Unformatted text preview: t: embedding depth = 6m. (3) Foundations
Two types of foundation
are considered.
One is a spread foundation,
(a) spread Foundation the other is a pile group
foundation. (b) Pile Group Foundation (4) Earthquake Disturbance
An earthquake disturbance is assumed to result from an
incident SH type wave propagating upward at a right
angle to the ground surface.
At the time of an earthquake, four kinds of earthquake
wave travel to the building. Among them, the Swave
comprises the principal shock of the earthquake motion
and it must be incorporated into the seismic design of a
structure.
φ4 As Swave velocities of strata
nearer to the ground surface
becomes normally lower, the
incident angle of the Swave
gets near a right angle to the
ground surface.(Snell’s Law) V4
φ3 φ2
φ1 V3
V2
V1 sin ϕ1 sin ϕ 2 sin ϕ3
=
=
V1
V2
V3 3.2 Response of Soil
Let take a look at soil response when a SH type
earthquake wave is propagating upward at a right angle
to the ground surface.
● Response of Layered Strata
First Natural Frequency f1 ith Natural Frequency fi Impedance Ratio α The figure shows magnification factor(M.F) US/2EO of the
ground surface response to the amplitude of the incident
wave EO. A denominator 2EO of M.F denotes the amplitude
of response at the surface of the bearing stratum when the
surface stratum is removed.
1.25Hz
3.75Hz Two resonance peaks are observed. The lower peak
corresponds to a first natural frequency 1.25Hz of the
surface stratum, while the higher to a second 3.75Hz. The ith natural frequency fi of the surface stratum is
formularized by :
(31)
When the surface stratum consists of multilayered
strata, the natural frequency is similarly calculated using
an equivalent shear wave velocity Vse given by:
(32)
where m is number of layers over the bearing stratum,
and VSj is shear wave velocity and dj thickness of the jth
stratum. Subst...
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 Summer '14
 DrGRD
 pile foundation, surface stratum

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