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a Zmin = minimum height used to ensure that the equivalent height z is the greater of 0.6h or zmin. For buildings with h < Znin, z shall be taken as z^,,. (From ASCE 7-02, Table 6.2.)

a Zmin = minimum height used to ensure that the equivalent height z is the greater of 0.6h or zmin. For buildings with h < Znin, z shall be taken as z^,,. (From ASCE 7-02, Table 6.2.)

Figure 1.12. Building height h, velocity pressure qz. (Adapted from ASCE 7-02.)

the building. These irregular pressures are the results of interference effects of nearby buildings and terrain, and dynamic effects on more flexible buildings. The ASCE 7-02 requirements represent of square and rectangular buildings with aspect ratios up to about 2.5. They may not cover all cases, even for symmetric and common building shapes where larger torsions have been observed. Therefore, the designer may wish to apply this level of eccentricity at full, rather than reduced, wind loading for certain more critical buildings, even though it is not required by the Standard.

In buildings with unusual structural systems, such as the one used for the City Corp. Tower in New York, more severe loading can occur when the resultant wind load acts diagonally to the building. To account for this effect and the fact that many buildings exhibit maximum response in the across-wind direction, a structure should be capable of resisting 75% of the design wind load applied simultaneously along each principal axis, as required by case 3 in Fig. 6.9 of ASCE 7-02.

For flexible buildings, dynamic effects can increase torsional loading. Additional tor-sional loading can occur because of eccentricity between the elastic shear center and the center of mass at each level of the structure. The new Eq. (1.18) given below accounts for this effect.

where eQ = eccentricity e as determined for rigid structures in Fig. 6.9 of ASCE 7-02 eR = distance between the elastic shear center and center of mass of each floor I. gQ, Q, gR shall be as defined in 6.5.8 of ASCE 7-02

The sign of the eccentricity e shall be plus or minus, whichever causes the more severe load effect.

The eccentricity e for flexible structures shall be considered for each principal axis (ex, ey).

The eccentricity is used for calculating torsional moment MT per unit height acting about a vertical axis of the building. The designer is referred to ASCE 7-02, Fig. 6.9 for additional information.

1.4.2.5. Gust Effect Factor

The gust effect factor accounts for additional dynamic amplification of loading in the along-wind direction due to wind turbulence and structure interaction. It does not include allowances for across-wind loading effects, vortex shedding, instability due to galloping or flutter, or dynamic torsional effects. Buildings susceptible to these effects should be designed using wind tunnel results.

Three methods are permitted for calculating G. The first two are for rigid structures and the third is for flexible or dynamically sensitive structures.

Gust Effect Factor G for Rigid Structure: Simplified Method. For rigid structures (defined as those having a natural frequency of vibration greater than 1 Hz), the engineer can use a single value of G = 0.85, irrespective of exposure category.

Gust Effect Factor G for Rigid Structure: Improved Method. As an option to using G = 0.85 the designer may calculate a more accurate value by including specific features of the wind environment at the building site. The procedure is as follows:

The gust effect factor G is given by

where Iz = the intensity of turbulence at height z and where z = the equivalent height of the structure defined as 0.6 h but not less than zmin for all building heights h. zmin and c are listed for each exposure in Table 1.9; gQ and gv shall be taken as 3.4. The background response Q is given by

Q=

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