Geoscience Reference
In-Depth Information
Table 10.2. Degradation of soilstiffnessaccording to ground acceleration from EC8Part 5
Ground acceleration ratio,
α
S
Damping ratio
V
s
/
V
smax
G
/
G
max
0.10
0.03
0.90
(
±
0
.
07
)
0.80
(
±
0
.
10
)
0.20
0.06
0.70
(
±
0
.
15
)
0.50
(
±
0
.
20
)
0.30
0.10
0.60
(
±
0
.
15
)
0.36
(
±
0
.
20
)
Table 10.3. Degradation of soil stiffness according to ground acceleration effective shear modulus
and shear wave velocity fromFEMA 273
Effective PGA (g) (at 5% damping)
0.10
0.70
Ratio of effective to initialshear modulus
(
G
/
G
o
)
0.50 0.20
Ratio of effective to initial shear wave velocity 0.71 0.45
The paragraph immediately following this table in FEMA 273 recommends that the equivalent
modulus is calculated according to the above, then response calculations done by taking an upper
and lower bound - twice the equivalent modulus and half the equivalent modulus.
Upper bound
4k
Lower bound
1
k
1
Deformation
Fig. 10.8. Suggested upper and lower bounds on soilstiffness given in theFEMA 273
document (cfTable 10.3)
Tables 10.2 and 10.3 allow for natural variability of the soil and uncertainties in site
investigation by giving bounds on the effect of the earthquake excitation on degradation
of the soil modulus. The FEMA 273 document is quite specific as shown in Figure 10.8.
With respect to damping EC8 gives single values, in contrast to the bounds on stiff-
ness,whereasFEMA273makesnomentionofsoildamping.Thedampingvaluesinthe
EC8tablerepresenthystereticdissipationinthesoil,radiationdamping,ifappropriate,is
additional.
