1,720,999 research outputs found
Analisi della dispersione delle leggi di attenuazione per mezzo di sismogrammi sintetici
No full text
A stereological posed mass balance for calculating the distributed efficiency of particle separation systems
No full text
Modelling of ground motion in the vicinity of massive structures
No full textA two-dimensional elastic Chebyshev spectral element method (SPEM) is used to model the seismic wavefield within a massive structure and in its vicinity. We consider 2-D models where a linear elastic structure, with quadrangular cross-section, resting on an elastic homogeneous half-space, is impinged upon by the waves generated by a surface impulse at some distance. The scattering of Rayleigh waves and the response of the structure are extensively analysed in a parametric way, varying size, mechanical parameters and shape of the load. Some of the models considered are representative of embankments and earth dams. The simulation shows that some models resonate, storing part of the incoming energy. With realistic parameters, the lowest resonance frequency is due to pure shear deformation and is controlled by the shear velocity and height of the load. Flexural modes are excited only at higher frequencies. The acceleration at the top of the structure may be five/seven times higher than at the base, depending on the mass of the structure. The gradual release of trapped energy produces a ground roll lasting several seconds after the wave front has passed. The ground-roll amplitude depends on the sturcture's mass and can be as large as 30% of the peak acceleration. Outside resonance conditions, the ground motion is almost unaffected by the presence of the artefact; the horizontal motion on top of it is nearly twice the motion at ground level. Similar results should be expected when the incident field is an upcoming shear wave. A qualitative discussion shows that the presence of anelastic attenuation in the embankment does not significantly alter the preceding conclusions, unless it is of very low values (e.g. Q < 15). The modelling results that we discuss indicate that the soil-structure interaction may substantially alter the 'free-field' ground motion. From a practical point of view, the main conclusions are: (1) careful analysis is necessary when interpreting seismic records collected in the vicinity of large artefacts; (2) seismic hazard at a site may depend on the presence of man-made structures such as embankments, dams, tall and massive buildings
Waves propagation in multilayered media: the effect of wave guides in oceanic and continental Earth models
No full textBDNF mRNA splice variants display activity-dependent targeting to distinct hippocampal laminae
No full textBrain-derived neurotrophic factor (BDNF) may exert contrasting effects depending on its different subcellular sites of action (soma, dendrites, axons). These contrasting effects may explain contradictory findings, for example that BDNF may favour or oppose epileptogenesis. We determined the distribution of five BDNF splice variants in the soma and dendrites of rat hippocampal principal neurons, after application of stimuli that prompt BDNF mRNA accumulation in dendrites (epileptogenic seizures). Under basal conditions, no BDNF mRNA splice variant was detectable in dendrites, while specific splice variants were found in dendrites in response to epileptogenic seizures. Three h after pilocarpine administration, exon VI and exon II splice variants were found in dendrites, while exons I and IV transcripts displayed a strictly somatic localization. Three h after kainate administration, only exon VI was found in dendrites. These data suggest that the regulated expression of different splice variants may provide a code to ensure the delivery of BDNF to precise destinations in the cell soma or along the dendrites
- …
