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92
Site
categories
A
B
C
D
Table 4 The adjusting coefficients suggested by Li Xiaojun (2001)
Acceleration from the zoning map for site category A
0.05g O.lg 0.15g 0.20g 0.30g 0.40g
1.0 1.0 1.0 1.0 1.0 1.0
1.5 1.45 1.40 1.33 1.25 1.18
1.10 LOO 0.9 0.8 0.7 0.6
0.8 0.7 0.6 0.55 0.50 0.45
The values in the above two tables commonly describe the amplifying effect of ground soil layers, and
the reduction effect of soil nonlinearity when acceleration is strong enough. Meanwhile one can find
there is quite a large difference between the values in the two tables, especially the trend of varying
with soil from stiff to soft and from thin to thick. Should it increase or decrease The most convincing
answer must come from the observed data. The maximum values of normalized response spectra of
760 free field ground motion records, and the corresponding magnitudes, distances and site conditions,
are summarized to work out the coefficient values in this paper.
To deal with the relation between the maximum values of normalized spectra and site condition, the
key point is to compare them at the same magnitude and distance. The biggest difficulty is that the data
sample is not large enough, and the data distribution in magnitude and distance is not homogeneous.
One can rarely find sets of data of the same magnitude (ideally the same earthquake) and exact same
distance for various site conditions. The qualification has to be relaxed to those of similar magnitude
and distance. The authors compare the mean maximum values in each magnitude and distance group
directly rather than compare the attenuation relationships for various site categories. The ratios of
mean values of maximum spectrum amplitudes for various sites to the value for rock site in every
group are shown hi table 5, where the number in bracket after the ratio is the sample size (i.e. number
of records. The records just on the boundaries between two neighboring groups are applied twice). The
term Ace. is for acceleration of maximum spectral amplitude on rock site divided by 2.5
Table 5 The ratios of mean values of maximum spectral amplitudes of various sites to one of rock site
Distance ikm)
' -— A CC - (§)
Site Categories----^^_
1
2
3
4
Distance (km)
' " — "•—--— .^Acc. (3)
Site Categories'——-^
1
2
3
4
Magnitude ^5.75
0-10 10-50
0.2543
1.00 (9)
0.1365
LOO (10)
139 (6)
0.79 (84) 0.79 (81)
Magnitude 5.75 - 6.5
0-30 30-50
0.199
1.00 (18)
1.47 (8)
1.96 (8)
1.35 (91)
0.1271
1.00 (6)
1.38 (6)
1.10 (4)
1.25 (112)
50-100
0.028
1.00 (4)
1.23 (22)
50-100
0.0291
1.00 (6)
1.53 (4)
2.21 (38)
100-200
0.0303
1.00 (2)
100-200
0.0357
1.00 (2)
0.68 (20)