The tendencies of resonance curves described above

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Unformatted text preview: on one peak with larger amplitude and the resonance phenomenon brings about between the coupling system and the surface stratum. The tendencies of resonance curves 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 building on the fixed base, but for the coupling 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 system becomes 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. Professor Graduate School of Engineering Hiroshima University...
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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.

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