Geoscience Reference
In-Depth Information
acceleration: m/s
2
2
acceleration: m/s
2
2
1
1
0
1
2
3
0
0
012345678
time: s
time: s
−
1
−1
−2
a.
c.
−
2
−3
2.0
5
acceleration: PSD
acceleration: PSD
4
1.5
b.
d.
3
1.0
2
0.5
1
0.0
0
0
10
20
30
40
50
0
10
20
30
40
50
frequency: Hz
frequency: Hz
Fig. 6.10. (a), (b) Friuli-San-Rocco earthquake: (a) timehistoryof acceleration
and (b) spectral power density; (c),(d) Vrancea earthquake: (c) timehistory
of acceleration and (d) spectral power density
terised by a single massive pulse at a frequency of about 3.5Hz; the Friuli-San-Rocco
history(Figure 6.10a, b) contains energy peaks at a number of frequencies.
The wider bandwidth of the Friuli-San-Rocco input motion induces a richer response
in the unmitigated state. Accelerations are amplified by a deposit which undergoes
appreciable shear strain. Excitation of the soil-structure interaction system results in
rocking accelerations and settlements of the model structure at around 12Hz. For this
input motion, the mitigation scheme brings substantial improvements to the response
(Figure6.11).Theamplificationofaccelerationwithinthesoildepositintheunmodified
model disappears. Acceleration attenuation occurs at frequencies above 7Hz, the reso-
nant frequency of the model mitigation scheme. Rocking accelerations and inter-storey
sways of the model structure are similarlyimproved.
However, with negligible energy at either the resonant frequency of the deposit (29Hz)
or the resonant frequency of the soil-structure interaction system (12Hz), the Vrancea
input motion produces small shear strains, settlements and structural rotations in the
unmodified situation. The soft caisson has a resonance at 7Hz and as a consequence
has a major response to the Vrancea energy spike at 3.5Hz. 'Mitigation' leads to
increasedaccelerationsinthefoundationsoil;therearebeneficialeffectsofsoil-structure
