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Unformatted text preview: on one
peak with larger
amplitude and the
brings about between the
coupling system and the
surface stratum. The tendencies of
described above are
similarly recognized in
the resonance curves of
the building on the pile
foundation . Because the rotational rigidity of the pile foundation is
much larger than that of the raft foundation, the natural
frequency of the coupling system for the pile foundation
becomes higher than that for the raft foundation. This
causes the differences in response characteristics of the
building between on the pile and on the raft foundation. The figure shows the
relation between the
number of floors n
and the maximum M.F
at the roof floor.
Of course, the building
with larger n has the
larger maximum M.F.
Comparing the raft to the pile foundation, M.F for the
building on the raft foundation becomes larger.
You can point out the number of floor n=15 for the raft
foundation at which M.F is the maximum. This indicates
that the natural frequency of the coupling system for n=15
approaches the nearest to that of the surface stratum. The first natural period T1 or the the natural frequency
f1(=1/T1) and the damping factor h1 at this period (is called
the modal damping) are the most important quantities
which affect the earthquake response of the building.
This period is
not for the
base, but for
system. The figure shows the relations between T1 , h1 and n. The
natural periods of the buildings under the fixed base
condition are given by T1=0.025n. T1 for the coupling
longer in almost
proportion to n.
T1 for the raft
foundation is longer
than that for the pile
foundation. Concerning the low or middle-rise buildings of n=5 to 10,
T1 are around 3 times for the raft foundation and around
2 times for the pile foundation as longer as T1 under the
fixed base condition. The superstructures
have 0.01 damping, but
h1 for the coupling
system is always much
bigger than 0.01.
With increase of n, the
h1 decreases gradually. Dynamic Soil Structure Interaction END
Chapter 3 :
Effect of SSI on
Dynamic Behaviors of Structure Kenji MIURA, Dr. Eng.
Graduate School of Engineering
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This note was uploaded on 03/14/2014 for the course CE 5680 taught by Professor Drgrd during the Summer '14 term at Indian Institute of Technology, Chennai.
- Summer '14