Equivalent-Linear Dynamic Impedance Functions of Surface Foundations
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 139, Issue 7
Abstract
An approximate linearization method using the familiar concept of and curves is presented for determining the dynamic impedance (stiffness and damping) coefficients of rigid surface footings accounting for nonlinear soil behavior. The method is based on subdivision of the soil mass under the footing into a number of horizontal layers of different shear modulus and damping ratio, compatible with the level of strain imposed by an earthquake motion or a dynamic load. In this way, the original homogeneous or inhomogeneous soil profile is replaced by a layered profile with strain-compatible properties within each layer, which do not vary in the horizontal sense. The system is solved in the frequency domain by a rigorous boundary-element formulation accounting for the radiation condition at infinity. For a given set of applied loads, characteristic strains are determined in each soil layer and the analysis is repeated in an iterative manner until convergence in material properties is achieved. Both kinematic and inertial interaction can be modeled simultaneously by the method, which thus encompasses primary and secondary material nonlinearity in a single step. The results are presented for a circular footing resting on: (1) a half space made of clay of different plasticity index and (2) a half space made of sand of different density, excited by a suite of recorded earthquake motions. Dimensionless graphs are provided for the variation of foundation stiffness and damping with frequency and excitation level in vertical, swaying, rocking, and torsional oscillations.
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Acknowledgments
This study was performed in the framework of the European research project New Methods for Mitigation of Seismic Rick of Existing Foundation [(NEMISREF) EC contract No G1RD-CT-2002-00702, EC project No GRD1-2001-40457]. The first author acknowledges Professor George Mylonakis for fruitful discussions on the topic throughout the last year. The authors also thank Fernando Lopez-Caballero for all the helpful discussions over the course of this research.
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Information & Authors
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Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 139 • Issue 7 • July 2013
Pages: 1130 - 1139
Copyright
© 2013 American Society of Civil Engineers.
History
Received: Jan 20, 2012
Accepted: Sep 4, 2012
Published online: Sep 5, 2012
Published in print: Jul 1, 2013
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