Investigation of Dynamic Response of Shallow Foundations on Sandy Soil to Horizontal Harmonic Loading by Physical Model Tests

Ghassemi, Ramin | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42216 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Jafarzadeh, Fardin
  7. Abstract:
  8. The most important step in current dynamic analysis of machine foundations is determining dynamic impedance functions. Impedance functions are defined as ratios of applied force or moment to induced displacement or rotation angle of foundation. In this research, horizontal dynamic response of foundation on homogeneous halfspace was investigated using physical modeling. Soil medium was built in a 1mx1mx0.8m steel box whose interior walls were covered by 0.1m of sawdust (a material with high damping). In order to prevent possible reflection of waves, this cover was extended to the box bottom with 0.2 m thickness. Sand raining method was used to build soil medium. In this method, with controlling falling height and sand flux, desired density of sand can be achieved. We used a sieve with a hole diameter of 6 mm and falling height of 0.7 m to reach relative density of 54%. In next stage, 0.1 meter wide square footing and the short column attached to that was place on homogenous soil medium. Harmonic loading with specified amplitude and frequency was produced by B&K Signal Analyzer 2035 and was applied using shaker in the horizontal direction through the column to the footing. The amplitude of applied force was recorded via loadcell located between column and shaker’s string. The foundation displacements in horizontal and vertical direction were recorded via accelerometers mounted on it. In order to carry out a comprehensive study on the effect of embedment depth ratio, foundation weight, dynamic force amplitude, and loading elevation on impedance functions, 860 different tests in 14 series were conducted. Using recorded data and related motion equations, foundation responses were analyzed. Finally, results were presented in graphs of horizontal impedance functions of square foundation on homogenous halfspace against dimensionless frequency. Comparison between horizontal dynamic impedance functions shows that an increase in embedment depth ratio results in the dynamic stiffness increase up to dimensionless frequency of 3.5 and the dashpot coefficient increase up to dimensionless frequency of five. The dynamic stiffness and dashpot coefficient are independent from level of dynamic force up to dimensionless frequency of 2.5 but in higher frequency, they are affected by the dynamic force level. Using massless impedance functions is valid up to dimensionless frequency of two but in higher frequency, the effect of mass on impedance functions should be considered. In summary, the dynamic stiffness and dashpot coefficient can be considered constant up to dimensionless frequency of 2, so traditional method of analyze of machine foundation which are based on constant and independent stiffness and dashpot coefficients are only valid in this frequency domain. Comparison between rocking dynamic impedance functions in dimensionless frequency range of 0 to 5 indicates that both real and imaginary parts of dynamic impedance function increase with increase in embedment depth ratio and foundation mass has no effect on impedance function, so the use of massless impedance functions is valid here. In addition, dynamic force amplitude has not any effect on impedance functions, either. In general, the dynamic stiffness and dashpot coefficients exhibit a strong sensitivity to variation in frequency parameter until dimension frequency of one but in higher frequency, they are essentially independent of frequency and can be considered constant without any appreciable error.

  9. Keywords:
  10. Impedance Function ; Dynamic Response ; Shallow Footing ; Physical Model ; Machine Foundation ; Horizontal Vibration

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