1,721,050 research outputs found
Seismic microzonation by means of finite element approaches
No full textWhen an earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth's crust. The motion recorded at the ground surface of an area could be really different, in terms of duration and frequency content, from the reference outcrop motion due to the following site conditions: sequence of soil layers, velocity contrast between soil layers, thickness of each layer, dynamic behaviour of the soils, topography and geometry of the sub-interface.
Concerning the site-effects, it is possible to discuss about one-dimensional (1D) effects, two-dimensional (2D) and three-dimensional (3D) ones. One-dimensional ef-fects are induced in case of horizontally layered deposits with a horizontal ground surface (vertically heterogeneous media). Two-dimensional site-effects are generated in case of a complex stratigraphic sequence (vertically and laterally heterogeneous media) and/or in case of an uneven ground surface. In presence of stratigraphic and topographical surface varying in any direction (vertically, laterally and transversally heterogeneous media), it is necessary to refer to three-dimensional site-effects.
Seismic microzonation (SM) studies are used to assess local geological and ge-otechnical site conditions and to identify earthquake characteristics. A SM study can be undertaken according to three different levels of details, as reported in the Italian guidelines. In particular, numerical analyses are requested for a level III SM to quantify the reference motion modification.
This work aims to evaluate complex site-effects for a real case study, i.e. the Bo-vino village, located in South of Italy. This case study has been chosen due to the presence of a soft soil valley surrounded by rock outcrop hills. As a consequence complex site-effects are expected.
Essential ingredients for predicting site effects are: topography, stratigraphy, interface between soil layers and dynamic soil behaviour. Moreover it is necessary to select the reference seismic event and to define properly the input motion used in the numerical analyses.
The present thesis addresses how to perform a site response analysis using the finite element (FE) method. Two different FE codes, in the time domain, have been used: QUAKE/W, based on the equivalent-linear approach, and PLAXIS 3D, which adopts a non-linear Hardening Soil model with small strain stiffness (HSs).
At first, some numerical approaches to simulate 1D site response are defined with reference to ideal case studies. These approaches have been validated by compari-son with results obtained with the code EERA, which is based on an equivalent-linear approach in the frequency domain. 1D schemes have also been used to clarify the definition of reference motion and numerical input motion.
The numerical simulations of seismic site response for the Bovino case study are then presented. Before discussing the results of these analyses, local geology and topography are described. The geotechnical model is subsequently defined, based on field investigations and laboratory data. The reference outcrop motion is then selected, according to the Italian probabilistic seismic hazard maps. Seven real accelerograms have been selected as reference outcrop motions.
The results of the 2D analyses performed using QUAKE/W, with reference to 22 sections, are presented first. The results of the 2D analyses are compared with those of 1D analyses performed with reference to 42 soil columns, extracted along two sections.
Finally, results of 1D, 2D and 3D analysis performed with PLAXIS 3D, assuming two reference motions, are discussed. The analyses allowed to identify the effects of dimensional scheme to seismic site response, the dependency of each amplification pattern to the selected reference motion, the ground motion modification due to dif-ferent topography and soil layers interface, the comparison between different consti-tutive approaches to the same problem (i.e. linear equivalent and non-linear)
Riduzione degli spostamenti indotti da scavi profondi mediante l'uso di pali a fondo scavo
No full textSeismic bearing capacity factors for shallow foundations through different methods of analysis
No full textPast earthquakes provided evidence of the susceptibility of shallow foundations to undergo large deformations and failure also in situations where liquefaction was not triggered. Several examples of these failures occurred after the 1971 San Fernando earthquake. the 1978 Miyagiken-Oki earthquake and the Michoacan earthquake that shook Mexico City in 1985. Experimental evidence of bearing capacity failures similar to those observed in the field was also provided by shaking table and centrifuge model tests on shallow foundations under dynamic excitation.
In this study, the evaluation of earthquake-induced reduction of the bearing capacity is carried out for a shallow strip foundation using two different approaches, both relying on the application of the method of characteristics, extended to the seismic condition through the pseudo-static approach.
Empirical equations, approximating the extensive numerical results, are given to compute corrective coefficients of static bearing capacity factors allowing to consider separately the effects of soil inertia and superstructure inertia. These equations can be introduced in current expressions of the bearing capacity to extend their applicability to seismic design of foundations
Evaluation of the seismic response of a homogeneous earth dam
No full textThe response of an earth dam to seismic loading is studied through displacement-based analyses and finite element, effective stress dynamic analyses. Displacement-based analyses are carried out using both empirical relationships and the decoupled approach in which the deformable response of the soil is accounted for through ground response analyses, and the resulting accelerograms are used in the sliding block analysis. The FE analyses are carried out using a constitutive model capable to reproduce soil non-linearity, calibrated against laboratory measurements of the stiffness at small strains. The influence of the assumed input motion and bedrock depth on the seismic response of the dam is also studied. The FE analyses permitted evaluation of the overall patterns of the dam behaviour in terms of displacements and of stress, strain and acceleration fields. For any of the seismic scenarios considered in the analyses the performance of the dam was satisfactory in that maximum computed settlements at the crest were considerably smaller than the service freeboard of the dam and limited shear strains were obtained within the dam during earthquake loading. Comparison of FE dynamic analyses with displacement-based analyses allowed predictions from different approaches to be compared. A fair agreement was obtained provided that ground response analysis was accounted for when using the displacement-based approach. (C) 2008 Elsevier Ltd. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
New developments in the treatment of medullary thyroid carcinoma
No full textMedullary thyroid carcinoma arises from the C cells, which produce a characteristic hormone, calcitonin. At present, surgery is the main treatment modality. Medullary thyroid carcinoma is usually treated with total thyroidectomy and with removal of nodes in the central portion of the neck and upper mediastinum. Cervical nodes dissection may be required for cancers affecting lateral neck nodes. Substitutive therapy with levo-thyroxine is indicated after surgery. External beam radiation therapy is not effective against advanced medullary thyroid carcinoma, while chemotherapy has a marginal activity. Biological therapy induces its anti-tumour activity through the inhibition of tumour cell growth without cytolysis and stimulating the antitumour immune response, in the absence of relevant side effects. On these bases, it can be suggested that chemo-refractory tumours could be still responsive to biological agents. In the last years somatostatin analogues and interferon have been used in the therapy of advanced and symptomatic medullary thyroid carcinoma, demonstrating an efficacious effect on neuroendocrine symptoms and on the production of calcitonin and an improving in quality of life. Even if there are no consistent data on the effects of biological agents on the reduction of tumour mass, the combined use of chemotherapy and biological therapy needs to be experimented in medullary thyroid carcinoma
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