364 research outputs found
Solar incubation cuts down parenmtal care in a burrow nesting tropical shorebird, the crab plover Dromas ardeola
No full textHomogenization models for nonlinear and limit analysis of FRP-strengthened masonry
No full textThis chapter discusses two advanced numerical approaches for the analysis of fiber reinforced polymer (FRP)-reinforced masonries, namely a novel adaptive upper bound limit analysis (UBLA) and a nonlinear procedure with rigid elements based on sequential quadratic programming. Both are tools for an effective structural analysis of FRP-reinfoced masonry, requiring a preliminary homogenization of the masonry material at the mesoscale. When dealing with the UB limit analysis approach, a genetic algorithm (GA)-nonuniform rational b-spline (NURBS)-based general framework suitable for a mesh adaptation-applied to curved masonry structures is discussed. A given FRP-reinforced masonry vault can be geometrically represented by a NURBS parametric surface, and a NURBS mesh of the given surface can be generated. Each element of the mesh is a NURBS surface itself and can be idealized as a rigid body. An UBLA formulation, which takes into account the main characteristics of masonry material and FRP reinforcement, is deduced, with internal dissipation allowed exclusively along element interfaces. GA helps in progressively adjusting the shape of the NURBS elements to closely approximate the real failure mechanism activating. When dealing with the nonlinear approach, finite element discretization without mesh adaptation constituted by both rigid wedge elements (masonry) and rigid triangular elements (FRP) interconnected by nonlinear homogenized interfaces is discussed. The step-by-step nonlinear problem is solved as constrained minimization of the quadratic energy function. Both approaches are capable of well predicting the load-bearing capacity of any kind of FRP-reinforced masonry structure (in particular, vaults of arbitrary shape, which are the most complex), with the nonlinear model having the additional features of accurately predicting initial stiffness, postpeak behavior, and displacements at failure. Both approaches are benchmarked through a number of numerical simulations applied to FRP-reinforced masonry structures tested in experiments taken from the literature
First report of non-marine ostracods (Crustacea, Ostracoda) from the Dahlak Archipelago (Eritrea), with the description of two new species
No full textThe information on Recent non-marine ostracod faunas of the north-eastern part of the Afrotropical region is still limited. Here we report the first record of ostracods from the Dahlak Archipelago, a group of small islands located in the southern part of the Red Sea off the Eritrean coast. Specimens were obtained by hatching in the laboratory of diapausing eggs occurring in dried mud collected in temporary freshwater ponds, following the so-called "Sars' method". A total of five species were found, including two species new to science, of which four belong to the family Cyprididae (Cypris galefensis, Plesiocypridopsis newtoni, Heterocypris sp., and Chrissia martensi n.sp.) and one to the family Ilyocyprididae (Ilyocypris dahlakensis n.sp.). A detailed description of the morphology of valves and soft parts is given, and COX1 sequences were obtained for four species. Cypris galefensis was so far only known for its type locality in Somalia with a population containing both males and females, whereas our samples consisted exclusively of females. Plesiocypridopsis newtoni, a species characterised by a wide geographic distribution and previously known to occur also in the Afrotropical region, shows marked variability in the prehensile palps of males, as evidenced by comparing the specimens from this study with descriptions available in the literature. The genus Heterocypris (with 15 species) and the genera of the two new species described here, Chrissia (with 17 species) and Ilyocypris (with four species), have been previously reported from this biogeographic region as well. The analysed specimens of Chrissia martensi n.sp. were all females, none of which had sperm inside the carapace, indicating the possibility of parthenogenetic reproduction in this species. A peculiar sexual dimorphism in the valve morphology characterises Ilyocypris dahlakensis n.sp., with females having a straight posterior margin, forming a right angle at the ventral anterior edge. Rehydration of dry sediments collected from arid areas where wet periods are short and often unpredictable has proven to be a successful method for describing aquatic invertebrate biodiversity
Fast kinematic limit analysis of FRP-reinforced masonry vaults. I: General genetic algorithm-NURBS-based formulation
No full textA new approach for the limit analysis of masonry vaults retrofitted with fiber-reinforced polymers (FRP) based on an upper bound formulation is presented in this paper. In particular, a new genetic algorithm (GA)-nonuniform rational b-spline (NURBS)-based general framework for the limit analysis of curved masonry structures tailored upon an upper bound formulation is discussed thoroughly in the present Part I. A given FRP-reinforced masonry vault can be geometrically represented by a NURBS parametric surface, and a NURBS mesh of the given surface can be generated. Each element of the mesh is a NURBS surface itself and can be idealized as a rigid body. An upper bound limit analysis formulation, which takes into account the main characteristics of masonry material and FRP reinforcement, is deduced, with internal dissipation allowed exclusively along element interfaces. The approach is capable of well predicting the load-bearing capacity of any reinforced masonry vault of arbitrary shape, provided that the initial mesh is adaptively adjusted by means of a metaheuristic approach (i.e., a suitable GA) to enforce that element edges accurately approximate the actual failure mechanism. The approach is validated and discussed in Part II, which is devoted to presenting a number of structural analyses of FRP-reinforced vaults
Analysis of Gothic Masonry Arches Through a Fully Three-Dimensional Kinematic Limit Analysis Approach
No full textA kinematic limit analysis approach for three-dimensional curved rigid blocks is presented. To maintain the exact geometry of curved structures in a three-dimensional context, NURBS solids are used. Under the hypotheses of rigid elements and frictional behavior at interfaces, a kinematic limit analysis problem is defined and solved through a linear programming formulation. The Gothic arches in the Carmo convent, Lisbon (Portugal), are studied to prove the efficiency of the method
Fast kinematic limit analysis of FRP-reinforced masonry vaults. II: Numerical simulations
No full textA new approach for limit analysis of masonry vaults retrofitted with fiber-reinforced polymers (FRPs) based on an upper bound formulation is presented. Part I of this paper was devoted to detailing the theory on which this approach relies. The main idea consists of exploiting properties of nonuniform rational b-spline (NURBS) functions to develop a computationally efficient adaptive limit analysis procedure, which allows quick evaluation of the collapse load multiplier of any given FRP-reinforced masonry vault starting from its three-dimensional (3D) model, which can be generated within any free-form modeler natively working with NURBS entities. A suitably devised genetic algorithm (GA) governs mesh adaption. The present Part II is devoted to validating and discussing through numerical simulations the proposed GA-NURBS procedure. Several structural examples of masonry vaults, including two distinct arches (a straight parabolic barrel vault and a skew parabolic arch, respectively), a hemispherical dome, and both cloister and cross vaults are investigated. Each example is analyzed considering both the unreinforced configuration and the presence of FRP reinforcements. Moreover, comparisons with both nonlinear finite-element (FE) simulations and data collected from experiments (where existing) are presented to assess the proposed GA-NURBS limit analysis procedure. It is shown that, for all cases analyzed, this model allows reliable prediction of both collapse mechanisms and failure loads. The present GA-NURBS approach turns out to be a promising tool that may be conveniently used by practitioners who seek a quick and reliable way to evaluate the outcome of restoration interventions based on the application of FRP composites
Nurbs-based upper bound limit analysis of FRP reinforced masonry vaults through an efficient mesh adaptation scheme
No full textMasonry vaults represent one of the typical structural typologies in historical masonry buildings. The study of the ultimate behavior of masonry vaults, together with the need to design adequate retrofitting techniques, is of high relevance in the optics of the preservation of the cultural heritage. In this paper, a new approach for the limit analysis of masonry construction is applied to FRP reinforced masonry vaults. This approach relies on the representation of geometry through NURBS surfaces, upper bound formulation of limit analysis, idealization of the structure as an assembly of rigid bodies with dissipation allowed only along interfaces, and optimization by means of a mesh adaptation scheme. The presence of FRP strips can be taken into account in easy way, because they can be included simply by adding NURBS surfaces and assigning them an adequate delamination stress value. The efficient mesh adaptation is performed by means of a Prey Predator Algorithm, which has been proven to be very suited for these problems. The strength of the proposed method lies in an accurate estimation of load-bearing capacity and collapse mechanism obtained with a model which requires a very low computational effort
Extended virtual element method for the Laplace problem with singularities and discontinuities
No full textIn this paper, we propose the extended virtual element method (X-VEM) to treat singularities and crack discontinuities that arise in the Laplace problem. The virtual element method (VEM) is a stabilized Galerkin formulation on arbitrary polytopal meshes, wherein the basis functions are implicit (virtual)-they are not known explicitly nor do they need to be computed within the problem domain. Suitable projection operators are used to decompose the bilinear form on each element into two parts: a consistent term that reproduces the first-order polynomial space and a correction term that ensures stability. A similar approach is pursued in the X-VEM with a few notable extensions. To capture singularities and discontinuities in the discrete space, we augment the standard virtual element space with an additional contribution that consists of the product of virtual nodal basis (partition-of-unity) functions with enrichment functions. For discontinuities, basis functions are discontinuous across the crack and for singularities a weakly singular enrichment function that satisfies the Laplace equation is chosen. For the Laplace problem with a singularity, we devise an extended projector that maps functions that lie in the extended virtual element space onto linear polynomials and the enrichment function, whereas for the discontinuous problem, the consistent element stiffness matrix has a block-structure that is readily computed. An adaptive homogeneous numerical integration method is used to accurately and efficiently (no element-partitioning is required) compute integrals with integrands that are weakly singular. Once the element projection matrix is computed, the same steps as in the standard VEM are followed to compute the element stabilization matrix. Numerical experiments are performed on quadrilateral and polygonal (convex and nonconvex elements) meshes for the problem of an L-shaped domain with a corner singularity and the problem of a cracked membrane under mode III loading, and results are presented that affirm the sound accuracy and demonstrate the optimal rates of convergence in the L-2 norm and energy of the proposed method. (C) 2019 Elsevier B.V. All rights reserved
A Genetic Algorithm NURBS-based new approach for fast kinematic limit analysis of masonry vaults
No full textThe present paper proposes a new Genetic Algorithm NURBS-based approach for the limit analysis of masonry vaults based on an upper bound formulation. A given masonry vault geometry can be represented by a NURBS (Non-Uniform Rational B-Spline) parametric surface and a NURBS mesh of the given surface can be generated. Each element of the mesh is a NURBS surface itself and can be idealized as a rigid body. An upper bound limit analysis formulation, which takes into account the main characteristics of masonry material is deduced, with internal dissipation allowed exclusively along element edges. The approach is capable of well predicting the load bearing capacity of any masonry vault of generic shape. It is proved that, even by using a mesh constituted by very few elements, a good estimate of the collapse load multiplier is obtained provided that the initial mesh is adjusted by means of a meta-heuristic approach (i.e. a Genetic Algorithm, GA) in order to enforce that element edges accurately represent the actual failure mechanism. The proposed method turns out to be both accurate and much less computationally expensive than existing methods for the limit analysis of masonry vaults
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