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• Numerical tool to study RC structures under seismic loads using fiber reinforced polymers

  X. Martinez, S. Oller, A. Barbat

  Intersections = Intersectii (2007). Vol. 4 (2), pp. 50-64

  
  

  Abstract

  Strengthening or retrofitting existing structures in order to increase their ductility and improve their seismic response has traditionally been accomplished using conventional materials and construction techniques. Composite materials of a polymeric matrix reinforced with long fibers (FRP) have emerged as an alternative to these methods. To view the performance of these reinforcements when seismic loads are applied this work studies the structural response of a frame joint when a horizontal load is applied to it. Under a seismic load, joints are one of the weakest parts of these structures. The response of a plain concrete frame joint is compared with the response obtained when it is reinforced using FRP. Different configurations of FRP reinforcements are considered to compare their behavior. The structural response of all structures considered is obtained with a numerical simulation. This is done using the finite element method. Composites are treated using the mixing theory, which obtains the composite behavior by means of the composition of each component material properties. Each component is simulated with its own constitutive equation. The anisotropy usually found in composite components is treated using a mapped space theory. The debounding effects found in composite materials are treated using the formulation developed by E. Car and S. Oller [Car, 2000]. The results obtained validate the use of FRP reinforcements to improve frames seismic response. The structure load capacity is increased in a 20% when lateral reinforcements are applied to the joint. This can avoid the structure to collapse in a seismic case. They result also show a good performance of the numerical tool developed.

  Abstract
  Strengthening or retrofitting existing structures in order to increase their ductility and improve their seismic response has traditionally been accomplished using conventional [...]

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• Factores de reducción de respuesta: estado del arte y estudio comparativo entre códigos

  J. Vielma-Perez, A. Barbat, S. Oller

  Revista Internacional de Ingeniería de Estructuras (2006). Vol. 11 (1), pp. 77-106

  
  

  Abstract

  En los procedimientos de análisis y diseño sismorresistentes de estructuras se hace necesario introducir simplificadores o aproximaciones que permitan estudiar la equivalencia entre el análisis elástico a partir del cual se efectúan los cálculos, con el comportamiento elástico plástico, que es el comportamiento real de las estructuras cuando son sometidas a la acción de terremotos fuertes. Una de estas simplificaciones consiste en proyectar las estructuras de manera que estas dispongan de una resistencia lateral inferior a la resistencia elástica, lo cual se consigue mediante la aplicación de cargas laterales calculadas a partir de espectros de diseño inelásticos, que no son más que los conocidos como factores R. En este artículo se realiza una revisión exhaustiva del estado del arte de los factores de reducción de respuesta, que comprende desde su propuesta original, pasando por su implementación en los códigos de diseño sismorresistente, hasta llegar a las nuevas propuestas de racionalización de su aplicación. Para poder alcanzar una mejor comprensión sobre el tema, es necesario realizar una comparativa entre aplicación de los factores de reducción de respuesta en los diferentes códigos de diseño sismorresistente a nivel mundial.  

  Abstract
  En los procedimientos de análisis y diseño sismorresistentes de estructuras se hace necesario introducir simplificadores o aproximaciones que permitan estudiar [...]

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• Vulnerability and ductility concepts in the seismic design of buildings

  A. Barbat, S. Oller, L. PUJADES

  Acero y Hormigón (2005). Vol. 238, pp. 61-73

  
  

  Abstract

  In spite of the every time better understanding and knowledge of the factors that influence in the seismic behaviour of the buildings, there is a great difference at the moment between the theoretical developments that are carried out continually in the field of earthquake engineering and the concepts that are applied in the seismic design codes of many countries. This fact is reflected clearly in the very irregular seismic behaviour of the buildings during the big earthquakes occurred in the world in the last years. The earthquake design codes make exhaustive recommendations concerning the seismic loads, the simplified methods of structural analysis to be applied, to the details that should incorporate in the seismic design and construction of the structural elements and, in lesser measure, to the conceptual design of the buildings. Therefore, special attention is paid in this work to the structural ductility and its influence on the seismic vulnerability of the buildings and on their seismic damage and risk.

  Abstract
  In spite of the every time better understanding and knowledge of the factors that influence in the seismic behaviour of the buildings, there is a great difference at the moment [...]

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• A Lagrangian nodal integration method for free-surface fluid flows

  A. Franci, M. Cremonesi, U. Perego, E. Oñate

  Comput. Methods Appl. Mech. Engrg., (2020). Vol. 361, 112816

  
  

  Abstract

  We present a Lagrangian nodal integration method for the simulation of Newtonian and non-Newtonian free-surface fluid flows. The proposed nodal Lagrangian method uses a finite element mesh to discretize the computational domain and to define the (linear) shape functions for the unknown nodal variables, as in the standard Particle Finite Element Method (PFEM). In this approach, however, the integrals are performed over nodal patches and not over elements, and strains/stresses are defined at nodes and not at Gauss points. This allows to limit the drawbacks associated with the remeshing and leads to a more accurate stress computation than in the classical elemental PFEM. Several numerical tests, in 2D and in 3D, are presented to validate the proposed nodal PFEM. In all cases, the method has shown a very good agreement with analytical solutions and with experimental and numerical results from the literature. A thorough comparison between nodal and elemental PFEMs is also presented, focusing on crucial issues, such as solution accuracy, convergence, mass conservation and sensitivity to mesh distortion.

  Abstract
  We present a Lagrangian nodal integration method for the simulation of Newtonian and non-Newtonian free-surface fluid flows. The proposed nodal Lagrangian method uses a finite [...]

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• Statical and dynamical behaviour of thin fibre reinforced composite laminates with different shapes

  L. Nallim, S. Oller, R. Grossi

  Comput. Methods Appl. Mech. Engrg., (2005). Vol. 194 (17), pp. 1797-1822

  
  

  Abstract

  Based on the classical laminated plate theory, a variational approach for the study of the statical and dynamical behaviour of arbitrary quadrilateral anisotropic plates with various boundary conditions is developed. The analytical formulation uses the Ritz method in conjunction with natural coordinates to express the geometry of general plates in a simple form. The deflection of the plate is approximated by a set of beam characteristic orthogonal polynomials generated using the Gram–Schmidt procedure. The algorithm developed is quite general and can be used to study fibre reinforced composite laminates with symmetric lay-ups, which may have general anisotropy and any combinations of clamped, simply supported and free edge support conditions. Various numerical applications are presented and some results are compared with existing values in the literature to demonstrate the accuracy and flexibility of the present method. New results were also determined for plates with different geometrical shapes, combinations of boundary conditions, several stacking sequences and various angles of fibre orientation.

  Abstract
  Based on the classical laminated plate theory, a variational approach for the study of the statical and dynamical behaviour of arbitrary quadrilateral anisotropic [...]

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• Estudio numérico del comportamiento de los hormigones reforzados con fibras cortas

  S. Oller, A. Barbat

  Hormigón y Acero (2004). Vol. 55 (232), pp. 113-123

  
  

  Abstract

  En este trabajo se presenta un acercamiento al estudio del comportamiento de los hormigones reforzados con fibras cortas. Se establece una formulación basada en una modificación de la teoría de mezclas clásica para determinar sus parámetros mecánicos a partir de las características de los materiales componentes (hormigón fibras cortas). También se muestran resultados numéricos obtenidos mediante el método de los elementos finitos y se comentan las posibilidades de un método analítico simplificado para el estudio de su comportamiento. El desarrollo del trabajo trata sobre el comportamiento del hormigón reforzado con fibras y las características que justifican su utilización. Se estudia el equilibrio interno y mecanismo de transmisión de tensiones entre el hormigón y las fibras cortas y la formulación general del problema e inserción dentro de la técnica de los elementos finitos. Se hace un breve comentario sobre el tratamiento simplificado del comportamiento de los hormigones reforzados con fibras cortas y se presenta un ejemplo de comprobación.

  Abstract
  En este trabajo se presenta un acercamiento al estudio del comportamiento de los hormigones reforzados con fibras cortas. Se establece una formulación basada en una [...]

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• Definition of a general implicit orthotropic yield criterion

  S. Oller, E. Car, J. Lubliner

  Comput. Methods Appl. Mech. Engrg., (2003). Vol. 192 (7-8), pp. 895-912

  
  

  Abstract

  The definition of an orthotropic yield criterion presents a serious challenge in the formulation of constitutive models based on such theories as elastoplasticity, viscoplasticity, damage, etc. The need to model the behavior of a real orthotropic material requires the formulation of orthotropic yield criteria, and these may be difficult to obtain. For metals, orthotropic yield functions have been formulated by Hill [Proc. Roy. Soc. Lon. Ser. A 193 (1948) 281; J. Mech. Phys. Solids 38(3) (1990) 200], Barlat [Int. J. Plasticity 5 (1989) 51; 7 (1991) 693], Chu [NUMISHEET 93 (1993) 199] and Dutko et al. [Comput. Methods Appl. Mech. Engrg. 109 (1993) 73], but in many cases these functions do not describe the true behavior of the metal. The situation is worse when one attempts to represent a nonmetal such as a polymer, ceramic or composite. In this paper, we present a general definition of an explicit orthotropic yield criterion together with a general method for defining implicit orthotropic yield functions. The latter formulation is based on the transformed-tensor method, whose principal advantage lies in the possibility of adjusting an arbitrary isotropic yield criterion to the behavior of an anisotropic material. As example we choose the adjustment to the Hill, Hoffman [J. Comp. Materials 1 (1967) 200] and Tsai–Wu [J. Comp. Materials 5 (1971) 58] criteria, but these particular cases serve to establish the methodology for achieving the desired function adjustment for any other well-known criterion or experimental set of data obtained from laboratory.

  Abstract
  The definition of an orthotropic yield criterion presents a serious challenge in the formulation of constitutive models based on such theories as elastoplasticity, viscoplasticity, [...]

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• Life prediction of structures. Numerical study and treatment of structural durability, a brief report

  S. Oller, E. Oñate

  Temes de Disseny (2002). Vol. 20, pp. 85-99

  
  

  Abstract

  This work is a brief report about the problem of life -or, durability study- of structural materials, undergoing mechanical, thermal and chemical actions. This document is approached from numerical techniques and, it highlights the potential of these types of tools un the study of structures undergoing very complex and coupled phenomena.

  Abstract
  This work is a brief report about the problem of life -or, durability study- of structural materials, undergoing mechanical, thermal and chemical actions. This document is [...]

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• A general framework for continuum damage models. II. Integration algorithms, with applications to the numerical simulation of porous metals

  F. Armero, S. Oller

  Int. J. Solids and Structures (2000). Vol. 37 (48-50), pp. 7437-7464

  
  

  Abstract

  In this paper, we develop numerical algorithms for the integration of the continuum plastic damage models formulated in the general framework identified in Part I of this work. More specifically, we focus our attention on a particular plastic damage model of porous metals, involving a classical von Mises yield criterion coupled with a pressure dependent damage surface to model the nucleation and growth of voids in the metallic matrix. Unilateral damage leading to a sudden change of stiffness in the material's response due to the closing/opening of these voids is also incorporated through the imposition of the unilateral constraint of a positive void fraction, thus, illustrating the clear physical significance added by this framework in the resulting constitutive models. The proposed integration algorithms fully use the modularity of the identified framework, leading in this way to independent integration algorithms for the elastoplastic part and each damage mechanism. Remarkably, all these individual integration schemes share the same formal structure as the classical return mapping algorithms employed in the numerical integration of elastoplastic models, namely an operator split structure consisting of a trial state and the return map imposing the plastic and damage consistency, respectively. A Newton iterative scheme imposes the equilibrium (equal stresses) among the different mechanisms of the response of the material. This modular structure allows to obtain the closed-form consistent linearization, involving in a simple form the algorithmic consistent tangents corresponding to each independent mechanism, thus resulting in a very modular and efficient computational implementation. The performance of the proposed algorithms is illustrated in several representative numerical simulations.

  Abstract
  In this paper, we develop numerical algorithms for the integration of the continuum plastic damage models formulated in the general framework identified in Part I of this [...]

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• A general framework for continuum damage models. I. Infinitesimal plastic damage models in stress space

  F. Armero, S. Oller

  Int. J. Solids and Structures (2000). Vol. 37 (48-50), pp. 7409-7436

  
  

  Abstract

  We identify in this paper a general framework for the development of continuum damage models in their fully coupled plastic damage form. The focus of this paper is directed to the general formulation of infinitesimal models defined by yield and damage surfaces in stress space. The main feature of the proposed formulation is the direct and independent consideration of the damage mechanisms (isotropic damage, cracking, etc.) degrading the stiffness of the material, thus allowing for a complete physical characterization of these effects. This modular structure is accomplished by a kinematic decomposition of the strains in an elastic, plastic and multiple damage parts, as belonging to each activated damage mechanism. An additive decomposition in the infinitesimal range of interest is considered. Based on this decomposition, the constitutive characterization alluded to above for each damage mechanism is carried out in a complete thermodynamically consistent framework. One of the virtues of the considered framework is the fact that it includes many of the diverse damage models existing in the literature as particular cases. In this way, the developments presented herein furnish a unified framework for the formulation of continuum damage models, including isotropic damage, compliance based formulations, effective stress anisotropic models, smeared crack models and the related formulations of cracking and damage based on strong discontinuities. Besides the clear physical significance added to these existing formulations, the proposed framework also defines a very convenient context for the efficient numerical integration of the resulting models. This aspect is explored in Part II of this work, as it is the application of the framework proposed herein to the numerical simulation of porous metals.

  Abstract
  We identify in this paper a general framework for the development of continuum damage models in their fully coupled plastic damage form. The focus of this paper is directed [...]

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