Environmental Engineering Reference
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duration of gusts vary randomly, but magnitude generally increases with decreasing
duration. The total gust environment can be defined in terms of the joint probability
distribution of magnitude and duration for gust events, together with some basic frequency-
of-occurrence data.
On the basis of 17 years of measurements of peak horizontal wind speeds at Cape
Kennedy, Florida, reported by Kaufman [1977], a gust model has been developed that
consists of two empirical factors: a mean gust factor , which is defined as the ratio of the
average wind speed during the gust to the steady wind speed averaged over a 10-minute
period or longer; and a statistical factor , to account for deviations from the mean gust.
Steady wind speeds for averaging periods longer than 10 min showed little variation from
the 10-min average. These factors are expected to be representative of winds over
reasonably flat, uniform terrain. In this model, the gust magnitude in Equation (8-20) is
given by
(8-23)
D u = ( F g F s - 1) U
where
F g
= empirical mean gust factor from Table 8-4
F s
= empirical statistical factor from Table 8-5
Table 8-4.
Mean Gust Factors, F g , for Calculating Extreme Gust Magnitudes
[Frost et al. 1978; data from Kaufmann 1977]
Ref. wind speed,
U r (m/s)
Duration of
gust, t (s)
Elevation, z (m)
10
20
40
80
160
300
5
1
1.450
1.398
1.350
1.307
1.270
1.239
5
1.380
1.336
1.297
1.263
1.232
1.207
10
1.335
1.300
1.266
1.235
1.208
1.186
50
1.219
1.197
1.176
1.158
1.141
1.127
100
1.160
1.144
1.129
1.116
1.105
1.094
300
1.059
1.054
1.049
1.044
1.040
1.036
10
1
1.390
1.331
1.282
1.239
1.203
1.175
5
1.322
1.276
1.236
1.201
1.172
1.149
10
1.288
1.248
1.211
1.179
1.153
1.132
50
1.186
1.163
1.141
1.121
1.104
1.086
100
1.135
1.119
1.105
1.090
1.077
1.066
300
1.049
1.043
1.038
1.047
1.038
1.025
1
1.382
1.322
1.271
1.227
1.192
1.162
³ 20
5
1.315
1.269
1.228
1.192
1.162
1.139
10
1.279
1.238
1.201
1.171
1.145
1.125
50
1.178
1.153
1.132
1.112
1.096
1.082
100
1.128
1.111
1.096
1.082
1.070
1.060
300
1.047
1.041
1.036
1.031
1.027
1.023
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