum basic wind speed provided in the standard is 85 mph (38 m/s). Increasing the minimum wind speed for special topographies such as mountain terrain, gorges, and ocean fronts is recommended.

The wind speed map for the United States and adjoining landmasses is based on data collected over a long period of time at weather stations located throughout the country. The maximum wind velocity expected at any location can be found simply by referring to the map.

The abandonment of the fastest-mile speed in favor of a 3-sec-gust speed first took place in the ASCE 7-1995 edition. The reasons are: 1) modern weather stations no longer measure wind speeds using the fastest-mile method; 2) a 3-sec-gust speed is closer to the sensational wind speeds often quoted by news media; and 3) it matches closely the wind speeds experienced by small buildings and by components of all buildings.

Method 1, the simplified procedure, and Method 3, the wind tunnel procedure, are not discussed here. The emphasis is on Method 2.

Method 2, the analytical procedure covered in this section, applies to a majority of buildings. It accounts for the following factors that influence the design wind forces:

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