47 research outputs found
Random lattice particle modeling of fracture processes in cementitious materials
No full textThe capability of representing fracture processes in non-homogeneous media is of great interest among the scientific community for at least two reasons: the first one stems from the fact that the use of composite materials is ubiquitous within structural applications, since the advantages of the constituents can be exploited to improve material performance; the second consists of the need to assess the non-linear post-peak behavior of such structures to properly determine margins of safety with respect to strong excitations (e.g. earthquakes, blast or impact loadings).
Different kinds of theories and methodologies have been developed in the last century in order to model such phenomena, starting from linear elastic equivalent methods, then moving to plastic theories and fracture mechanics.
Among the different modeling techniques available, in recent years lattice models have established themselves as a powerful tool for simulating failure modes and crack paths in heterogeneous materials. The basic idea dates back to the pioneeristic work of Hrennikoff: a continuum medium can be modeled through the interaction of unidimensional elements (e.g. springs or beams) spatially arranged in different ways. The set of nodes that interconnect the elements can be regularly or irregularly placed inside the domain, leading to regular or random lattices. It has been shown~\cite{bola} that lattices with regular geometry can strongly bias the direction of cracking, leading to incorrect results.
A variety of lattice models have been developed. Such models have seen a wide field of applications, ranging from aerodynamics (using Lattice-Boltzman models) to heat transfer, crystallography and many others.
Every material used in civil and infrastructure engineering is constituted of different phases. This is due to the fact that the different features of different elements are usually coupled in order to obtain greater advantages with respect to the original constituents. Even structural steel, which is usually thought of as a homogeneous continuum-type medium, includes carbon particles that can be seen as inhomogeneities at the microscopic level. The mechanical behavior of concrete, which is the main object of the present work, is strongly affected not only by the presence of inclusions (i.e. the aggregates pieces) but also by their arrangement. For this reason, the explicit, statistical representation of their presence is of great interest in the simulations of concrete behavior. Lattice models can directly account for the presence of different phases, and so are advantageous from this perspective. The definition of such models, their implementation in a computer program, together with validation on laboratory tests will be presented.
The present work will briefly review the state of the art and the basic principles of these models, starting from the geometrical and computing tools needed to build the simulations. The implementation of this technique in the Matlab environment will be presented, highlighting the theoretical background. The numerical results will be validated based on two complementary experimental campaigns,which focused on the meso- and macro-scales of concrete.
Whereas the aim of this work is the representation of the quasi-brittle fracture processes in cementitious materials such as concrete, the discussed approach is general, and therefore valid for the representation of damage and crack growth in a variety of different materials
Robustness evaluation of RC frame buildings to progressive collapse
No full textA new procedure derived from non-linear static and dynamic analyses is proposed for comparing the relative robustness of RC frame buildings against progressive collapse. The methodology offers a formal way to assess the collapse resistance of structures following the sudden loss of one or more vertical load carrying member/s. The novelty of the proposed methodology lies in the sequential dynamic + static procedure that tracks the redistribution of axial forces following each critical column removal. The basic tool used in the procedure is the ‘‘pushdown’’ analysis, which permits estimation of the residual strength in the structure after it has been damaged by the loss of a critical vertical element. The proposed procedure identifies the critical sequence of column removals and is central to the methodology and the definition of the robustness indices. Referred to as Local Robustness Evaluation (LRE), the methodology is used to develop two robustness indices and applied to two buildings to demonstrate the effectiveness of the procedure in providing a basis for comparing their resistance to progressive collapse
Disproportionate collapse simulations
No full textIn recent years the ability to simulate and predict disproportionate collapse has seen growing interest among the scientific community. This is a challenging matter, since many authors have been dealing with the modeling of progressive collapse, and it is now well-established that such problem requires the use of many different non-standard modeling techniques together with extensive calibration. Despite the many papers in the literature, there is still a lack of methodologies tailored for the quantification of the structural robustness and its acceptable level, which clearly depends on the importance and function of the structure. Further, consideration of Performance Criteria as well as Decision Making Strategies, have to be supported by robust (but also efficient) modeling methods that have to include three-dimensional and geometrical non-linear effects. Some of the previously outlined principles about robustness will be discussed following which the authors present a methodology for taking into account that progressive collapse phenomena are governed not only by the dynamic response of the bays adjacent to the triggering event, but also by the residual plastic resources the structure conserves after the initial damage
Random Lattice Modeling of Quasi-brittle Fracture in Cementitious Materials: A State of the Art
No full textLattice models established themselves as a powerful tool to simulate fracture processes in cementitious materials such as concrete. The paper presents the main features of this method,together with the advancements in the modeling of fracture of concrete materials. A historical
perspective is also given, highlighting advantages and drawbacks of the existing fracture mechanics theories and numerical methods
In Brown Norway rats, MPP+ is accumulated in the nigrostriatal dopaminergic terminals but it is not neurotoxic: a model of natural resistance to MPTP toxicity
No full textWeb-flange behavior of pultruded GFRP I-beams. A lattice model for the interpretation of experimental results
No full textGlass fiber reinforced polymer (GFRP) I-beams have seen growing interest in the last decades, so that they are now being used in many civil applications. For this reason various experimental campaigns have been performed to study the structural response of such elements. In particular, experimental tests performed by Feo et al. [1] highlighted the need to study the local problem of the web-flange junction when pultruded I-beams are subjected to loads acting in the web plane: the experimental results dispersion stimulated numerical analyses and the need to study the problem by means of a nonlinear mesoscale lattice model approach that helped in the experimental result interpretation. The lattice model proposed has several appealing features that make it suitable for the simulation of orthotropic materials like GFRP. The different steps needed to build the model, and the constitutive law used will be explained and the achieved main results will be given in order to conclude that fluctuations in the effective contact area and local material non linearity can be the reasons for the measured dispersion for
both element stiffness end strength
Random lattice particle modeling of damage localization in concrete members under compression
No full textThe ability to predict the localization of damage in concrete members subject to uniaxial
compression is investigated by means of a recently developed random lattice particle model. Such
capability is of great interest in the modeling of concrete structures, since most of the existing mod els rely on the a-priori definition of a zone in which the nonlinear behavior is concentrated. Lattice
particle models, by explicitly representing the mesoscale structure of the material, are capable of sim ulating the localization of damage. Herein, aggregate particles are represented by poly-sized spheres
embedded in a cementitious matrix. The connectivity among particles is defined by a Delaunay tetra hedralization of the sphere centers; the resisting areas of the lattice struts are evaluated by a graph that
is dual to the tetrahedralization. The mesoscale mechanical properties used in the simulations were
measured as part of a multiscale experimental campaign, which also served to validate the numerical
macroscopic response of concrete elements subjected to uniaxial compression
Experimental dynamic testing and numerical modeling of historical belfry
No full textThe experimental characterization of historical bell towers and wall belfries can provide important information for the calibration of numerical models as well as to implement proper restoration strategies. Within this framework, the presented study is concerned with the experimental dynamic assessment of an ancient belfry dating back to 1537. The structure is part of the “SantaMaria in Aracoeli Church” (Rome, Italy), an important heritage construction placed on the summit of the Capitoline Hill, close to the building that hosts the Major’s office. Several field tests have been conducted using accelerometers, and records obtained under different dynamic loading scenarios have been examined.
Moreover, experimental accelerations have been elaborated to estimate the most important modal features of the structure and to validate a finite element model. Field tests have confirmed that severe vibrations are induced when the bells swing, and thus a slight reduction of the swing angle has been suggested in order to provide an immediate and inexpensive benefit to the structure. A new set of field
tests demonstrates that the new swing angle is sufficient to reduce the induced vibrations while preserving the original sound
Delamination processes in RC beams strengthened with SRG/SRP
No full textSteel reinforced polymer (SRP) and steel reinforced grout (SRG) systems have emerged as viable and cost-effective solutions for the flexural strengthening of RC beams/slabs. Experimental tests recently performed at the University of Salerno have allowed for verifying the satisfactory behaviour exhibited by RC slabs strengthened in bending by SRG/SRP systems, both in terms of strength and deformability. Based on the results of this experimental campaign, a simple and efficient OpenSees model is presented herein with the twofold objective of: (1) simulating the behavior of shallow RC beams flexurally strengthened with SRG/SRP; (2) calibrating a constitutive law of the SRG/SRP-concrete interface based on the local deformation tensor evaluated. The preliminary comparison between experimental results and numerical predictions have allowed for verifying the accuracy of the developed model
