1,720,978 research outputs found
The equivalent linear nature of the dynamic Soil-Foundation-Superstructure Interaction (SFSI) of bridge-piers on caisson foundations
No full textThis study presents a simplified approach for evaluating the Foundation Input Motion (FIM) of embedded caissons, considering the complex interaction between the caisson and the surrounding soil. The proposed solution is based on a parametric study using the finite difference code FLAC3D. The analysis explores different embedment-to-radius aspect ratios while incorporating the nonlinear response of the surrounding soil. In FLAC3D the caisson was modelled as a cylindrical element with the mechanical properties of concrete, while the soil was modelled using both a linear viscoelastic and a nonlinear constitutive model. Numerical results in the nonlinear soil regime were compared with both the proposed solution and equivalent linear numerical results, where mobilized values of soil shear modulus and damping ratio (inferred from free-field analyses) were applied. These comparisons shed light on the equivalent linear nature of the soil-caisson interaction. Additionally, several soil-caisson-bridge pier system configurations were studied in linear viscoelastic, equivalent linear, and nonlinear soil regimes. A modified version of the “substructure approach”, in which the FIM was evaluated with the proposed solution, was applied to derive the maximum acceleration of the bridge deck and the drift between the deck and the caisson and the results were compared with those obtained with FLAC3D. The results confirm that the modified “substructure approach” captures the dynamic response of soil-caisson-bridge pier systems. Furthermore, as observed in the soil-caisson interaction case, the findings support the equivalent linear nature of the soil-caisson-bridge pier interaction. The proposed solution was also compared with other methodologies available in the literature
Simplified assessment of pile-head kinematic demand in layered soil
No full textSoil-pile kinematic interaction develops as a consequence of soil movements during seismic excitations. Severe bending strains exceeding the strain limits of the pile material can then develop. The highest kinematic bending moments occur at the interface of layered soil, characterized by consecutive layers with a high Vs2/Vs1 ratio between their shear wave velocities, or at the pile-head of fixed-head piles. Seismic design codes are generally worried about the kinematic bending developing at the interface of two layers with different stiffnesses and suggest evaluating kinematic effects on strategic buildings in seismic prone areas. Recent research has shown that the kinematic demand at the pile-head is important in the design of reinforced concrete piles in soft deposits, and relatively simple formulas exist for its assessment, both in homogenous and inhomogeneous soil conditions. However, there are no simplified solutions for the assessment of the kinematic demand at the pile-head in two-layered soil with a relatively shallow interface depth. In this study a recently developed BEM-based code (KIN SP) is used for this, and the relevance of the threshold values of the average shear wave velocity parameter suggested in seismic codes is discussed. The influence of pile and soil nonlinearities is also investigated with KIN SP, which has been enhanced to account for these nonlinearities
SEMI-ANALYTICAL SOLUTIONS FOR EVALUATING THE FIM FOR PILE FOUNDATIONS
No full textIn this contribution, the kinematic filtering effect of free-head and fixed-head single piles in layered soils is investigated to define new simplified formulas for evaluating the foundation input motion (FIM). These solutions have been derived numerically by employing a boundary element method code (KIN SP, Stacul and Squeglia [11]) and a finite element method code (VERSAT-P3D, Wu [12]). The proposed solutions allow to compute the transfer functions in translation (Iu) and rotation (I?), representing the ratio between pile-head and free-field motion as a function of the excitation frequency, for the following soil profiles: homogeneous, two-layered, parabolic and Gibson soil profiles. These solutions have been developed assuming a linear elastic behavior for the pile material and a linear viscoelastic soil model. Nevertheless, these solutions are expected to be valid also in the case of non-linear soil response and large earthquake-induced shear strains. In fact, as shown in a recent work (Stacul et al. [10]), kinematic soil-pile interaction is a stiffness-controlled mechanism while dynamic and non-linear effects simply modify soil response at free-field conditions
Valutazione semplificata del momento flettente cinematico considerando il comportamento non lineare del palo e del terreno
No full textNel presente contributo viene studiato il problema del momento flettente cinematico indotto alla testa di un palo in calcestruzzo armato (impedito di ruotare in testa) per effetto del passaggio delle onde sismiche nel caso di elevati livelli di deformazione di taglio nel terreno, tenendo conto anche del comportamento non lineare del palo. A tal fine viene preso in esame un palo immerso in un deposito di argilla NC, alla base del quale vengono applicati sette accelerogrammi caratterizzati da diverso contenuto in frequenza. Tali input sismici sono stati scalati a tre livelli crescenti di accelerazione di picco su roccia. In tal modo è possibile studiare il momento cinematico al crescere dei livelli deformativi nel terreno, specialmente, nel caso in cui questi ultimi superano i valori oltre i quali il comportamento del terreno non può più essere approssimato con un modello lineare equivalente. Viene quindi illustrata una procedura analitica semplificata per valutare il momento cinematico alla testa del palo, i cui risultati verranno confrontati con quelli ottenuti da analisi numeriche più sofisticate
PRaFULL: A method for the analysis of piled raft foundation under lateral load
No full textA new code, called PRaFULL (Piled Raft Foundation Under Lateral Load), was developed for the analysis of laterally loaded Combined Pile Raft Foundation (CPRF). The proposed code considers the contribution offered by the raft-soil contact and the interactions between all the CPRF system components. The nonlinear behaviour of the reinforced concrete pile and the soil are accounted. As shallower soil layers are of great relevance in the lateral response of a pile foundation, PRaFULL includes the possibility to consider layered soil profiles with appropriate properties. The shadowing effect on the ultimate soil pressure is accounted, when dealing with pile groups, as proposed by the Strain Wedge Model. PRaFULL BEM code obviously requires less computational resources compared to FEM (Finite Element Method) or FDM (Finite Difference Method) codes. The proposed code was validated in the linear elastic range by comparisons with the code APRAF (Analysis of Piled Raft Foundations). The reliability of the procedure to predict piled raft performance was then verified in nonlinear range by comparisons with both centrifuge tests and computer code PRAB
Numerical Assessment of Laterally Loaded Pile Group Efficiency
No full textThe key factors governing the lateral response of a pile group include the pile-head connection rigidity, the pile-soil relative stiffness, the pile spacing ratio, the pile group size and the pile/soil nonlinear response. Since the earlier experimental observations of the existence of group effects it is commonly assumed that the efficiency of a pile group is less than unity and group effects tend to disappear for spacing ratios larger than 6–7. Despite the availability of advanced computational tools, laterally loaded groups are studied in practice via BNWF approaches and researches about the assessment of the pile group efficiency as a function of the displacement level (i.e. load level) are extremely limited. This paper is aimed at numerically evaluating the influence of displacement level, spacing ratio and pile group size on the lateral efficiency of in-line pile groups in a sandy soil. This will be done by using both a BEM code recently developed by the authors and a more advanced and rigorous commercial code. Presented results may be useful for the design of laterally loaded pile groups in a displacement-based design approach
Effect of non-linear soil response and pile post-cracking behavior on seismically induced bending moments in fixed-head long piles
No full textRecent studies showed that kinematic interaction is relevant for the design of large-diameter concrete piles in soft soils both in homogeneous and layered deposits. Especially, it was found that exists a range of admissible pile diameters able to resist to combined inertial and kinematic bending at the pile-head. Most of these studies assume that the soil is a linear-viscoelastic material and the pile nonlinear behavior is neglected. Here a BEM-based code, called KIN SP, is used to provide new insights about pile-soil interaction under seismic loads. The analyses are performed in the time domain and nonlinear soil response is modelled with the Ramberg-Osgood constitutive law. Moreover, a constitutive model for reinforced concrete piles has been introduced to consider the cyclic variation of the pile stiffness after the first cracking of the cross-sectional area. This paper highlights the importance of considering both soil and pile nonlinear behavior in kinematic interaction analyses
Numerical simulation of seismic response of earth dams
No full textThe paper aims at evaluating the capabilities and limitations of some numerical tools for estimating the seismic response of earth dams. In particular, the recorded acceleration time histories and permanent displacements are compared against those predicted by using a 2D FEM code (Quake/W). Acceleration time histories and displacements were recorded during centrifuge tests of reduced scale model of an earth-core rockfill dam (ECRD). Such tests were carried out by researchers at KAIST (Korea Advanced Institute of Science & Technology) and results were recently published. This paper summarizes the essential aspects of these experiments for the sake of clarity
Use of 1D and 2D seismic response analyses of soil deposits for seismic Microzonation of urban areas in Tuscany (Italy)
No full textThe paper firstly summarizes the main factors affecting seismic response of natural soil deposits with special attention to soil nonlinearity. Then, the available simplified approaches to assess stratigraphic and topographic amplification effects in various codes are discussed.In the second part, the paper shows the results of 1D and 2D Seismic Response Analyses (SRAs), that were performed for the urban centre of Castelnuovo di Garfagnana which is in one of the most seismic areas of Tuscany (Italy). More specifically, the construction of the geotechnical models for seismic response analyses carried out for three cross-sections is illustrated, based on geological and geotechnical information and data available in the Tuscany Region database developed in the framework of the VEL Project. The paper also describes this interactive, georeferenced database. The last part of the paper presents the results of the numerical analyses on selected representative points located along the surface of soil deposit in terms of bar plots of amplification factors. Various available criteria to express 2D amplification effects were considered and summarised. Anyway, the possible aggravation effect was computed by means of an ad - hoc defined parameter. Moreover, possible criteria to extend the SRA results that were obtained for a given geological section over the whole study area were discussed. Use of the average shear wave velocity parameter, V-s,V-30, revealed the most appropriate and cost-effective criterion
Kinematic pile-head bending under large earthquake-induced shear strains
No full textThe problem of kinematic bending moments imposed at the head of a single pile during the passage of seismic waves is explored under large shear strains in the surrounding soil. To this end, non-linear soil response at free-field conditions is derived numerically by a freely-available 1D code and then utilized to calibrate the constitutive law of soil introduced in a rigorous 3D Finite-Difference (FD) model of the soil-pile system employed to obtain pile's head bending moments. The pile is considered embedded to a normally-consolidated clay and seven earthquake records with different amplitude and frequency content are imposed as input motions at the base of the soil layer, thus allowing the investigation of pile kinematic bending with increasing levels of shear strains in the soil, exceeding the limit of equivalent-linear soil behavior. The performance of a simple analytical expression for predicting the kinematic bending moment at the pile-head is compared to the rigorous FD solution. It is concluded that this simple solution is still applicable, with slight modifications, for high shear strains related to non-linear soil behavior close to shear failure, provided that the proper mobilized soil properties from 1D soil response analysis are introduced
- …
