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Figure 1.9. Basic wind speed map, 3-sec gust wind speed: (a) map of the United States; (b) western Gulf of Mexico hurricane coastline (enlarged); (c) eastern Gulf of Mexico and southeastern U.S. hurricane coastline (enlarged); (d) mid- and north-Atlantic hurricane coastline (enlarged). (Adapted from ASCE 7-02.)

Figure 1.9. (Continued)

1. The basic wind speed.

2. The mean recurrence interval of the wind speed considered appropriate for the design.

3. The characteristics of the terrain surrounding the building.

4. The height at which the wind load is being determined.

5. Directional properties of the wind climate.

6. The size, geometry, and aerodynamics of the building.

7. The positions of the area acted on by the wind flow.

8. The magnitude of the area of interest.

9. The porosity of the building envelope.

10. The structural properties that may make the building susceptible to dynamic effects.

11. The speed-up effect of certain topographic features such as hills and escarpments.

1.4.2.1. Wind Loads on Main Wind-Force-Resisting System: Overview of Analytical Procedure

The analytical procedure has two steps. The first step considers the properties of the wind flow and the second accounts for the properties of the structure and its dynamic response to the longitudinal (along-wind) wind turbulence. The effects of across-wind response are not explicitly considered in the ASCE 7-02, Methods 1 and 2.

The velocity pressure at elevation z is given by the equation qz = 0.00256KzKztKdV 2I (qz in psf, V in mph) (1.8)

The basic wind speed V corresponds to a 50-year mean recurrence interval. It represents the speed from any direction at an elevation 33 ft (10 m) aboveground in flat open country (exposure C).

The velocity pressure exposure coefficient Kz depends on the velocity, terrain roughness (i.e., exposure category), and the height aboveground.

Three exposure categories—B, C, and D—are defined. Exposure A, at one time intended for heavily built-up city centers, was deleted in the 2002 edition of ASCE 7. The exposure for each wind direction is now defined as the worst case of the two 45° sectors on either side of the wind direction being considered.

In summary, exposure B corresponds to surface roughness B typical of urban and suburban areas, exposure C to surface roughness C in flat open country, and exposure D to surface roughness D representative of flat unobstructed area and water surfaces outside hurricane-prone regions. Exposure C applies to all cases where exposures B and D do not apply. Interpolation between exposure categories is now permitted for the first time in the ASCE 7-02. Formal definitions of exposure categories are given later in Section 1.4.2.9.

The importance factor I is a factor that accounts for the degree of hazard to human life and damage to property. For category II buildings (See Table 1.7), or other structures representative of typical occupancy, I = 1.0. For category I buildings or other structures representing low hazard in the event of failure (e.g., agriculture facilities), I = 0.87 or 0.77, depending upon whether the building site is located in hurricane-prone regions. For buildings and other structures in category III posing a substantial hazard to human life in the event of failure (e.g., buildings where more than 300 people congregate in one area, and essential facilities such as fire stations), I = 1.15. For category IV buildings or other structures deemed as essential facilities, I = 1.15, the same as for category III.

The topographic factor Kzt is given by

It reflects the speed-up effect over hills and escarpments. The multipliers K1, K2, and K3 are given in Fig. 6.4 of the Standard (Figs. 1.11 and 1.11a of this text).

Wind directionality is explicitly accounted for by introducing a new factor Kd. It is no longer a component of the wind load factor. Kd varies depending upon the type of structure. Prior to introduction of exposure factor Kd, the load factor for wind was 1.3. Now it is 1.6, obtained by dividing 1.3 by the Kd factor equal to 0.85 for most buildings. Thus the new load factor = 1.3/0.85 = 1.53 rounded to 1.6.

Internal pressures and suctions on side walls and the roof of buildings do not affect the value of wind load for the main wind-force-resisting system (MWFRS). Therefore, pressures and suctions, both denoted by Pz, are calculated for the MWFRS using the following equations:

Pz = qzGfCp (for positive pressures) (1.10)

Pz = qhCfCp (for negative pressures) (1.11)

instead of the more general equation:

P = q(GCp) - qi (GCpi) [ASCE 7-02 Eq. (6.23)] (1.11a)

The overall wind load is the summation of positive pressures on the windward wall, and negative pressure or suction, on the leeward wall. In the above equations Gf is a gust factor equal to 0.85 for rigid buildings, and Cp is an external coefficient, typically equal to 0.8 and 0.5 for the windward and leeward walls, respectively.

Thus, for a typical rigid buildings, the total design wind pressure at height Z above ground level is given by

1.4.2.2. Analytical Procedure: Step-by-Step Process

Design wind pressure or suction on a building surface is given by the equation:

where

Pz = design wind pressure or suction, in psf, at height z, above ground level qz = velocity pressure, in psf, determined at height z above ground Gf = gust effect factor, dimensionless

Cp = external pressure coefficient, which varies with building height acting as pressure (positive load) on windward face, and as suction (negative load) on nonwindward faces and roof. The values of Cp, unchanged from the previous edition of the Standard, are shown in Figs. 1.10 and 1.10a for various ratios of building width to depth.

The velocity pressure and suction qz and qh are given by qz = 0.00256KzKztKdV 2I (1.14)

qh = 0.00256KhKztKdV 2I (1.15)

where

Kh and Kz = combined velocity pressure exposure coefficients (dimensionless), which take into account changes in wind speed aboveground and the nature of the terrain (exposure category B, C, or D). (See Fig. 1.11b and Table 1.6.)

Figure 1.10. Horizontal variation of external wind pressure coefficient Cp with respect to plan aspect ratio LIB: (a) 0 < LIB < 1; (b) L/B = 2; (c) LIB > 4. (Adapted from ASCE 7-02)

Ktt = topographic factor, introduced in ASCE 7-95 for the first time I = importance factor, a dimensionless parameter that accounts for the degree of hazard to human life and damage to property (Tables 1.7 and 1.7a) V = basic wind speed, Fig. 1.10 in miles per hour that corresponds to a 3-sec gust speed at 33-ft (10 m) aboveground, exposure category C, for a 50-year mean recurrence interval Kd = wind directionality factor that varies from 0.85 to 0.95 depending on the structure type (Table 1.8, ASCE 7-02 Table 6.4)

The wind directionality factor identified as Kd in ASCE 7-02 accounts for two effects:

• The reduced probability of maximum winds flowing from any given direction

• The reduced probability of the maximum pressure coefficient occurring for any given direction

This factor, which was hidden in the load factors of the previous editions of the Standard, is now explicity included in the equation for velocity pressure:

qz = 0.00256KzK2tKdV2I

The value of Kd is equal to 0.85 for most types of structures, including buildings. Therefore, qz calculated from the previous equation is equal to 85% of the value designers were used to, prior to publication of ASCE 7-02. However, the load factors specified in ASCE 7-02 have been

Figure 1.10a. Vertical variation of external wind pressure coefficient Cp with respect to plan aspect ratio LIB. (a) 0 < LIB < 1; (b) LIB = 2; (c) LIB > 4.

adjusted upward, so the wind loads are about the same as before. Thus, for LRFD or strength design, the new load factor is 1.6, which previously was 1.3. The factor 1.6, when multiplied by the directionality factor Kd = 0.85, gives an effective load factor equal to 1.6 x 0.85 = 1.36 approximately equal to the previous factor of 1.3.

ESCARPMENT 2-D RIDGE OR 3-D AXISYMMETRICAL HILL

Topographic Multipliers for Exposure C

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