Geoscience Reference
In-Depth Information
0.10
0.10
inter-storey sway: PSD
inter-storey sway: PSD
0.08
0.08
0.06
0.06
0.04
0.04
a.
c.
0.02
0.02
0.00
0.00
0
10
20
30
40
50
0
10
20
30
40
50
frequency: Hz
frequency: Hz
0.10
0.10
inter-storey sway: PSD
inter-storey sway: PSD
0.08
0.08
0.06
0.06
0.04
b.
0.04
d.
0.02
0.02
0.00
0.00
0
10
20
30
40
50
0
10
20
30
40
50
frequency: Hz
frequency: Hz
Fig. 6.11. Inter-storey sway power spectral density estimates:
(a),(b) the Friuli-San-Rocco input motion; (c),(d)the Vrancea input motion;
(a), (c) without softcaisson;(b),(d) with soft caisson
interaction but these do not outweigh the adverse effects of acceleration amplification:
the frequency analysis of inter-storey sway is shown in Figure 6.11. For the Vrancea
input motion, themitigation scheme has an adverse effect.
It is not surprising that the mitigation scheme is successful when the frequency content
of the input motion lies clearly above the resonant frequency of the mitigation scheme.
Subjecttosatisfyingconditionsofstaticstability,reducingthestiffnessofthesoftcaisson
improvesitsbenefitandwidensitsrangeofapplication.Thissoundseasierintheorythan
in practice where the frequency content of the earthquake motion is almost certain to be
a surprise.
5. Macroelement analysis
The (apparent) randomness of natural earthquake motions means that any one motion
maynotbetypical:agivensystemmayrespondinacompletelydifferentwaytotwodif-
ferentinputhistories(Figure6.11).Inaddition,thedegreeofnonlinearityinanygeotech-
nical system is such that the responses to apparently only slightly different inputs may
be significantly different: the response is chaotic. One solution is to perform model tests
using a very large number of different input motions, all of which are somehow related
