Identification of Fixed-Base and Rigid Body Frequencies of Vibration of Soil-Structure Systems from Recorded Response with Minimum Instrumentation

Abstract eng:
Civil engineering structures are founded on soils, which are not infinitely rigid, and alter the structural dynamic response by modifying its frequencies of vibration and introducing additional degrees of freedom and dissipation. The energy of the vibrational response of the coupled soil-structure system is concentrated around the frequencies of vibration of the system, which depend on the properties of the soil, structure and foundation. Consequently, observed changes in the frequencies of vibration, as identified via Fourier type of analyses, can be due to changes in any of these quantities. For structural health monitoring, it is essential to be able to isolate the changes in the fixed-base frequencies - task thought to be impossible unless the structural instrumentation had been specially designed, which is not the case for most instrumented structures. This paper shows how the fundamental fixed-base frequency of a structure deforming primarily in shear can be measured, using impulse response functions, and then the rigid body rocking frequency can be computed using a relationship between fixed-base, rigid-body and system frequencies. This requires data from only two sensors recording horizontal motion - at ground level and at the roof. Results are shown for the NS response of Millikan Library in Pasadena, California, during several earthquakes between 1970 and 2003, which reveal to which degree the observed “wonder” of this building NS system frequency has been due to structural deterioration vs. nonlinear elastic behavior of the building and nonlinear behavior of the foundation soil.

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