70 research outputs found
Frequency and damping evolution during experimental seismic response of civil engineering structures
The results of the seismic tests on several reinforced-concrete shear walls and a four-storey frame are analysed in this paper. Each specimen was submitted to the action of a horizontal accelerogram, with successive growing amplitudes, using the pseudodynamic method. An analysis of the results allows knowing the evolution of the eigen frequency and damping ratio during the earthquakes thanks to an identification method working in the time domain. The method is formulated as a spatial model in which the stiffness and damping matrices are directly identified from the experimental displacements, velocities and restoring forces. The obtained matrices are then combined with the theoretical mass in order to obtain the eigen frequencies, damping ratios and modes. Those parameters have a great relevance for the design of this type of structures
A multi-material transport problem and its convex relaxation via rectifiable -currents
In this paper we study a variant of the branched transportation problem, that
we call multi-material transport problem. This is a transportation problem,
where distinct commodities are transported simultaneously along a network. The
cost of the transportation depends on the network used to move the masses, as
it is common in models studied in branched transportation. The main novelty is
that in our model the cost per unit length of the network does not depend only
on the total flow, but on the actual quantity of each commodity. This allows to
take into account different interactions between the transported goods. We
propose an Eulerian formulation of the discrete problem, describing the flow of
each commodity through every point of the network. We provide minimal
assumptions on the cost, under which existence of solutions can be proved.
Moreover, we prove that, under mild additional assumptions, the problem can be
rephrased as a mass minimization problem in a class of rectifiable currents
with coefficients in a group, allowing to introduce a notion of calibration.
The latter result is new even in the well studied framework of the
"single-material" branched transportation.Comment: Accepted: SIAM J. Math. Ana
Two stress update algorithms for large strains: accuracy analysis and numerical implementation
Two algorithms for the stress update (i.e., time integration of the constitutive equation) in large-strain solid mechanics are compared from an analytical point of view. The order of the truncation error associated to the numerical integration is deduced for each algorithm a priori, using standard numerical analysis. This accuracy analysis has been performed by means of a convected frame formalism, which also allows a unified derivation of both algorithms in spite of their inherent differences. Then the two algorithms are adapted
from convected frames to a fixed Cartesian frame and implemented in a small-strain finite element code. The implementation is validated by means of a set of simple deformation paths (simple shear, extension, extension and compression, extension and rotation) and two benchmark tests in non-linear mechanics (the necking of a circular bar and a shell under ring loads). In these numerical tests, the observed order of convergence is in very good agreement with the theoretical order of convergence, thus corroborating the accuracy analysis.Peer ReviewedPostprint (author’s final draft
Actividades do laboratĂłrio ELSA no domĂnio da avaliação e reforço de estruturas em zonas sĂsmicas
O LaboratĂłrio ELSA (LaboratĂłrio Europeu de Avaliação Estrutural), do Centro Comum de Investigação (CCI) da ComissĂŁo Europeia, em Ispra, Itália, desenvolve desde 1992 uma intensa actividade de investigação de carácter experimental e numĂ©rico no domĂnio do comportamento sĂsmico de estruturas. Este trabalho de investigação Ă© desenvolvido em estreita colaboração com outros organismos de investigação (universidades, laboratĂłrios nacionais,...) pertencentes aos Estados Membros ou outros Estados com os quais existam acordos de cooperação. O laboratĂłrio ELSA dedica atenção especial Ă s áreas de investigação na avaliação do comportamento de estruturas sujeitas Ă acção dos sismos. Para alĂ©m da avaliação da vulnerabilidade estrutural, sĂŁo tambĂ©m investigadas tĂ©cnicas de reparação e reforço adequadas a cada tipo estrutural. A avaliação da eficiĂŞncia destas tĂ©cnicas Ă© feita atravĂ©s da realização de ensaios Pseudo-Dinâmicos em modelos de grande escala de estruturas de edifĂcios, pontes, bem como partes de edifĂcios com valor cultural (interesse histĂłrico e monumentos)
Hybrid simulations of a multi-span rc viaduct with plain bars and sliding bearings
This paper deals with the seismic response assessment of an old reinforced concrete viaduct and the effectiveness of friction-based retrofitting systems. Emphasis was laid on an old bridge, not properly designed to resist seismic action, consisting of 12 portal piers that support a 13-span bay deck for each independent roadway. On the basis of an OpenSEES finite element frame pier model, calibrated in a previous experimental campaign with cyclic displacement on three 1:4 scale frame piers, a more complex experimental activity using hybrid simulation has been devised. The aim of the simulation was twofold: (i) to increase knowledge of non-linear behavior of reinforced concrete frame piers with plain steel rebars and detailing dating from the late 1950s; and (ii) to study the effectiveness of sliding bearings for seismic response mitigation. Hence, to explore the performance of the as built bridge layout and also of the viaduct retrofitted with friction-based devices, at both serviceability and ultimate limit state conditions, hybrid simulation tests were carried out. In particular, two frame piers were experimentally controlled with eight-actuator channels in the as built case while two frame piers and eight sliding bearings were controlled with 18-actuator channels in the isolated case. The remaining frame piers were part of numerical substructures and were updated offline to accurately track damage evolution
Modèle de béton armé équivalent pour l'analyse du comportement des voiles de très faible élancement. Programme SAFE
Within the SAFE research project squat reinforced concrete walls have been tested pseudo-dynamically at ISPRA. This paper describes the modeling of the structures T5 and T12 of the program. Numerical calculations done by LMT are presented. Modeling is based upon "equivalent reinforced concrete medium" coupled with damage mechanics and plasticity. Beams and truss elements are used in order to elaborate an efficient, non time consuming simplified method. Comparison with experimental results demonstrate the efficiency of the modeling tools to understand the nonlinear behavior of very squat walls.Le programme SAFE concerne des essais pseudodynamiques effectués à ISPRA sur des voiles en béton armé très faiblement élancés. La modélisation choisie au LMT pour calculer le comportement non linéaire des structures T5 et T12 du programme est ici présentée. Les maquettes sont modélisées selon le principe du «béton armé équivalent». Les maillages du béton et de l'acier sont découplés et des éléments barres ou poutres sont utilisés pour modéliser la structure. La loi constitutionnelle du béton est basée sur la mécanique de l'endommagement tandis que celle de l'acier suit une loi élastoplastique. La comparaison entre les résultats expérimentaux et numériques prouve la capacité de la nouvelle méthode de simuler le comportement non linéaire des voiles de très faible élancement
Cyclic behaviour of stone and brick masonry under uniaxial compressive loading
An experimental research concerning the uniaxial compressive behaviour of stone and brick specimens, as well as masonry prisms, is presented. Local sandstone and clay brick materials were used in order to obtain results representative with respect to local constructions. Aiming at a comprehensive material description, a set of displacementcontrolled
experiments were carried out, both under monotonic and cyclic compressive loading. The procedure adopted for testing is described and the results are discussed, namely material brittleness, intrinsic variability, energy dissipation and stiffness degradation.Dans cet article une recherche expérimentale
à propos du comportement en compression uniaxial de spécimens de pierre et de la brique, aussi bien que prismes de maçonnerie, est présenté. Grès et brique de l’argile locale ont été utilisés pour obtenir des résultats représentatifs en ce qui concerne les constructions locales. Avec l’objective de obtenir une description matérielle complète, un ensemble de tests contrôlé par déplacement a été emporté, sous chargement de compression monotonic et cyclique. La procédure adoptée pour tester est décrite et les résultats sont discutés, nommément la fragilité matérielle, variabilité intrinsèque des matériaux, dissipation d’énergie et déchéance de la raideur
5G network slicing with QKD and quantum-safe security
We demonstrate how the 5G network slicing model can be extended to address
data security requirements. In this work we demonstrate two different slice
configurations, with different encryption requirements, representing two
diverse use-cases for 5G networking: namely, an enterprise application hosted
at a metro network site, and a content delivery network. We create a modified
software-defined networking (SDN) orchestrator which calculates and provisions
network slices according to the requirements, including encryption backed by
quantum key distribution (QKD), or other methods. Slices are automatically
provisioned by SDN orchestration of network resources, allowing selection of
encrypted links as appropriate, including those which use standard
Diffie-Hellman key exchange, QKD and quantum-resistant algorithms (QRAs), as
well as no encryption at all. We show that the set-up and tear-down times of
the network slices takes of the order of 1-2 minutes, which is an order of
magnitude improvement over manually provisioning a link today
Homogenization and seismic assessment : review and recent trends
The mechanics of masonry structures has been for long
underdeveloped in comparison with other fields of knowledge. Presently,
non-linear analysis is a popular field in masonry research and
advanced computer codes are available for researchers and practitioners.
The chapter presents a discussion of masonry behaviour and
clarifies how to obtain the non-linear data required by the computations.
The chapter also addresses different homogenisation techniques
available in the literature in the linear and rigid-plastic case,
aiming at defining a catalogue and at discussing the advantages and
disadvantages of the different approaches. Special attention is given
to stress assumed models based either on a polynomial expansion
of the micro-stress field or in the discretization of the unit cell by
means of a few constant stress finite elements CST with joints reduced
to interfaces. Finally, the aspects of seismic assessment are
presented and case studies involving the use of macro-block analysis,
static (pushover) analysis and time integration analysis are
discussed.(undefined
Efficient and reliable nonlocal damage models
We present an efficient and reliable approach for the numerical modelling of failure with nonlocal damage models. The two major numerical challenges––the strongly nonlinear, highly localized and parameter-dependent structural response of quasi-brittle materials, and the interaction between nonadjacent finite elements associated to nonlocality––are addressed in detail. Reliability of the numerical results is ensured by an h-adaptive strategy based on error estimation. We use a residual-type error estimator for nonlinear FE analysis based on local computations, which, at the same time, accounts for the nonlocality of the damage model. Efficiency is achieved by a proper combination of load-stepping control technique and iterative solver for the nonlinear equilibrium equations. A major issue is the computation of the consistent tangent matrix, which is nontrivial due to nonlocal interaction between Gauss points. With computational efficiency in mind, we also present a new nonlocal damage model based on the nonlocal average of displacements. For this new model, the consistent tangent matrix is considerably simpler to compute than for current models. The various ideas discussed in the paper are illustrated by means of three application examples: the uniaxial tension test, the three-point bending test and the single-edge notched beam test.Peer ReviewedPostprint (author’s final draft
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