1,721,012 research outputs found
PILE-HEAD KINEMATIC BENDING IN TWO-LAYERED SOIL
No full textPile-soil kinematic interaction arises because of the soil deformations induced by seismic actions. These deformations interact with the pile foundation and can cause severe bending moments that may exceed the flexural capacity of the pile section. The highest bending moments occur at the interface of two adjacent soil layers having a high stiffness contrast and at the pile-head in the case of fixed-head pile. Most of seismic codes recommend assessing the kinematic bending in high seismicity areas for important (i.e. strategic) structures and only in presence of a high stiffness contrast in layered soil. However, several studies demonstrated that pile-head kinematic bending is relevant for the design of piles in soft soils both homogeneous and inhomogeneous. Simplified solutions exist to evaluate the pile-head kinematic bending in the case of homogenous soil and in the case of inhomogeneous soil (if the soil shear modulus continuously increase with depth following a generalized power law function). Nevertheless, no simplified solutions or procedures exist to evaluate the pile-head kinematic bending in two-layered soil with shallow interface. Here the code KIN SP is used to develop a new simplified procedure to solve the latter issue
Relevance of the Foundation Input Motion in the assessment of the seismic response of pile-supported bridge piers
No full textThe seismic response of pile-supported bridge piers is commonly studied avoiding the assessment of the Foundation Input Motion (FIM) (i.e., the motion at the foundation level, which is different compared to that in the free-field, due to the filtering effect exerted by the piles) and assuming fixed-base conditions. A better approach is represented by the substructure method which consists of 3 steps: 1) the assessment of the FIM; 2) the computation of the impedance functions of the soil-foundation system, 3) the analysis of the superstructure response resting on a compliant-base at which is applied the FIM. In many cases this method is not used in a rigorous way (as the assessment of the FIM is not commonly considered an easy task by the structural engineers) and the seismic motion at the base of the model is assumed to be the same as in the free-field. Thus, the effect of Soil-Structure Interaction (SSI) is neglected. Nevertheless, this effect might be beneficial, as piles can filter out some high-frequency components of the free-field motion, especially in the case of large-diameter piles and soft soil conditions (i.e., in those cases in which piles are generally necessary). On the other hand, special attention should be given to those foundations composed by very small pile groups, since the FIM will consist of both a horizontal and a rotational component (which would play a detrimental role). Moreover, it should be remembered that considering SSI would lengthening the fundamental period of the system compared to that under fixed-base hypothesis. This aspect would lead to both positive and negative effects based on the seismic motion properties. This contribution will attempt to show the relevance of a proper assessment of the FIM while studying the seismic response of pile-supported bridge piers. To this end a specific software has been developed to properly compute the FIM considering the filtering effect exerted by the piles
Available methods for the identification of the foundation system and depth in case of existing bridges
No full textThe new Italian “Guidelines for classifying and managing the risk, assessing the safety and monitoring existing bridges” (LLG) are based on a multi-level and multi-risk approach. The Level 0 of this approach is focused on collecting the basic information about both the bridge under consideration and the environment in which it is located (for the assessment of seismic, hydraulic and landslide hazards). These can be retrieved in the project documentation, via available seismic/hydraulic/landslide hazard maps, accelerometric and hydraulic monitoring stations and interferometric data. The Level 1, instead, consists of carrying out visual inspections and compiling some inspection forms for each risk (structure-foundation, seismic, hydraulic and landslide). Then, in the Level 2 all the collected data are organized and used for defining the so-called Class of Attention (5 classes: low, low-medium, medium, medium-high, high) for each risk and the overall Class of Attention. Based on the experiences gained in the last three years by the geotechnical staff of the University of Pisa (member of the FABRE Consortium) one of the main issues encountered in the Levels 0 and 1 (and therefore in the Level 2) was the difficulty to properly identifying the type, geometry, layout, material, and depth of the foundation system of existing bridges. This contribution is focused on describing Non-Destructive Tests, such as Low-Strain Surface tests, Low-Strain Down-hole tests, Parallel Seismic testing (PS) and Electrical Resistivity Tomography (ERT) with the aim of showing main capabilities, limitations, and field of application. These techniques can be useful to improve the classification of existing bridges according to the new Italian Guidelines
The restoration of San Paolo Church in Pisa: Geotechnical aspects
Full text linkSan Paolo a Riva d'Arno is an important medieval church located in Pisa, on the south bank of the Arno river. Its existence is documented before 1032, most probably around 925: its actual configuration, however, was reached only at the end of 14th century. Structural diseases, following damages by bombing during World War II, have affected the church over the last decades; cracks in the perimeter walls and problems to the wooden frames of the roof are progressively appearing. After recent earthquakes the evolution of the statical situation has forced the authorities to close the church to the public. A comprehensive investigation on both the subsoil and the structure has been carried out with the aim of conceiving, designing and implementing suitable remedial works. Particular attention has been given to the collection of quantitative data about the foundation: layout, depth, thickness and state of conservation of the masonry. Geophysical techniques, such as electric tomography and ground penetrating radar, have been attempted with doubtful results; a special technique, consisting in small diameter holes drilled through the masonry with an instrumented drilling machine, was eventually developed. To investigate earthquake effects, a thorough analysis of local amplification of seismic action has been performed. Local seismic response of the subsoil has been analysed by different techniques applied to two different subsoil models; the results obtained evidence of significant differences among the different analyses and in comparison with the prescriptions of the Italian Seismic Code
Analysis method for laterally loaded pile groups using an advanced modeling of reinforced concrete sections
Full text linkA Boundary Element Method (BEM) approach was developed for the analysis of pile groups. The proposed method includes: the non-linear behavior of the soil by a hyperbolic modulus reduction curve; the non-linear response of reinforced concrete pile sections, also taking into account the influence of tension stiffening; the influence of suction by increasing the stiffness of shallow portions of soil and modeled using the Modified Kovacs model; pile group shadowing effect, modeled using an approach similar to that proposed in the Strain Wedge Model for pile groups analyses. The proposed BEM method saves computational effort compared to more sophisticated codes such as VERSAT-P3D, PLAXIS 3D and FLAC-3D, and provides reliable results using input data from a standard site investigation. The reliability of this method was verified by comparing results from data from full scale and centrifuge tests on single piles and pile groups. A comparison is presented between measured and computed data on a laterally loaded fixed-head pile group composed by reinforced concrete bored piles. The results of the proposed method are shown to be in good agreement with those obtained in situ
On the use and effectiveness of the Italian Landslide Hazard Map for the assessment of the Landslide Class of Attention according to the Italian Guidelines for classifying the risk of existing bridges
No full textIn the last three years the Geotechnical and Geological team of the University of Pisa, as member of the FABRE Consortium, provided a technical and scientific support to the first experiences of application of the new Italian Guidelines for classifying the risk of existing bridges (LLG). This contribution deals with the landslide risk and will be focused on the use and effectiveness of the Italian Landslide Hazard Map (LHM) for the assessment of the Landslide Class of Attention. LLG are based on a multi-level approach. In the Level 0 some basic landslide risk information of the area in which the considered bridge is located are collected, while in the Level 1 a visual inspection is performed and in the Level 2 the Landslide Class of Attention is evaluated. On several occasions, geological and geotechnical data are not directly available in the project documentation of the bridge, thus, the only available data for a preliminary screening (desk-study) of the study area come from some National Geo-databases in which the information have been generally organized and developed for different purposes. In Italy, the most relevant maps for identifying the areas interested by recognized or potential landslides are distributed by the ISPRA Institute. The ISPRA LHM uses for the entire Italian territory 5 hazard classes: PF1 (moderate), PF2 (medium), PF3 (high), P4 (very high) and AA (Area of Attention). This Map is the result of the activities performed by all the District Basin Authorities which provide to ISPRA the data related to the landslide hazard areas. Nevertheless, each District Basin Authority uses its own method (qualitative, quantitative, geomorphological or hybrid methods) for the assessment of landslide hazard. As a result, the final LHM might be affected by some inhomogeneities. This contribution will present and discuss a first application of the Italian LHM in the assessment of the Landslide Class of Attention for a representative sample of bridges
Analysis of Laterally Loaded Piled Raft Foundation
No full textA BEM (Boundary Element Method) based method has been developed for the analysis of laterally loaded piled raft foundation. This method considers the raft-soil contact contribution and all the interactions between the piles, the soil and the raft. The nonlinear soil response is accounted by a hyperbolic modulus reduction curve, while the nonlinear response of reinforced concrete piles is modelled accounting also for the influence of tension stiffening. The behaviour of laterally loaded pile foundation is strongly affected by shallower soil layers, which in turn are frequently influenced by suction. The latter aspect has been considered by implementing the Modified Kovacs model in the proposed BEM method. Moreover, the shadowing effect has been modelled using an approach similar to that described in the Strain Wedge Model. The proposed method saves computational effort compared to more sophisticated FEM (Finite Element Method) or FDM (Finite Difference Method) codes and provides reliable results. The validation of the method in the linear elastic range has been carried out by comparing parametric analysis results with those obtained by using the code APRAF, and a comparison with centrifuge test data is shown to verify its reliability
Analisi di fondazioni su pali soggette a forze orizzontali
No full textCome è ben noto sono poche le evidenze sperimentali ed i metodi di analisi di fondazioni miste su pali soggette a carichi orizzontali. Si è visto nel recente passato come l’impiego e l’evoluzione di metodi di analisi e progettazione più razionali per le fondazioni miste platee su pali, soggette prevalentemente a carichi verticali, abbia portato non solo ad enormi benefici in termini di riduzione dei cedimenti assoluti e differenziali ma anche ad un risparmio di materiali grazie a layout delle palificate sempre più ottimizzati. Fino a non molto tempo fa la progettazione di questi sistemi di fondazione è stata largamente limitata dalle normative tecniche, come il D.M. 1988 e le NTC 2005, le quali portavano il progettista a realizzare sistemi di fondazione miste con un numero di pali decisamente superiore a quello che una buona e razionale progettazione avrebbero richiesto. Le Nuove Norme Tecniche per le Costruzioni (NTC, 2008), l’Eurocodice 7: Parte-1 (EC7-1, 2004) e le più recenti International Composed Pile-Raft Foundations Guideline (Katzenbach, 2012) consentono al progettista la possibilità di effettuare (parlando di fondazioni miste sottoposte ad azioni prevalentemente verticali) delle analisi di interazione tra terreno e sistema di fondazione al fine di determinare l’aliquota di carico trasferita al terreno direttamente dalla struttura di collegamento e di quella trasferita dai pali. Poco o niente si dice, invece, circa un più razionale approccio per la progettazione di sistemi di fondazione miste su pali soggette a carichi trasversali. Viene, pertanto, presentato con questo documento l’inizio di un lavoro che si propone quale obiettivo principale lo sviluppo di un nuovo metodo di analisi, per lo studio di fondazioni miste su pali sotto azioni orizzontali, di tipo ibrido BEM-curve “p-y”, che consenta di tener conto del contributo offerto dal contatto struttura di collegamento-terreno, degli effetti delle interazioni tra i vari elementi costituenti il sistema di fondazione, del comportamento marcatamente non-lineare del terreno e dei pali in calcestruzzo armato
KIN SP: A boundary element method based code for single pile kinematic bending in layered soil
Full text linkIn high seismicity areas, it is important to consider kinematic effects to properly design pile foundations. Kinematic effects are due to the interaction between pile and soil deformations induced by seismic waves. One of the effect is the arise of significant strains in weak soils that induce bending moments on piles. These moments can be significant in presence of a high stiffness contrast in a soil deposit. The single pile kinematic interaction problem is generally solved with beam on dynamic Winkler foundation approaches (BDWF) or using continuous models. In this work, a new boundary element method (BEM) based computer code (KIN SP) is presented where the kinematic analysis is preceded by a free-field response analysis. The analysis results of this method, in terms of bending moments at the pile-head and at the interface of a two-layered soil, are influenced by many factors including the soil-pile interface discretization. A parametric study is presented with the aim to suggest the minimum number of boundary elements to guarantee the accuracy of a BEM solution, for typical pile-soil relative stiffness values as a function of the pile diameter, the location of the interface of a two-layered soil and of the stiffness contrast. KIN SP results have been compared with simplified solutions in literature and with those obtained using a quasi-three-dimensional (3D) finite element code
On the influence of pile discretization in single pile kinematic analysis using a boundary element method (BEM) based approach
No full textIn high seismicity areas it’s important to consider kinematic effects to properly design pile foundations. Kinematic effects are due to the interaction between pile and soil deformations induced by seismic waves. One of the effect is the arise of significant strains in weak soils that induce bending moments on piles. These moments can be significant in case of presence of high stiffness contrast in a soil deposit. The single pile kinematic interaction problem is generally solved with beam on dynamic Winkler foundation approaches (BDWF) or using continuous models (BEM, FEM). In this work it is presented a new BEM-based computer code (KIN SP) where the kinematic analysis is preceded by a free-field response analysis. The analysis results of this method, in terms of bending moments at the pile-head and at the interface of a two-layered soil, are influenced by many factors including the soil-pile interface discretization. Finally, it is shown a parametric study having the aim to suggest the minimum number of boundary elements to guarantee the accuracy of a BEM solution, for typical pile-soil relative stiffness values as a function of the pile diameter, the location of the interface of a two-layered soil and of the stiffness contrast
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