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I have performed an experiment using a shaking table to simulate the response of a soil base structure to an earthquake. A multi-floor structure was placed on a sand (90% relative density) bed and the bed was shaken horizontally at frequencies of 1-4 Hz for amplitudes of 1 mm, 2 mm and 3 mm. The acceleration of the base of the structure and of its floors was recorded, Frequency - Relative acceleration (= floor acceleration/base acceleration) graphs were drawn and the damping ratio and resonant frequency were extracted for each case.

What I found is that, as the amplitude of the shaking is increased, the damping ratio decreases, as well as the resonant frequency. Theoretically, for a linear system, the amplitude should not influence these two parameters, which led me to the conclusion that sand behaves in a non-linear way. However, I do not understand exactly what aspects of the sand's behaviour and of the sand-structure interaction lead to a lower damping ratio and resonant frequency. Any help on clarifying this would be appreciated!

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closed as unclear what you're asking by Buzz, Jon Custer, Kyle Kanos, Chair, glS Feb 14 at 20:06

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The resonances depend on the stiffness of the bedding material. If the material is stiff, the resonant frequencies will increase; if the bedding material is not stiff, then the frequencies will decrease.

What you are probably seeing here is a decrease in the effective stiffness of the sand under the building model which would occur when the shaking amplitude is sufficient to cause the sand particles to momentarily "unlock" and allow more movement.

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