
 DOESTD102395
(3) Determine control points and connect control points by straight line segments in log
log space to form the spectrum plot.
Control
Spectral Acceleration
Point
(Hz)
(g)
E
0.1
0.395 dmax/g
D
vmax/(2 dmax)
(vmax)2/(g dmax)
(g amax)/(2 vmax)
amax
C
B
8.0
amax
A
33.0
PGA
A'
100.0
PGA
(4) The vertical design response spectrum is typically taken as 2/3 of the horizontal
spectrum provided that the site is not one of the "nearfield" sites, i.e. closer than
approximately 15 km (9.3 mi) to a seismic source. For nearfield sites, the guidance in
ASCE 4 (ASCE, 1998b) should be followed. Local site conditions should be
considered when developing appropriate vertical design response spectra.
A3.2.2e.
An acceptable method to estimate the annual probability that specified wind speeds at the site
will be exceeded is included in Coats and Murray, 1985, and described by the following:
Step 1:
Select a data set of annual extreme wind speeds from a weather station near the
site of interest.
Step 2:
Correct the annual extreme wind speeds to an anemometer height of 33 ft (10
meters) above ground in flat, open terrain using appropriate methodologies. For
example, a power law (Simiu and Scanlan, 1986) could be used to make an
adjustment, if needed. No recorded wind speeds from anemometers located on
building roofs near the edges, sheltered by parapets or neighboring buildings, or
too close to the roof surface (less than 5 feet (1.5 meters)) shall be used.
Step 3:
Estimate the annual probability of exceedance of selected wind speeds with
associated uncertainty.
Data sets of historical extreme winds shall be obtained from weather stations close enough to
sites to represent the site conditions as described in DOESTD102294 If more than one
station is available, they may be combined, provided they represent the same conditions as
those at the site.
Several statistical models may be used to estimate frequency of winds. An estimate of the
models fitting the data shall be performed. If only one statistical model is to be used, the
FisherTippet Type I extreme value distribution (also named Gumbel distribution) (Coats and
Murray, 1985) shall be used, unless justified otherwise. Additional guidance may be found in
Ramsdell, Elliott, et al. (1986).
A8

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