1,721,061 research outputs found
Key issues in Seismic Microzonation studies: Lessons from recent experiences in Italy
No full textThe Seismic Microzonation (SM) is nowadays a world-wide accepted tool for the mitigation of seismic risk. Despite the large number of SM studies in the literature and the publication of national and international guidelines, some open questions still exist in SM studies and are addressed in this paper. These key issues are discussed after a brief history of SM in Italy and the presentation of Italian and international guidelines on the matter. The SM is a complex process involving different disciplines ranging from Geology and Applied Seismology to Structural and Geotechnical Engineering. The outcome of a SM is presented on a zoning map in terms of ground shaking intensity and susceptibility to main ground instability (soil liquefaction, landslides, fault ruptures). In an advanced SM study for a given area, four main interdisciplinary steps can be recognized: 1) definition of the reference input motions, 2) construction of the subsoil model, 3) performing of numerical analyses, 4) identification of zones with different geotechnical hazard potential and drawing up of the SM map. The key issues and the controversial aspects of these steps are deeply discussed in the paper based on the experience of the Author gained in three recent Italian case studies: Middle Aterno valley, Central Archaeological Area of Rome and Fivizzano. Earthquake-induced permanent soil deformations are out of the scope of the paper being the attention focused on soil amplification phenomena. The paper closes with some remarks on the differences between local seismic hazard assessment for SM mapping and for the seismic design (according to the Italian building code NTC08), and with some proposals on the use of SM output in supporting design
Numerical modelling of seismic site response at large strains: a parametric study.
Full text linkThe numerical analysis of seismic site response at large strains should adopt constitutive models able to guarantee not only a correct modelling of stiffness and damping properties but also a compatibility with the shear strength of the materials. The traditional hyperbolic models used in nonlinear analyses are generally calibrated on stiffness and damping curves and therefore does not necessarily match the soil shear strength. An inaccurate modelling of shear strength can lead to unrealistic predictions of the seismic site response with results that are not necessarily conservative: underestimation or overestimation of the computed surface response depends on the difference between the maximum shear stress implied by the adopted hyperbolic nonlinear model and the real soil shear strength. In this paper, over 1900 one-dimensional parametric analyses on ideal sand and clay deposits were executed with DEEPSOIL software. A first comparison was undertaken between equivalent linear and nonlinear analyses; then the nonlinear analyses were addressed to study the influence of shear strength as an input parameter on the results of numerical site response analyses. In particular two strategies to take into account the soil shear strength were considered: an adjustment procedure associated to the standard MKZ hyperbolic model and the GQ/H model which allows the shear strength to be explicitly defined as input parameter of the analyses. This parametric study made it possible to define preliminary threshold shear strain values, beyond which it is necessary to execute numerical analyses with more advanced models or procedures, able to capture the real behavior of the soil at large strains. Indicatively above shear strains of 0.1%, traditional nonlinear models neglecting soil strength can provide unrealistic results, with important overestimation of the seismic motion (up to 30% in terms of PGA at the surface)
Dynamic properties of earth-core Italian dams from field and laboratory tests
Full text linkThe seismic design of new earth-core dams as well as the seismic re-assessment of existing ones with advanced numerical simulations require the knowledge of dynamic properties of core materials in a wide range of shear strains. The key parameters to be determined are the shear wave velocity Vs (or the maximum shear modulus G0) and the normalized modulus reduction and damping ratio curves (G/G0-c and D-c). Indeed, limited data do exist in the scientific literature on the in-field measured Vs profiles and on the laboratory cyclic/dynamic tests on undisturbed core samples. In this paper the dynamic properties of core materials of six zoned Italian dams are described and the main peculiarities are discussed, also in the light of the dynamic behaviour of natural soils
Validazione di una procedura numerica per l’analisi di interazione dinamica terreno-fondazione-struttura di telai in acciaio
No full textLa risposta sismica di una struttura è il risultato di complessi fenomeni di mutua interazione che avvengono tra tre sistemi interconnessi: terreno di fondazione, struttura di fondazione e sovrastruttura.
La presente nota illustra, con riferimento a due differenti tipologie strutturali in acciaio, la validazione di una procedura di simulazione numerica dell’interazione dinamica terreno-fondazione-struttura, attraverso l’implementazione del cosiddetto metodo diretto, il quale prevede un unico modello che include il terreno, la fondazione e la sovrastruttura.
La procedura numerica è validata mediante la riproduzione di risultati di prove su tavola vibrante, utilizzando il programma di calcolo alle differenze finite FLAC2D. La modellazione del sottosuolo è implementata con elementi a comportamento isteretico non lineare e criterio di rottura alla Mohr-Coulomb. Le fondazioni, superficiali e profonde, sono modellate come elementi a comportamento elastico e sono connesse al terreno mediante l’utilizzo di elementi di interfaccia. La sovrastruttura è costituita da due tipologie strutturali in acciaio, caratterizzate da diverso comportamento dissipativo non lineare e tipologia di fondazione: struttura intelaiata (MRF) e struttura a controventi concentrici (CBF)
Analisi d’interazione dinamica terreno-struttura di edifici in acciaio intelaiati e a controventi concentrici
No full textEffects of ground motion characteristics on seismic response of earth dams: some remarks on duration and vertical shaking
No full textAcceleration records used as input motion for nonlinear dynamic analyses of earth
dams can significantly affect the outcome of the analyses. The selection of an adequate set
of records is therefore an essential step of the study. Customary approaches rely on
matching the target spectrum and the average response spectrum of the selected records.
Furthermore, vertical motion is often considered to have a modest influence on dam
response. In this paper, FLAC was used to conduct dynamic analyses of an earth dam
located in Central Italy. The response was assessed in terms of permanent crest settlements
and correlation were attempted with several IMs. The analyses were conducted with and
without vertical component of motions. It was found that Arias Intensity may be considered
an additional parameter to guide selection of input motions. The inclusion of the vertical
components lead to a general increase, on average 75%, of the crest settlement
Effects of uncertainties and soil heterogeneity on the seismic response of archaeological areas. A case study
No full textThe paper deals with a numerical study aimed at the evaluation of the seismic response of the Central Archaeological Area of Rome including Palatine hill, Roman Forum, and Coliseum. A relevant role is played by buried morphology and mechanical properties of the anthropogenic cover which reaches a thickness of 20 m. This layer is constituted by "dominant masonry" and "dominant infill" zones strongly variable in the space thus exhibiting wide spatial heterogeneity in dynamic properties whose definition is affected by high degree of uncertainty. In order to investigate the effects of the uncertainty in dynamic soil properties of the anthropogenic unit on ground motion at surface, 1D and 2D stochastic analyses were carried out. The numerical results were then compared with those obtained from a standard deterministic model defined by using "average" values of dynamic properties. The influence of uncertainty in shear wave velocity distribution as well as in nonlinear properties (i.e. normalized shear modulus and damping curves) of the anthropogenic cover is highlighted in terms of Housner Intensity amplification factors profiles and acceleration response spectra in representative nodes at surface
Laboratory investigation on nonlinear dynamic properties of core materials of Italian dams
Full text linkThis paper presents the results of a series of laboratory investigations into the
dynamic properties of core materials of Italian earth-core rockfill dams. All tests have been
performed on specimens of undisturbed samples reconsolidated in a wide range of effective
confining stresses. Most of the data were obtained from cyclic simple shear tests and few data
from resonant column tests. It is shown that the shifting of the G/G0- c and D- c curves with
plastic index and effective confining stress is not as significant as it is well established for
natural fine-grained soils. Generic literature curves do not predict properly the dynamic
behaviour of the core materials, especially in the small-to-medium strain range. The
importance of conducting site-specific measurements in order to accurately model the
behaviour of core materials for dynamic analyses of embankment dams is therefore
highlighted
The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies
No full textThe possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part
Earthquake induced floor accelerations on a high-rise building: Scale model tests on a shaking table
No full textThe paper discusses results of shaking table tests on an in-scale high-rise building model. The purpose was to calibrate a dynamic numerical model for multi-hazard analyses to investigate the effects of floor acceleration. Accelerations, because of vibration of non-structural elements, affect both the comfort and safety of people. The research investigates the acceleration effects of both seismic and wind forces on an aeroelastic in-scale model of a multi-story building. The paper discusses the first phase of experiments and gives results of floor accelerations induced by several different base seismic impulses. Structural analyses were first performed on the full-scale prototype to take soil-structure interaction into account. Subsequently the scale model was designed through aeroelastic scale laws. Shaking table experiments were then carried out under different base accelerations. The response of the model and, in particular, amplification of effects from base to top are discussed
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