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Study of Rocking of Shallow Foundations Subjected to Dynamic Loading by Consideration of Soil-Structure Interaction

Amiri, Amin | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51710 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Haeri, Mohsen
  7. Abstract:
  8. Foundation which is lowest load-bearing part of structure, transfers load of superstructure to substructure. Because structures are usually placed on soils, the response of the soils influences the motion of the structures and the motion of structures affects the response of the soils. This phenomenon is termed Soil Structure Interaction (SSI). One of these mutual effects is the rocking of shallow foundations in which the base of the foundation under dynamic loading may separate from the underlying ground, by rotating about the longitudinal axis. This rotational mode elongates natural period of the soil-structure system and decreases the base shear demands of the superstructure. Indeed during rocking of the foundation, plastic hinge spreads in the underneath soil instead of the structure, therefor it acts as a shield for superstructure and renders structure safety damage, however, it leads to residual deformations in the soil, resulting in settlement and/or tilting of the foundation. In this research rocking of shallow foundations of tall buildings subjected to dynamic loads has been simulated by finite element analysis using ABAQUS. Four behavioral models namely linear and nonlinear (elastic and elastoplastic) were taken for modeling of the soil. Two nonlinear soil models such as Duncan-Chang model and Nonlinear Cyclic analysis with Ramberg-Osgood hyperbolic and Masing rule are considered for the material behavior and were implemented in associated subroutines coded in FORTRAN. Results of this investigation revealed that although eccentricity of the applied load exceeded from the middle third of the foundation, structures did not overturn and remained stable. Separation of the base from the underneath soil showed to have great potential for dissipation of a large amount of energy during dynamic loading. Moment-rotation hysteresis plots gets less steep by increasing of rotation. This nonlinear relation between moment and rotation is observed even in linear and elastic soil which is the result of uplift and larger displacement of footings. Even though linear models for soil presents valuable information such as capacity moment, rotation of footing and even damping ratio, they are not useful for determination of residual rotation and settlement. The results of the study indicates that, as the height of structure increases, the rotation of footing and structure decreases. Also low damping ratio is achieved for slender structures. In addition, rocking stiffness is dependent on the soil properties such as shear modulus and increases with increase in width of the foundation
  9. Keywords:
  10. Rocking Behavior ; Soil-Structure Interaction ; Nonlinear Cyclic Analysis ; Foundation Uplift ; Ramberg-Osgood Hardening Behavior ; Shallow Foundation

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