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In order to validate the conclusion above,
the ground motion recorded at soft soil site
in Chi-Chi earthquake are processed in the
same way. The result is shown in figure 4
for the whole penod band, where the black
curve is the average spectrum with
descending rate of T 1 , the grey ones are the
spectra of the original records. It is also
clear that the reliability of the average
spectrum at period range longer than 6
seconds is higher than that at short period
range.
0 5 10 15
T(sec.)
Fig 4 Comparison of the average spectrum with those of the
original records on soft soil sites from Chi-Chi earthquake
The maximum value of design spectrum adjusted from the site condition
Before and during the revision of the code for the seismic design of buildings in China, an
improvement had been adopted in some seismic design codes of other countries in that the maximum
values of design spectra determined from the zoning maps were further adjusted from site condition.
For example, in the NEHRP recommended provisions, the maximum value of the design spectrum
equals the value from the zoning map for site category B (soft rock site) multiplied by a site factor.
This improvement is consistent with the experience that earthquake damage on soil site is generally
more serious than on rock site, and is supported by some observed data (TLDobry et al., 2000). The
values of the adjusting coefficients mainly depend on statistical data for acceleration 0.1 g and less on
site in category B, and on a lot of numerical results from site response analyses for stronger
accelerations, as shown in table 3.
Site
categories
A
B
C
D
E
Table 3 The adjusting coefficients of various sites
Acceleration from the zoning map for site category B