Dynamic pile impedances for fixed-tip piles Anoyatis, George; Laora, Raffaele Di; Lemnitzer, Anne
Soil dynamics and earthquake engineering (1984),
June 2017, 2017-06-00, 20170601, Letnik:
97
Journal Article
Recenzirano
Odprti dostop
The behavior of a laterally loaded pile with fixed-tip boundary condition (i.e., displacement and rotation, are perfectly restrained) is evaluated using a recently proposed, improved Tajimi-type ...model. The model performance in both, static and dynamic regime is first validated against rigorous finite element solutions and subsequently compared with Winkler model results for a selected range of pile-soil system parameters. In addition, pile impedances for fixed-tip piles are compared with previously proposed impedance expressions for hinged-tip piles. Results indicate that pile tip fixity has moderate impact on the pile stiffness in rotation but show stronger influence for pile stiffness in swaying and cross-swaying. The effect of tip fixity on pile impedances diminishes when piles are longer than approximately ten pile diameters. The proposed expressions for damping were evaluated across a wide range of frequencies, and damping was found to be most pronounced in rotation across the entire spectrum of pile-soil stiffness ratios examined. Winkler based formulations from literature almost exclusively over-predict damping for fixed tip piles.
•An extension of a recently proposed model for lateral dynamic SSI is offered.•The effect of tip fixity on pile impedances is examined.•Closed form expressions for pile stiffness and damping are proposed.•Proposed results are validated using FE analyses.•The performance of available Winkler moduli to predict impedances is investigate.
AbstractThis paper proposes a new simple approach for assessing dynamic soil–structure interaction effects on structures supported by embedded massive foundations. The classical substructure method ...was revisited by proposing an exact decomposition approach which allows performing the inertial interaction analysis of the superstructure without modeling foundation and soil, which are replaced by properly defined impedances. The proposed approach requires redefinition of simplified formulas for both the dynamic stiffnesses and the kinematic interaction factors, which are referred to the top of the massive foundation. Accurate expressions for the complex impedances were derived based on rigorous finite-element results. The same formulas were used to calibrate a four-spring model for the analysis of the kinematic interaction problem, resulting in different expressions for the kinematic interaction factors depending on the adopted assumptions. The proposed approach, in conjunction with the novel formulas for impedances and kinematic factors, was applied to the case of bridge piers on caisson foundations, and the results were more accurate than those of the existing simplified procedures.
AbstractThis work aimed at providing analytical closed-form solutions for the design of thermal piles. To this end, a model in which a cylindrical pile is attached along the shaft to a series of ...distributed vertical springs representing soil stiffness is proposed. The pile has constant section and elastic properties; the restraints provided by the superstructure and base stiffness are represented through concentrated springs. The model allows derivation of exact solutions for homogeneous, two-layer soil and soil with linearly increasing stiffness with depth. In addition, approximate energy solutions are derived via the principle of virtual work for more general subsoil conditions with spring stiffness calibrated through finite element results. Expressions for the axial force and shear stress at the pile–soil interface are provided for typical soil stiffness distributions. A successful comparison to literature studies, involving complex transient-coupled numerical analyses and two field tests, corroborate model reliability. The proposed analytical solutions provide insight into the behavior of thermally loaded piles and can be used as a simple tool for ultimate limit state design.
The kinematic bending and filtering potential of a fixed-head pile are explored when large shear strains are generated in the surrounding soil during the passage of seismic waves. The problem is ...treated numerically by employing a freely available 1D code to derive soil response at free-field conditions and an advanced 3D finite-difference (FD) model of the soil-pile system. Three idealized soil profiles with varying stiffness and strength and a real layered site are considered under earthquake excitations of increasing intensity, allowing investigation of the pile’s non-linear kinematic response under shear strains exceeding the threshold of an equivalent-linear approximation. Simple analytical solutions are revisited in the context of soil response close to failure, by means of the FD solution, and an equivalent linear approach is proposed for assessing kinematic pile-head bending and filtering action in the presence of large earthquake-induced shear strains in the soil and non-linear pile behavior. A practice-oriented procedure requiring only a pertinent 1D soil response analysis is proposed to address kinematic effects in seismic design of piles.
This work investigates the effect of the rotational component of input motion induced by the kinematic interaction between a pile group and the surrounding soil on the seismic behaviour of a ...structure. To this end, a simple analytical model is developed by deriving the pile group behaviour from the seismic response of a single pile, taking into account equilibrium and compatibility of displacements at piles’ heads. Closed-form solutions in the frequency domain are provided for both the translational and the rotational motion of a group of unevenly distributed identical piles, rigidly connected at the top and displaced by the surrounding soil, which is subjected to purely translational oscillations. The proposed solutions, applicable to any subsoil conditions, highlight that pile group layout is the crucial parameter governing the magnitude of the foundation rotation. Further, new transfer functions from the soil surface in free field conditions to the top of a SDOF system are introduced, which take into account the translational and/or rotational kinematic effects. An application of the above concepts to a case study is presented, highlighting that the rotational component of input motion may be important for tall structures on small pile groups.
•The Foundation Input Motion caused by pile-soil kinematic interaction under seismic shaking is explored.•A novel solution for both displacement and rotation of the pile group is derived.•The simplified proposed method compares well with rigorous numerical analyses.•Group width and pile axial stiffness strongly affect the rotational component.
Pile foundations supporting tall structures, such as wind turbines, chimneys, silos, elevated water tanks or bridge piers, are subjected during their life span to remarkably eccentric loads. These ...may lead to significant rotations which, however, cannot exceed the limiting values corresponding to the safe operation of the structure. A physically motivated mathematical framework aimed at the prediction of the serviceability performance of such kind of structures is herein presented and discussed. Piles are idealized as uniaxial nonlinear elements characterized by two yielding loads, one in compression and one in uplift, while pile-to-pile interaction effects are modeled by means of superposition, through an approximate solution. The axial load–moment capacity of the pile group is preliminary determined from a recent closed form, exact solution based on upper and lower bound theorems, allowing the analysis to be performed under load control. The model is capable of accounting for the dependence of the moment–rotation response from the dead load of the structure and the ‘coupling effect’ between generalized loads and displacements. The prediction performance of the proposed calculation method is validated against both numerical and experimental benchmarks. Finally, a parametric study allowed to assess the importance of pile-to-pile interaction on the foundation response under eccentric loads.
This paper presents a novel elastodynamic model for the kinematic response of single, end-bearing piles embedded in vertically inhomogeneous soils, excited by vertically propagating P-waves. The ...response in terms of displacements is expressed in the form of a generalized Fourier series, and extends a previous work of some of the authors to account for soil inhomogeneity. Contrary to formulations for homogeneous soils, the associated Fourier coefficients are now coupled, and can be obtained as a solution to a system of algebraic equations of rank equal to the number of soil modes considered in the analyses. The pile is modelled as a rod, using the strength-of-materials solution, and the soil as an approximate continuum of the Tajimi type. For the axisymmetric problem at hand, the Tajimi approximation lies in adopting the physical motivating assumption that the vertical normal and vertical shear stresses in the soil are controlled exclusively by the vertical component of the soil displacement. This approximation results in reducing the number of governing elastodynamic equations to one, which satisfies the equilibrium in the vertical direction. The proposed model can predict the steady-state and transient response of piles in inhomogeneous soil strata over a rigid rock, subjected to vertically-propagating harmonic compressional waves and actual earthquake recordings, respectively. The predictive power of the model is verified through comparisons with finite element analyses. Results are presented in terms of a kinematic response factor which relates the motion of the pile head to the free-field surface motion. It is shown that a pile foundation may significantly alter the motion transmitted to the base of a structure.
The paper investigates the problem of Soil‐Foundation‐Structure Interaction (SFSI) for buildings supported on piles through the comparative analysis between the fixed base and the compliant base ...assumptions. The structure, a nine‐storey residential building (with or without infills), is modelled in non‐linear regime while the piled foundation is idealized by means of independent lumped parameters models, either linear or non‐linear. In this last case, the soil‐foundation system is replaced by an assembly of viscous‐dampers, fictitious masses and non‐linear springs modelled according to the classical Bouc‐Wen formulation, so as to account for the hysteretic behaviour of the foundation. A detailed calibration procedure for both linear and non‐linear foundation models is also presented and discussed. Two different natural soil deposits are considered, a pyroclastic deposit and a deep layer of lacustrine clay. The results undertaken in the context of a probabilistic analysis show that SFSI may lead to a significant reduction of the seismic demand in infilled buildings at low and intermediate earthquake intensity levels. Conversely, at higher intensity earthquakes the seismic demand is not affected by the non‐linear springs. It is shown that a proper modelling of radiation mechanism at foundation level is crucial for a reliable and sustainable prediction of SFSI effects.
The effect on seismic demand of the soil-foundation compliance is investigated for a reinforced concrete frame building representative of Italian constructions from the 1970s. Design accounts for ...regulations, materials, preferred structure and foundation layouts and analysis methods of the period. Inelastic response history analysis for multiple motions at three intensities is carried out. For the compliant-base case, each foundation is replaced with a nonlinear inertial macro-element. Results suggest that the effect of interaction is minor, lower than expected from similar analysis on a new building, due to the larger degree of conservatism associated with foundation design in the considered period and the corresponding lower strength and stiffness of the superstructure.
PDF
