1,720,979 research outputs found
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
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
Foundation motion filtered by piles: Effect of soil Inhomogeneity
No full textThe effect of soil inhomogeneity on kinematic response of single flexural elastic piles to vertically-propagating seismic SH waves is explored. The system under consideration consists of a fixed-head long pile embedded in a viscoelastic soil layer underlain by a rigid bedrock; soil stiffness is assumed to increase linearly or constant. Both harmonic and real earthquake motions are employed to investigate soil-pile kinematic interaction in frequency and time domain. Pile response in inhomogcncous soil is analysed in terms of kinematic interaction coefficients relating pile-head to free-field soil lateral motion and compared to its homogeneous counterpart. The problem is tackled numerically by means of both rigorous elastodynamic Finite-Element analyses and Beam-on-Dynamic-Winkler-Foundation (BDWF) formulations. The role of model parameters such as pile diameter, rate at which soil stiffness increases with depth and average shear wave velocity VSi30 referring to soil type C or D according to ECS is elucidated. Results indicate that: (a) the horizontal displacement of fixed-head piles under harmonic excitation is essentially governed by a single dimensionless frequency parameter based on an average Winkler wavenumber incorporating pile-to-soil stiffness ratio, pile slenderness and soil inhomogeneity and (b) piles-induced filtering effect tends to increase by increasing the degree of soil inhomogeneity and pile diameter, revealing a substantially reduced seismic demand on the superstructure compared to that pertaining to die free-field motion. The above filtering action although neglected in seismic codes may of importance in pile design practice
ON THE SSI PERIOD AND DAMPING FOR BRIDGE PIERS ON CAISSON FOUNDATIONS
No full textThe vibrational period and damping of structure-foundation-soil systems are explored, with focus on the case of bridge piers on caisson foundations. To this end, a SDOF model resting on flexible supports is considered as reference model. A parametric study is then performed by considering different dimensions of the caisson, heights of the pier, mass of the deck and subsoil. Rigorous results are compared with predictions of earlier solutions, obtained in the context of the “replacement oscillator” approach, to highlight the role of the double damping terms, foundation mass, reference system for the calculation of impedances and the cross swaying-rocking stiffness
A numerical study on the filtering action of piles in the soft clay of Maliakos Gulf, Central Greece
No full textThe potential of piles to reduce the seismic motion transmitted to the superstructure with respect to the free field motion, due to the kinematic interplay between soil and pile, is explored by considering a single pile embedded in the soft marine clay of Maliakos Gulf subsoil in central Greece. The soil profile consists of a very soft normally consolidated clay layer with nearly-zero stiffness at ground surface, increasing proportionally with depth, underlain by a stiffer layer of sandy clay. The problem is tackled numerically by means of Equivalent linear (EL) and strength-controlled non-linear (NL) constitutive laws to model the behavior of the soil under seismic excitation. The latter is chosen on a magnitude-epicentral distance (M-R) basis from the European Strong-motion Database (ESD), taking into account the seismotectonic regime of the broader area under study. Seven (7) motions recorded at soil type A, according to EC8, were chosen to define an average code-compatible spectrum. The average mobilized stiffness of the free field soil obtained from EL and NL analyses is then introduced into a finite-element model of the soil-pile system to investigate the kinematic-induced filtering action of the pile as affected by pile cap embedment depth and pile diameter. The above is quantified in terms of ratios of spectral accelerations at the pile head and free field at ground surface. Results highlight the importance of considering NL response for such type of soils and conclude on the beneficial role of piles as reflected in a large reduction of the free field spectral acceleration, even for piles with relatively small diameter
Effetto filtro esercitato dai pali in terreni non omogenei
No full textNella nota si affronta il problema della modifica del moto sismico prodotta dall’interazione cinematica fra pali e
terreno nel caso di sottosuolo con rigidezza variabile continuamente con la profondità con legge potenza
generalizzata e pali impediti di ruotare in testa. Il problema è affrontato numericamente con il classico modello di
Winkler dinamico (BDWF). Viene suggerita un’espressione approssimata nel dominio delle frequenze del fattore
di interazione cinematica palo-terreno di validità generale, che può essere utilizzata convenientemente ai fini della
definizione del moto sismico di una fondazione su pali per i casi di sottosuolo esaminati. I risultati ottenuti
mostrano in modo incontrovertibile che – a parità di velocità equivalente di propagazione delle onde di taglio - la
riduzione delle ampiezze del moto sismico nel caso di terreno eterogeneo è molto più pronunciata di quella che si
verifica del caso di sottosuolo omogeneo
Quasi-kinematic response of embedded foundations: evidence of foundation mass effect from numerical analyses and instrumented structures
Full text linkKinematic Soil-Pile Interaction under Earthquake-Induced Nonlinear Soil and Pile Behavior: An Equivalent-Linear Approach
Full text linkThe 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
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