Civil Engineering Reference
In-Depth Information
(2.16)
Prediction of the
R
-year return period wind speed,
U
R
,
can then be calculated from:
U
R
=
u
0+
σ
[1−(λ
R
)
−
k
]/
k
(2.17)
In Equation (2.17), the shape factor,
k,
is normally found to be positive (usually around
0.1). As
R
increases to very large values, the upper limit to
U
R
of
u
0
+
(σ
/
k)
is gradually
approached.
When
k
is zero, it can be shown mathematically that Equation (2.17) reduces to
Equation (2.18):
U
R
=
u
0
+
σloge(λR)
(2.18)
The similarity between Equations (2.7) and (2.18) should be noted.
The highest threshold level,
u
n
,
should be set so that it is exceeded by at least 10 wind
speeds. An example of this method is given in the following section.
2.4.1
Example of the use of the 'peaks over threshold' method
Daily wind gusts at several stations in Melbourne, Australia, have been recorded since
1940. Those at the four airport locations of Essendon, Moorabbin, Melbourne Airport
(Tullamarine) and Laverton are the most useful as the anemometers are located at
positions most closely matching the ideal open country conditions and away from the
direct influence of buildings. Table 2.4 summarizes the data available from these four
stations.
The two most common types of events producing extreme wind in the Melbourne area
are gales produced by the passage of large low-pressure or frontal systems ('synoptic'
winds—see Section 1.3.1) and severe thunderstorm 'downbursts' (Section 1.3.3).
Downbursts are usually accompanied by thunder, but the occurrence of thunder does not
necessarily mean that an extreme gust has been generated by a downburst. The
occurrences of downbursts in the data from the four stations were identified by inspection
of the charts stored by the Australian Bureau of Meteorology or the National Archives.
Table 2.4 shows that the rate of occurrence of downbursts greater than 21 m/s is quite
low (around one per year at each station); however, as will be seen they are significant
contributors to the largest gusts.
