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The Role of Higher Modes of Vibration on Seismic Response of Soil-Structure Systems

Ghodrati, Emad | 2007

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41166 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ghannad, Mohammad Ali
  7. Abstract:
  8. It is several decades that the effect of soil-structure interaction (SSI) has been known to engineers. Many methods have been developed to estimate this effect, but modal analysis of soil-structure systems is one of the most popular techniques due to its simplicity and better insight. Due to this reasons, modal analysis found its place in codes and provisions such as NEHRP. Presence of soil beneath the structure not only would alter the natural periods and damping ratio of structure but also would impose some new vibration modes to the system due to soil’s degrees of freedom. Consequently, response of structure is a combination of the effects of all these modes. But, in practice, usually effects of SSI are just considered on the first mode of vibration. This thesis discusses the effects of SSI on various modes of vibration and participation of the higher modes on the structure response. A parametric study is done for a wide range of non-dimensional parameters, which completely define the problem. Finally a simplified 3 DOF model is suggested as a soil-structure system by replacing the structure by SDOF model using the modal coordinates of the fixed-based structure. It is concluded that the effect of SSI on the period of the higher modes of vibration is negligible, but the SSI effect on damping ratios of higher mode is considerable. Also it is concluded that the effect of SSI on response of higher modes of vibration is minimal when the structure is located on soft soil, but SSI effect on the response of the higher modes is notable when the structure is located on alluvium and rock. Finally, the simplified 3 DOF model estimates the modal parameters and responses of the various modes of vibration with acceptable accuracy
  9. Keywords:
  10. Soil-Structure Interaction ; Higher Mode Effect ; Seismic Response ; Nonclassical Modal Analysis ; Simplified Method

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