The calculated value is below the 3% limit. Its along-wind response is unlikely to disturb the comfort and equanimity of building's occupants.

Comparison of Along-wind and Across-wind Accelerations. To get a sense for along-wind and across-wind, the results for two buildings are given in summary form in Fig. 1.20. One is representative of a 30-story rectangular building, shown in Fig.1.20a and 1.20b, and is examined for wind along both its principle axes. The other, shown in Fig.1.20c, is square with a height corresponding approximately to a 60-story-plus building. Results for both are given for suburban exposure B.

Response characteristics were also evaluated for the other two types of exposure categories. From the calculations performed but not shown here, it appears that the type of exposure has a significant effect on both along-wind and across-wind response. Accelerations were about 20 to 50% greater for an open-terrain exposure A. The reductions for an urban setting, exposure C, were of the same order of magnitude.

Observe that in Fig. 1.20, the maximum acceleration of the building occurs in a direction perpendicular to the wind (across-wind direction) because the building is considerably more slender in the across-wind than in the along-wind direction. Across-wind accelerations control the design if the building is slender about both axes, that is, if WD/H is less than one-third, where W and D are the across-wind and along-wind plan dimensions and H is the building height.

Figure 1.20. Wind-induced peak accelerations; 1995 NBCC procedure: (a) 30-story building, wind on narrow face; (b) 30-story building, wind on broad face; (c) 60-story building.

Since Eqs (1.58) and (1.59) for along-wind and across-wind accelerations are sensitive to the natural frequency of the building, use of approximate formulas for period calculations are not appropriate. Therefore, results of more rigorous methods such as computer dynamic analyses are recommended for use in these formulas.

In addition to acceleration, many other factors such as visual cues, body position and orientation, and state of mind of occupants during windstorms influence human perception of motion. However, research has shown that when the amplitude of acceleration is in the range of 0.5 to 1.5% of acceleration due to gravity, movement of buildings becomes perceptible to most building occupants. Based on this and other information, a tentative acceleration limit of 1 to 3% of gravity is recommended. The lower value is considered appropriate for apartment buildings, the higher values for office buildings.

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