EM 1110-2-1100 (Part II)
30 Apr 02
Example Problem II-8-1 (Concluded)
These results and those calculated for the other samples are summarized in Table II-8-11.
Table II-8-11
Wind Speed at 25- and 50-year Return Period
U25 , m/s
U50 , m/s
σrm , m/s
Years
1971-73
30.8
32.6
2.5
1974-76
34.4
36.9
3.4
1977-79
28.5
30.5
2.6
1980-82
30.1
32.3
2.9
Mar 69 - Feb 83
31.8
34.0
1.4
Option 3: (use Figure II-8-7)
Figure II-8-7 is the least desirable option, since it does not make use of the available measurements at the
site. However, it provides a reference to regional behavior against which the data analyses can be interpreted.
For this example, the figure indicates that
U50 . 80 mph = 35.8 m/s
(fastest mile)
From Table II-8-6,
U25 = 0.9535.8 = 34.0 m/s
(fastest mile)
These wind speeds are converted from fastest mile to 1-hr averages using Table II-8-7:
U25 = 0.7834.0 = 26.5 m/s
(1-hr average)
U50 = 0.7835.8 = 27.9 m/s
(1-hr average)
Finally, the 1-hr averages are converted to 10-min averages to be comparable to the results of Options 1 and 2.
Using Table II-8-8,
U25 = 1.0526.5 = 27.8 m/s
(10-min average)
U50 = 1.0527.9 = 29.3 m/s
(10-min average)
Discussion: Option 1, which is generally preferred, gives the smallest values for U25 and U50. Option 3 gives wind
speeds about 10 percent higher and Option 2 over 20 percent higher than Option 1. Use of Option 1 with a partial
duration series of wind speed maxima, which is recommended in practice rather than the annual maximum series,
could be expected to give more realistic estimates.
(Sheet 4 of 4)
II-8-24
Hydrodynamic Analysis and Design Conditions
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