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In the paper some topics relevant to the design procedure and the seismic performance of steel concentrically braced frames (CBFs) are addressed. The design proposal of Eurocode 8 for CBFs is analysed in view of the application to fairly complex structural systems. Application of the “tension only” concept, to be used in strength verifications against ultimate-limit-state seismic effects, is applied employing a new strategy based upon the definition of a set of static equivalent seismic forces, computed from response spectrum analysis. Energy dissipation due to interaction between beams and diagonals in macro-bracing systems is also addressed. The dynamic response of a complete 3-D structural system, encompassing several frames with different bracing solutions, is studied by means of a step-by-step numerical procedure implemented in an in house developed computer code. Non-linear response of bracing bars due to element buckling and inelastic material behaviour, as well as non-linear response of beams interacting with macro-bracing systems is considered. Results, given either in terms of displacements and interstory drift envelopes or braces and beams hysteretic behavior, show higher modes and torsional effects importance. Effects of connection detailing, which may jeopardize bracing ductility is also considered through the introduction of a “death” option for bracing diagonals exceeding a predeterminate ductility in tension.

Proportional electroluminescence (EL) in noble gases is used in two-phase detectors for dark matter searches to record (in the gas phase) the ionization signal induced by particle scattering in the liquid phase. The "standard" EL mechanism is considered to be due to noble gas excimer emission in the vacuum ultraviolet (VUV). In addition, there are two alternative mechanisms, producing light in the visible and near infrared (NIR) ranges. The first is due to bremsstrahlung of electrons scattered on neutral atoms ("neutral bremsstrahlung", NBrS). The second, responsible for electron avalanche scintillation in the NIR at higher electric fields, is due to transitions between excited atomic states. In this work, we have for the first time demonstrated two alternative techniques of the optical readout of two-phase argon detectors, in the visible and NIR range, using a silicon photomultiplier matrix and electroluminescence due to either neutral bremsstrahlung or avalanche scintillation. The amplitude yield and position resolution were measured for these readout techniques, which allowed to assess the detection threshold for electron and nuclear recoils in two-phase argon detectors for dark matter searches. To the best of our knowledge, this is the first practical application of the NBrS effect in detection science., Comment: 26 pages, 22 figures, 3 tables

Aria is a plant hosting a$${350}\,\hbox {m}$$350mcryogenic isotopic distillation column, the tallest ever built, which is being installed in a mine shaft at Carbosulcis S.p.A., Nuraxi-Figus (SU), Italy. Aria is one of the pillars of the argon dark-matter search experimental program, lead by the Global Argon Dark Matter Collaboration. It was designed to reduce the isotopic abundance of$${^{39}\hbox {Ar}}$$39Arin argon extracted from underground sources, called Underground Argon (UAr), which is used for dark-matter searches. Indeed,$${^{39}\hbox {Ar}}$$39Aris a$$\beta $$β-emitter of cosmogenic origin, whose activity poses background and pile-up concerns in the detectors. In this paper, we discuss the requirements, design, construction, tests, and projected performance of the plant for the isotopic cryogenic distillation of argon. We also present the successful results of the isotopic cryogenic distillation of nitrogen with a prototype plant.

Assessment of Aeolian vibrations of overhead electrical conductors and guard wires is a relevant technical problem, usually solved through an application of the Energy Balance Method (EBM). The reliability of the EBM results can be strongly affected by the criteria adopted to define the input data (e.g. the wind power) as well as by the modeling assumptions adopted to describe the dynamics of the line. The present paper investigates the effects of different modeling strategies on the outcomes of the EBM. The transmission line spans are modelled through a dynamic substructuring approach: the mechanical impedance matrix of each structural element (e.g. the cables) is defined through a continuum approach and then assembled to lumped components (e.g. the dampers) to obtain the impedance of the overall system. Natural frequencies and mode shapes are then determined through a forced vibration method, which allows to easily handle non-proportional damping, typical for the problem at study. Different structural models to describe both the cables and the passive damping devices are presented. Several different strategies to model the wind power input and the self-damping of the cables are also considered for comparison purposes. The proposed formulation is then applied, within the framework of the EBM, to estimate the aeolian vibration amplitude of some well documented power lines, for which experimental data are available in the literature.

The modeling of iced suspended cables under wind excitation is developed basing on aerodynamic elements superimposed on corotational beam finite elements. Eccentricity of the ice coating is accounted for and aerodynamic loads include the aerodynamic moment. The proposed formulation, applied to a suspended cables of the literature, highlights the importance of the eccentricity of the ice coating on the critical conditions for galloping and on the post-critical behavior.

Permeable double skin facades (permeable DSF) are part of modern architectural concepts. An accurate assessment of the wind loads and the porosity effects on this kind of facades is crucial for a correct design and performance evaluation. Measuring and predicting DSF airflow is not a straightforward task due to the interaction between the wind turbulence and the outer porous skin. Moreover, Eurocode and many other National Codes do not supply any prescription about such issue. In this paper the comprehensive experimental study of wind loads acting on the porous double skin facade of the New Bocconi Campus, currently under construction in Milan, is reported. Cladding loads assessment has been carried out performing wind tunnel tests on properly scaled rigid models able to reproduce the aerodynamic behaviour of the porous medium. Peak pressures for the inner facade have been estimated and compared to the ones expected forcing a standard glazed facade.

This paper deals with control strategies for the mitigation of unwanted vibrations in typical medium and long-span cable-supported bridges, aiming to underline general observations and side effects related to the adoption of different control strategies. Two finite-element models of medium and long-span bridges, namely a suspension and a cable-stayed bridge, are developed for simulating the structural response under wind and earthquake excitation. Passive and semi-active control strategies are then implemented in the models for mitigating dynamic effects. Such control schemes have been designed for wind excitation on the suspension bridge, and seismic excitation on the cable-stayed bridge. Attention is initially focused on high-intensity loading conditions, having a low probability of occurrence. The same control strategies and arrangements have then been re-assessed by changing the input intensity; in addition, a cross-check has been performed by addressing earthquake actions on the suspension bridge and wind forces on the cable-stayed bridge. The results have been also analysed in terms of indirect effects, such as fatigue damage.

Building on the successful experience in operating the DarkSide-50 detector, the DarkSide Collaboration is going to construct DarkSide-20k, a direct WIMP search detector using a two-phase Liquid Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 t (20 t). This paper describes a preliminary design for the experiment, in which the DarkSide-20k LAr TPC is deployed within a shield/veto with a spherical Liquid Scintillator Veto (LSV) inside a cylindrical Water Cherenkov Veto (WCV). This preliminary design provides a baseline for the experiment to achieve its physics goals, while further development work will lead to the final optimization of the detector parameters and an eventual technical design. Operation of DarkSide-50 demonstrated a major reduction in the dominant 39Ar background when using argon extracted from an underground source, before applying pulse shape analysis. Data from DarkSide-50, in combination with MC simulation and analytical modeling, shows that a rejection factor for discrimination between electron and nuclear recoils of $>3 \times 10^{9}$ is achievable. This, along with the use of the veto system and utilizing silicon photomultipliers in the LAr TPC, are the keys to unlocking the path to large LAr TPC detector masses, while maintaining an experiment in which less than $< 0.1$ events (other than $\nu$ -induced nuclear recoils) is expected to occur within the WIMP search region during the planned exposure. DarkSide-20k will have ultra-low backgrounds than can be measured in situ, giving sensitivity to WIMP-nucleon cross sections of $1.2 \times 10^{-47}$ cm2 ( $1.1 \times 10^{-46}$ cm2) for WIMPs of 1 TeV/c2 (10 TeV/c2) mass, to be achieved during a 5 yr run producing an exposure of 100 t yr free from any instrumental background.

This work deals with the evaluation of the effects of the spatial variability of earthquake input motion on the dynamic response of structures. The variability of the free-field motion both over the area of a single foundation as well as over the distance between independent foundations of large structures is considered. Two significant applications of the proposed numerical procedure are presented. In the first one a base isolated nuclear reactor building is considered where the issue of the rocking excitation on the peak value of axial forces in the isolation devices and acceleration in the building structure is investigated. Extended structures having tall members are considered as second application. A cable-supported bridge model, formerly studied within a simpler representation of the seismic ground motion, serves as case study in which spatial variability acts at different scales.

The structure studied in this paper is a 85 meters high tapered obelisk, made of reinforced concrete for the first 9 meters and of steel welded plates for the upper 76 meters. Its bluff cross section consists of two closely spaced triangular sections that are scaled linearly with the height from ground. Having a complex bluff section and being a light structure, aerodynamic instabilities and vortex induced vibrations were investigated using a mixed experimental-numerical approach, in order to define wind loads for the structural design, and possible interventions aimed at mitigating the dynamic effects due to wind action. Exploiting the regular tapering of the obelisk section, a rigid sectional model with constant section was initially tested in wind tunnel: static aerodynamic coefficients and vortex-induced response as a function of the Scruton number were measured for several angles of attack. The experimental sectional results were successively used to assess numerically the response of the structure considering tapering, the gradient of wind speed, the modal shapes, and the correlation of wind actions. Even though the numerical analysis showed that the tapering drastically reduces aerodynamic issues, the achieved results underlined the pressing need to increase the damping of the structure in order to avoid galloping and vortex induced vibrations for wind speeds within the design range; to this aim tuned mass dampers were designed.

The problem of vibration transmitted by train traffic to the soil in the case of railway lines in urban areas is gaining increasing attention in environmental impact analysis. An efficient method to consider both the train-track interaction in detail and the vibration transmitted to the soil nearby with an affordable computational cost is desirable. The paper proposes a numerical procedure based on a substructuring approach, in which the system is divided into three main subdomains: the train running on the track, the rail subjected to the loads coming from the train and the reactions from the sleepers and the “ground” sub-system, composed by the sleepers, the ballast with its subgrade and the actual ground. The overall procedure is divided into subsequent steps: first, the finite element modelling of the sleeper-ballast-subgrade combined system, characterized within the linear elastic field by means of frequency response functions at rail-sleeper interfaces. In a second step, moving loads transmitted to the track are computed by numerical time domain integration of the equations of motion of the train running on a model of the track only, in which the subgrade is modelled as a series of spring-damper elements, whose parameters are tuned according to the results of the FE model used in the first step and therefore consistent with it. Non-linear behavior of the rail-wheel interaction can be accounted for by the time-domain procedure. The track dynamics is finally computed via direct frequency domain analysis; the track is again modelled by Finite Elements, loaded by the forces transmitted by the train wheels and by the supporting sleepers. Finally, the vibrations propagated through the soil to a general receiver point are evaluated. The procedure can exploit favorable properties of frequency domain analysis in treating moving loads; in addition, frequency dependent properties of materials can be introduced.

The maximum expected amplitude of aeolian vibration is commonly evaluated, in overhead power lines, by the Energy Balance Principle. Within this approach empirical power laws are used to express energy dissipation, and usually require experimental tests to define their coefficients. Starting from a previous formulation for the hysteretic bending of overhead electrical lines, the authors propose first a unified non-dimensional expression for the dissipated energy per unit of length of ACSR conductors, and, second, a closed form expression of the upper-bound estimate of the cable self damping. The proposed self-damping model is applied to the aeolian vibrations of a full-scale experimental test span comparing the predictions with application of the empirical power laws and with experimental data. The results highlight the paramount role played by the self damping model.

This paper deals with the numerical simulation of the dynamic response of frame structures undergoing large displacements and 3D rotations.The corotational and the geometrically exact approaches are combined in the formulation of a 3D beam element. Inertia forces are evaluated starting from an exact expression of the element kinetic energy. This last is defined only in terms of nodal variables in the global inertial frame of the mechanical problem, so to avoid cumbersome calculations coming from the corotational decomposition of the motion. The proposed procedure is formally independent from the definition of the corotated frame. The performance of the proposed formulation is discussed with reference to some classic benchmarks involving slender frame structures undergoing large 3D rotations and displacements.

Aim of the paper is to propose a new approach for the determination of the so termed self-damping, or internal damping, of metallic cables. The formulation is developed starting from a recent mechanical model of a strand, from which the hysteretic bending behavior of stranded cables is derived. Each wire of the cable is individually modeled as an elastic curved thin rod. A kinematic model is defined to relate the axial strain and bending curvature of the strand to the generalized strains of the wire. The interaction among the wires belonging to adjacent layers is then studied by neglecting deformations of the contact surfaces and assuming a classic Amontons–Coulomb friction law. In the adopted strand mechanical model a function is derived, which defines the domain of admissible values of the wire axial force to prevent sliding. A simplified model of the cable hysteretic bending behavior is then derived from the cyclic response predicted with the adopted mechanical formulation of the strand, leading to a closed-form upper-bound estimate of the energy dissipated when the cable cross section is subjected to alternate bending. This expression is used as the starting block for the definition of an analytical equation giving an upper-bound estimate of the cable self-damping. The predictions of the proposed model are compared to available data resulting from experiments and empirical literature equations: the comparison is extended to a wide range of strands and parameters that characterize practically most of the configuration commonly used in overhead electrical lines.

DANIELI's Coil Quality Estimator (DANIELI-CQE) is an online system meant for estimation of properties of the hot-rolled coil produced in a hot-strip mill. The system predicts the mechanical properties, especially the yield strength (YS), tensile strength (TS), elongation (EL), and hardness (HV) at different locations over the length and through the thickness of the strip as soon as the coil is rolled. The estimation is based on the final ferrite grain size and the volume fraction of the microstructural constituents such as ferrite, pearlite, and bainite. The system is available for different grades of steel such as low, medium, and high carbon steels, and high strength low alloy (HSLA) steels microalloyed with niobium, vanadium, and titanium. The prediction of properties is based on a series of physically based interconnected models. Different models—Thermal Model, Deformation Model, Microstructural Model, Precipitation Model, Phase Transformation Model, and Structure-Property Correlation Model together f...

Past research has shown that, for seismically isolated structures, rocking excitation has little effect on the peak value of shear deformations in the isolation devices and accelerations in the building structure. Recently, however, seismic isolation has been considered for the reactor buildings of almost all new generation nuclear power plants (NPPs). The safety issues related to NPPs require an assessment of previous results, considering that the seismic isolation needs to be designed in order to both guarantee high performance in case of "beyond-design" earthquakes and to ensure the continuous functionality of the structure. Thus, all factors affecting the isolators performance assume the utmost importance in risk identification and evaluation. The work, described in this paper, aims at the evaluation of the effects of the rotational components of the foundations motion, excited by earthquakes, on seismic isolation systems. The case study is the building housing a Generation 3+, mediumsize nuclear reactor, equipped with HDRB devices. The rubber bearings are simulated in the non-linear field through a phenomenological model from the literature, implemented within a multipurpose finite element code (ABAQUS). The bearings model is non-linear and cyclic with respect to the horizontal displacements and bi-linear-elastic, with different stiffness in tension and compression, for the vertical displacements. © Civil-Comp Press,2015

This paper is concerned with the evaluation of the effects of earthquake excited rotational components of foundation motion in structures having tall structural members. The case study, presented in the paper, is a cable-supported bridge, previously studied within a simpler representation of the seismic ground motion. The study has been developed at the numerical level using a bridge finite element model that is a refined version of a well-known model part of an international benchmark for structural control. The effects of the rotational components of the earthquake ground motion could lead to an amplification of the bridge seismic response. Assessment of the extent is part of this research work. © Civil-Comp Press,2015

The recent developments in numerical tools and computing resources seem to provide a suitable environment to perform numerical analyses of Fluid-Structure Interaction problems. The Co-Simulation technique, in particular, develops the idea of coupling a CFD software with a structural one in order to simulate complex FSI phenomena with a partitioned approach, stressing the concept of software modularity. In this way, it is possible to adopt software tools at the cutting edge of technology. Nonetheless, several difficulties may arise in the choice of the partitioning scheme and of the algorithmic details for the step-by-step time integration. This paper deals with the application of the Co-Simulation technique to a benchmark case experimentally investigated in previous works: the vortex-induced vibrations (VIV) of a beam supported sphere (that is, a sphere fixed to the end of a slender cantilever beam) in a free surface flow. This problem is challenging although apparently simple and it seems quite absent from literature so far. In this paper, the computational issues are thoroughly investigated and the model is validated by comparison with the experimental data. In this way, a robust framework is created in order to deal with VIV problems.

In this paper, a numerical procedure is presented for the dynamic analysis of elastic cables subjected to turbulent wind excitation in quasi-steady conditions. The proposed methodology, which takes geometrical and aerodynamic non-linearities into account, is based on artificial simulation of turbulence, on finite-element modeling of the cables and on a step-by-step implicit procedure for the integration of the dynamic equilibrium equations. As a first application, the dynamic behaviour of a cable in 1 : 2 internal resonance conditions is studied, focusing on some aspects of the influence of wind turbulence on galloping oscillations.

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Three prototype frames are studied within the context of Eurocode 8 (EC8) provisions; these are MRFs which are traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frames, which are not considered in the collapse resistance of the structural system. The non-linear behaviour of these systems under ultimate seismic environment is studied and compared to the one of the unbraced MRFs. The influence of second order effects is also investigated.

The seven-story RC building hosting a University dorm and named “Casa dello Studente”, was built between 1965 and 1967 for a mixed use (residential/commercial). The building plan is subdivided in three “wings” connected through the stairwell, with a basement common to all the wings and a second underground level below the South wing. In 1979 the building was bought by the “ Opera Universitaria ” of the L’Aquila University, to be used as student house, and in 1982 became a property of the Abruzzo Region, in force of State and Region laws. A wide refurbishment intervention, not affecting the structural system, was performed during the years 1999–2002. During the April 6th 2009 L’Aquila (Italy) earthquake a partial collapse of the building took place, limited to the North wing. A first exam of this wing pointed out the occurrence of two distinct collapse zones: the failure of all columns at the ground floor, due to a soft/weak story mechanism, and that of three columns from the base to the top of the building. The aim of the studies here presented, that were performed for legal reasons, has been twofold: to explain the collapse mechanism and to determine the causes of the collapse. This second aim has been pursued with a multidisciplinary approach, investigating the material properties, the ground motion at the building site, the dynamic behavior of the building in its original configuration and after refurbishment. In the light of the design criteria adopted in 1965, a collapse explanation is proposed, compatible with all the experimental data coming from the analysis of damage in the part of building that survived the earthquake and allowing to determine the mechanical causes of the collapse itself.

The research work here described is devoted to the development of a numerical procedure for the computation of seismic fragilities for equipment and structural components in Nuclear Power Plants; in particular, reference is made, in the present paper, to the case of isolated buildings. The proposed procedure for fragility computation makes use of the Response Surface Methodology to model the influence of the random variables on the dynamic response. To account for stochastic loading, the latter is computed by means of a simulation procedure. Given the Response Surface, the Monte Carlo method is used to compute the failure probability. The procedure is here applied to the preliminary design of the Nuclear Power Plant reactor building within the International Reactor Innovative and Secure international project; the building is equipped with a base isolation system based on the introduction of High Damping Rubber Bearing elements showing a markedly non linear mechanical behavior. The fragility analysis is performed assuming that the isolation devices become the critical elements in terms of seismic risk and that, once base-isolation is introduced, the dynamic behavior of the building can be captured by low-dimensional numerical models.

The paper deals with the dynamic non-linear behaviour of concentrically braced steel moment-resisting frames under severe earthquake conditions. The first part of the paper is devoted to the description of a non-linear beam element which has been developed starting from the classical ‘one-component’ formulation, having concentrated inelastic hinges at the ends. The hysteretic behaviour of the end hinges is calibrated on the basis of a set of experimental tests previously performed at Politecnico di Milano. The non-linear brace element used to model the diagonal bars and the procedure for step-by-step dynamic analysis are then briefly summarized. In the second part of the paper two examples of application are presented; the first one concerns a six-storey MRF designed in accordance with Eurocode 8, in which concentric braces are introduced only to satisfy the serviceability limit state requirements (interstory drifts) but are not taken into account in the ultimate conditions. The numerically computed seismic behaviour of the system is analysed and compared to that of the unbraced frame. The second example is a six-storey CBF designed according to the Eurocode 8 specifications; the non-linear behaviour under seismic loading is studied especially to the aim of analysing the effect of the columns flexural stiffness and resistance, which was not considered in the design, on the CBF response.

The paper deals with the use of complex modal analysis for computing the response of linear systems having non-proportional damping. The problem of the approximate evaluation of complex modes is first addressed: a second-order perturbation technique, proposed by other researchers, is adopted and modified in view of the application to the analysis of systems having a large number of degrees of freedom. Frequency domain algorithms for the computation of modal response are then tested and a technique for reducing the computational effort due to the performance of inverse Fourier Transforms is proposed. Two examples of application are finally given.

In the paper the activity of the research group in the field of the dynamic behaviour of Archimede’s Bridges is reviewed. The scope is twofold; on one hand the implications, in terms of structural analysis and design, of the results of this activity are summarized and discussed. On the other hand the needs for future research are defined. The general criteria adopted in the simulation of the dynamic response of the Archimede’s Bridge are first summarized. Attention is focused on “slow” dynamic actions; loading conditions due to impacts and internal or external explosions are thus not considered. The problem of seismic response is subsequently addressed; the problem of transverse response is discussed, especially in light of the anchoring system typology. It is then commented how another critical issue can be represented by the longitudinal motion of very long tunnels and, on design grounds, by the way of providing adequate restraint. Finally the aspects related to hydrodynamic excitation due to seabed motion (tsunamis and “seaquake”) are briefly presented especially in light of research development. The dynamic behaviour under wave and current excitation is addressed in the second part; the adopted wave models are briefly described along with the criteria for defining hydrodynamic forces; some results are shown illustrating the response to exceptional wind waves, with particular reference to the motion of the anchoring elements. The issue of vortex induced vibration is then treated, showing how long anchoring elements can be dangerously prone to activation of large oscillations, with the additional problem of high Reynolds numbers characterizing the fluid-structure interaction. The numerical procedure which has been developed for simulating the phenomenon is illustrated along with some example of application to simple problems. Finally, some general considerations relating dynamic performance and basic design choices are proposed.

The safety-by-design™ approach adopted for the design of the International Reactor Innovative and Secure (IRIS) resulted in the elimination by design of some of the main accident scenarios classically applicable to Pressurized Water Reactors (PWR) and to the reduction of either consequences or frequency of the remaining classical at-power accident initiators. As a result of such strategy the Core Damage Frequency (CDF) from at-power internal initiating events was reduced to the 10−8 /ry order of magnitude, thus elevating CDF from external events (seismic above all) to an even more significant contributor than what currently experienced in the existing PWR fleet. The same safety-by-design™ approach was then exported from the design of the IRIS reactor and of its safety systems to the design of the IRIS Nuclear Steam Supply System (NSSS) building, with the goal of reducing the impact of seismically induced scenarios. The small footprint of the IRIS NSSS building, which includes all Engineered Safety Features (ESF), all the emergency heat sink and all the required support systems makes the idea of seismic isolation of the entire nuclear island a relatively easy and economically competitive solution. The seismically isolated IRIS NSSS building dramatically reduces the seismic excitation perceived by the reactor vessel, the containment structure and all the main IRIS ESF components, thus virtually eliminating the seismic-induced CDF. This solution is also contributing to the standardization of the IRIS plant, with a single design compatible with a variety of sites covering a wide spectrum of seismic conditions. The conceptual IRIS seismic isolation system is herein presented, along with a selection of the preliminary seismic analyses confirming the drastic reduction of the seismic excitation to the IRIS NSSS building. Along with the adoption of the seismic isolation system, a more refined approach to the computation of the fragility analysis of the components is also being developed, in order to reduce the undue conservatism historically affecting seismic analysis. The new fragility analysis methodology will be particularly focused on the analysis of the isolators themselves, which will now be the limiting components in the evaluation of the overall seismic induced CDF.Copyright © 2009 by ASME

A numerical procedure is proposed in the paper for computing seismic fragility functions for equipment components in Nuclear Power Plants. The procedure is based on the hypothesis, which is typical when seismic excitation of components is addressed, of linear behaviour of the building. Given the large size of the FE element models adopted for the building, which makes direct Monte Carlo simulation impossible, the response surface methodology is used to model the influence of the random variables on the dynamic response. To account for stochastic loading the latter is estimated by means of a simulation procedure. Once the response surfaces defining the statistical properties of the response are available, the Monte Carlo method is used to compute the failure probability. A procedure for refining the RS estimation is also proposed, based on the evaluation of risk for a prototype site. A validation example is given, regarding the simplified modelling of a reactor building resting on a base-isolation system; results obtained by plain Monte Carlo analysis are compared to those computed via the proposed procedure The latter is finally applied to a real life case, taken from the preliminary design of the auxiliary building within the IRIS international project.

In this paper, after a general presentation of the Submerged Floating Tunnel (SFT) as an attractive technical solution for waterway crossings, the SFT prototype to be built in Qiandao Lake (People's Republic of China) is introduced. The main peculiarities of such an innovative system are briefly summarized and the importance of realizing a SFT prototype is underlined. The types of action that the SFTs can be generally subjected to are illustrated and the specific load conditions for the SFT prototype in Qiandao Lake are presented. The numerical analyses carried out for investigating the system behaviour in presence of the environmental loads, such as waves, currents and earthquakes, are focused. Three cables configurations are considered for the hydrodynamic analyses, in order to select the most performing one, which is subsequently analyzed also under seismic loads. The model assumptions and the results of the performed analyses are shown and critically discussed. Finally, the displacement and strength safety checks are shown, which allow to state that the designed SFT prototype is able to withstand the environmental design actions in Qiandao Lake.

The modeling of tethering elements of seabed anchored floating structures is addressed, with particular reference to the so-called Archimedes Bridge (submerged floating tunnel, SFT) solution for deep water crossing; attention is devoted to the design solution encompassing slender bars as anchor elements. Two numerical tools are proposed: firstly, a geometrically nonlinear finite element (NWB model), developed in previous work, has been refined in order to capture the effect of higher flexural modes of anchor bars. Secondly, a 3D beam element, based on the classical corotational formulation (CR model), has been developed and coded. Both elements are implemented in a numerical procedure for the dynamic time domain step-by-step analysis of nonlinear discretized systems; seismic loading is introduced by generating artificial time histories of spatially variable seismic motion. An example of application of the NWB element is shown regarding the behavior of the dynamic model of a complete SFT. The model was subjected to extreme multiple-support seismic loading. The seismic behavior is here illustrated and commented, especially in light of the effect of higher local vibration modes of the anchor bars. Finally, a comparison between the performances of the two modeling approaches is presented. Both harmonic and seismic excitations are considered in the test; the results justify the use of the simpler NWB approach, especially in the SFT design phase. Copyright © 2008 John Wiley & Sons, Ltd.

In the paper a numerical procedure is described for the dynamic analysis of seabed anchored floating structures, with particular reference to the so-called Archimedes bridge solution for deep water crossings; attention is devoted to the design solution encompassing slender bars as anchor elements. A geometrically nonlinear finite element, developed in previous work, is here refined extending its capabilities to full 3-D analysis and to nonlinear modelling of hydrodynamic loads due to steady current and wind waves. The element is implemented in a numerical procedure for the dynamic time domain step-by-step analysis of nonlinear discretized systems; consistently, hydrodynamic and seismic loading are introduced by generating artificial time-histories of spatially variable seismic motion and wind waves. An example of on application is shown regarding the behavior of the dynamic model of a submerged tunnel proposed for the Messina Strait crossing. The model is subjected to an extreme multiple-support seismic loading having a PGA equal to 0.64 g and to an extreme wave loading with significant wave height of 16 m. The dynamic behaviour in the two loading situations is illustrated and compared, showing interesting facets, especially in terms of interaction between the tunnel and anchoring bars oscillations.

The seismic behaviour of large-size structures subjected to multiple-support excitation is studied by means of a random vibration approach. Ground motion is modelled as a non-stationary stochastic process having an evolutionary spectral density. To describe the spatial variability of the input motion a model is adopted which accounts for finite propagation velocity and for loss of correlation as a function of distance and frequency. Structural analysis is entirely performed in the frequency domain and full advantage is taken of FFT properties and capabilities. An example is given regarding the behaviour of a 800 m long viaduct. Results are shown in terms of response evolutionary spectral density, variance and reliability with respect to a fixed threshold.

In cables, turbulent wind may cause large amplitude oscillations. The prediction of cable response under wind action requires the use of high-dimensional numerical models either to describe the spatial wind field or to model the expected large cable oscillations. The paper discusses the ability of reduction techniques, for loading and cable descriptions, in reproducing accurately the dynamic response of a suspended cable excited by an artificially generated 3D turbulent wind field. Both the mechanical system and the spatially varying wind velocities are projected on the basis of cable eigenfunctions, retaining in the reduced models few degrees-of-freedom associated with the low-frequency modes. A numerical investigation performed by a refined finite element model provides novel findings on the cable response to wind and permits to demonstrate the effectiveness of the reduced models in the description of cable dynamics.

In cables, near resonance time-varying loading causes large amplitude oscillations mainly involving the resonant mode. A relevant contribution of higher modes may arise as a result of nonlinear coupling phenomena. In this work, analytical and finite element models are used to study the modal interactions in both planar and spatial responses to harmonic in-plane and out-of-plane loads. The aim of the investigation is to compare the two approaches by examining the effectiveness of the analytical model in describing the response with few degrees of freedom and the ability of the nonlinear finite element procedure adopted to capture the complex features of cable dynamics, albeit limited to stable oscillation branches. The analyses first explore a moderately taut cable and are then extended to the behavior of a slacker cable, in order to validate the simplifying kinematic assumptions introduced in the analytical models by comparing the obtained results with those furnished by the richer finite element models. 2003 Elsevier B.V. All rights reserved.

In this paper a procedure for analysing the seismic response of seabed anchored floating tunnels is presented. The first step of the research was the development of an ‘ad hoc’ finite element for modelling the behaviour of anchor elements, with particular reference to the problem of transverse oscillations under time varying axial loads. The element was subsequently inserted in a step-by-step procedure for the numerical analysis of non-linear response to multiple-support seismic input; the procedure encompasses simplified modelling of fluid–structure and soil–structure interaction effects. An example of an application is given concerning two 4680 m long floating tunnels with different seabed profiles. Copyright © 2000 John Wiley & Sons, Ltd.

The paper deals with the numerical treatment of the direct frequency domain (DFD) analysis of linear mechanical systems. Attention is mainly focused on the solution of the complex system of equations needed for each analyzed frequency. Strategies for transforming the system into the “shifted” form ( T −σ I ) z = d are proposed and discussed, where σ is related to the frequency. For two formulations of the shifted system the performance of some Krylov sub-space iterative solvers is tested and compared to that of a multi-frontal direct method. Advantage is taken of the shifted form in solving simultaneously a large number of systems resulting from different values of the shift (frequency). Numerical experiments on some prototype structural dynamics problems are reported; the results shown demonstrate how the devised strategies for the iterative solution can outperform, in many cases, the direct solver.

The existence of a valuable historical heritage of the Medieval age in Bologna has motivated a deeper study on the seismic behaviour of the Asinelli Tower, which, beyond being the town symbol, has a structural configuration prone to seismic damage. Accordingly, in the present paper, the seismicity of Bologna is reviewed and a synthesis of the damage observed in the urban area, based on an accurate historical research, is given, with the objective of evaluating the behavior of the Asinelli Tower within the framework of the effects localized in the municipality. An assessment of the tower's stability with respect to compatible seismic events, at least qualitatively, with the region seismicity is then carried out by means of a non-linear dynamic analysis on a simplified model.

In this paper, we propose a system aimed at interpreting 3-D scenes from 2-D image sequences in the case of scenes containing solid objects that can be described as being made up of polyhedral subparts. The major problems addressed are the definition of a suitable search strategy to reduce the computational complexity of the problem, and the exploitation of the information provided by motion analysis applied to the image sequence. The object models' representation is based on the definition of view classes. Experimental results on simple real scenes are presented.

The paper deals with the non-linear dynamic analysis of concentrically braced pinned frames under earthquake conditions. A model is first presented for the behaviour of bars subjected to cyclic axial force, which takes into account the overall and local buckling phenomena, section shape, residual stresses, material behaviour and damage accumulation due to low-cycle fatigue and fracture. Dynamic analyses of bracing systems are then described: these have been performed, on the basis of the axially loaded bar model, within the framework of a simplified procedure for the determination of optimal design factors. The results obtained are first reported for the case of one-storey systems; an example of the behaviour of a four-storey system is given finally.

This paper analyses the uncertainties in probabilistic interpretation of short-term earthquake precursors, even when the statistical information commonly indicated in the literature as sufficient to define the characteristics of these precursors is assumed to be known. The wide margins for uncertainty in the interpretation of such data are pointed out. One of the principal causes of uncertainty, as an example, lies in the physical origin of false alarms. Depending on this physical origin, the conditional probability of an earthquake, other conditions being equal, may vary in certain cases from values around 0–1 to as much as 0–7 or even higher. The utility of an alarm system, defined as the ratio between the mean number of favourable cases (successful alarms) and the mean number of unfavourable cases (false and missed alarms) is analysed. It is shown that an alarm system based on monitoring n ≥ 2 precursors, if the alarm is set off only after a combination of all precursors, may be considerably less useful than a system based on monitoring only one of the considered precursors.

In this paper we suggest a methodology for detecting arcs and circles in a digitized image, by using as inputs the original image and the gradient image.