Escudero-Torres, C.; Oller, S.; Martinez, X.; Barbat, A. H. Journal of engineering mechanics Vol. 143, num. 9, p. 04017080-1-04017080-19 DOI: 10.1061/(ASCE)EM.1943-7889.0001275 Data de publicació: 2017-09 Article en revista
The construction of confined masonry buildings has become a good choice to meet the housing needs of low-income families inbig cities. Despite this, current building codes for such construction allow the use of highly simplified analysis techniques that have hardlychanged in the last 40 years. This paper is based on numerical simulation and discusses the need to combine and improve existing techniquesin finite-element method (FEM) analysis for composite materials, to assess the overall structural behavior of reinforced concrete structureswith masonry in-fills, and consequently to support the derivation of rational rules for analysis and design. Through the use of a simpleyet powerful shell finite element (FE), state-of-the-art theories of mixtures to analyze composite materials, a computational tool to generatethe volume fraction of composites, and the Mexican building code, this paper attempts to be a guide to numerical reproduction ofthe overall behavior of confined masonry structures.
Rahimi-Aghdam, S.; P. Bažant, Z.; Caner, F.C. Journal of engineering mechanics Vol. 142, num. 10, p. 04016109-1-04016109-10 DOI: 10.1061/(ASCE)EM.1943-7889.0001185 Data de publicació: 2016-10-28 Article en revista
The theory for the material and structural damage due to the alkali-silica reaction (ASR) in concrete is calibrated and validated by
finite element fitting of the main test results from the literature. The fracture mechanics aspects, and particularly the localization limiter, are
handled by the crack band model. It is shown that the theory can capture the following features quite well: (1) the effects of various loading
conditions and stress states on the ASR-induced expansion and its direction; (2) degradation of the mechanical properties of concrete, par-
ticularly its tensile and compressive strength, and elastic modulus; (3) the effect of temperature on ASR-induced expansion; and (4) the effect
of drying on the ASR, with or without simultaneous temperature effect. The finite element simulations use microplane model M7. The aging
creep, embedded in M7, is found to mitigate the predicted ASR damage significantly. The crack band model is used to handle quasi-brittle
fracture mechanics and serve as the localization limiter. The moisture diffusivity, both the global one for external drying and the local one
for mortar near the aggregate, decreases by one to two orders of magnitude as the pore humidity drops. The fits of each experimenter’s
data use the same material parameters. Close fits are achieved and the model appears ready for predicting the ASR effects in large structures.
Paredes, J.; Oller, S.; Barbat, A. H. Journal of engineering mechanics Vol. 142, num. 10, p. 04016072-1-04016072-16 DOI: 10.1061/(ASCE)EM.1943-7889.0001130 Data de publicació: 2016-10 Article en revista
A new damage model, based on continuum damage mechanics and simulating the opening, closing, and reopening of cracks in concrete using only one surface of discontinuity, is proposed in this article. The model complies with the thermodynamics principles of nonreversible, isothermal, and adiabatic processes. Two scalar internal variables have been defined: a tensile damage variable d+d+ and a compressive damage variable d-d-; the threshold of damage is controlled by only one surface of discontinuity and a new parameter controlling the damage variable that should be activated. This new parameter represents the ratio of tensile stress to compressive stress in the damaged material. The continuity of response under complex loads, which is one of the aims of this work, is ensured. An adequate response under different types of loads leads to the conclusion that the proposed model provides a powerful tool to numerically analyze reinforced concrete structures. Validation and illustrative examples are included in the article.
The microplane material model for concrete, formulated mathematically in the preceding Part I, is here calibrated by material test data from all the typical laboratory tests taken from the literature. Then the model is verified by finite elements simulations of data for some characteristic tests with highly nonuniform strain fields. The scaling properties of model M7 are determined. With the volumetric stress effect taken from the previous load step, the M7 numerical algorithm is explicit, delivering in each load step the stress tensor from the strain tensor, with no iterative loop. This makes the model robust and suitable for large-scale finite element computations. There are 5 free, easily adjustable material parameters, which make it possible to match the given compressive strength, the corresponding strain, the given hydrostatic compression curve, and certain triaxial aspects. Besides, there are many fixed, hard-to-adjust, parameters, which can be taken the same for all concretes. The optimum values of material parameters are determined by fitting a particularly broad range of test results, including the important tests of compression-tension load cycles, mixed-mode fracture, tension-shear failure of double-edge-notched specimens, and vertex effect when axial compression is followed by torsion. Because of the lack of information on the material characteristic length or fracture energy, which can be obtained only by size effect tests on the same concrete, and on the precise boundary conditions and precise gauge locations, the finite element fitting of the present test data can hardly be expected to give better results than single-point simulations of specimens with approximately homogeneous strain states within the gauge length. Nevertheless, tensile test data with severe localization are delocalized on the basis of assumed material length. Model M7 is shown to fit a considerably broader range of test data than the preceding models M1–M6.
Linearized continuum models of a suspended span with unloaded backstays and of a symmetric three-span suspension bridge are used to study the effects of the flexibility of the hangers on the vertical vibrations of suspension bridges. The models include elastic parabolic cables, flexible distributed hangers with variable length, and a stiffening girder represented by an elastic beam. It is shown that the free vibrations of a suspended span with unloaded backstays are controlled by five dimensionless parameters, while six dimensionless
parameters control the response of a symmetric three-span suspension bridge. The results idicate that the flexibility of the hangers has a significant effect on the natural frequencies of the higher modes only when the relative stiffness of the girder is very high. The effects of hanger flexibility on the response of a suspension bridge to localized impulsive loads are also found to be small. These findings confirm the traditional, albeit previously untested, assumption of inextensible hangers. Finally, the threshold amplitudes of free vibrations that
would result in the incipient slackening of the hangers are determined.
Andreu, A.; Gil, L.; Roca, P. Journal of engineering mechanics Vol. 133, num. 4, p. 473-480 DOI: 10.1061/(ASCE)0733-9399(2007)133:4(473)#sthash.qmTDHedR.dpuf Data de publicació: 2007-04 Article en revista
This paper presents a computational approach for the assessment of masonry structures based on the well known analogy between the equilibrium of arches and that of hanging strings or cables working in tension. According to the analogy, the hanging strings model the inverted shape of the equilibrium lines (or thrust lines) describing the locus of the equilibrium forces acting across the sections of the arch. The approach proposed combines two developments. First, a new cable element is proposed to numerically model the strings used to describe the equilibrium lines. The formulation proposed, obtained as a modification of the conventional equations for inextensible cables, is based on an exact analytical derivation. Compared to other available numerical approaches, it has the advantage of ensuring the exact equilibrium of the cable net after deformation. Second, complementary algorithms are proposed for the assessment of the strength of masonry structures by the application of the limit theorems of plasticity (static approach). These algorithms are intended to find optimized solutions complying with the so-called safe (or lower-bound) and uniqueness theorems. Two examples of application are described to illustrate the accuracy of the method and its ability to handle masonry structural systems.
The behavior of two stress update algorithms for shear-free large deformation paths is analyzed. The first algorithm has a truncation error of order 1. The second algorithm has a truncation error of order 2. As a consequence, the global performance of the second algorithm is clearly superior. However, for the particular case of shear-free deformation paths, the first algorithm correctly predicts null shear stresses, while the second one does not. This behavior was reported in a previous paper for an extension-rotation test. In this note a general shear-free deformation path is considered in full detail.
Two algorithms for the stress update (i.e., time integration of the constitutive equation) in large-strain solid mechanics are discussed, with particular emphasis on two issues: (1) The incremental objectivity;
and (2) the implementation aspects. It is shown that both algorithms are incrementally objective (i.e., they treat rigid rotations properly) and that they can be employed to add large-strain capabilities to a small-strain finite element (FE) code in a simple way. A set of benchmark tests, consisting of simple large deformation paths, have
been used to test and compare the two algorithms, both for elastic and plastic analyses. These tests evidence different time-integration accuracy for each algorithm. However, it is also shown that the algorithm that is less accurate in general gives exact results for shear-free deformation paths.
A simple but general model for normal/shear cracking in quasi-brittle materials is presented. It is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A crack surface in stress space determines crack initiation under pure tension, shear-tension, or shear-compression loading. Two independent fracture energy parameters are used: the classical Mode I fracture energy GIfGfI, and the asymptotic Mode II fracture energy GIIafGfIIa under very high shear-compression and no dilatancy. The cracking model proposed can be implemented in two ways: directly as the constitutive law of an interface element in the context of discrete crack analysis, or as the law of a generic cracking plane in a multicrack formulation in the context of smeared crack analysis. In this paper, the first approach is presented and examples are given of numerical constitutive testing and verification with experimental data.
A simple but general model for normal/shear cracking in quasi-brittle materials is presented. It is defined in terms of the normal and shear stresses on the average plane of the crack and the corresponding normal and shear relative displacements. A crack surface in stress space determines crack initiation under pure tension, shear-tension, or shear-compression loading. Two independent fracture energy parameters are used: the classical Mode I fracture energy GIf, and the asymptotic Mode II fracture energy GIIaf under very high shear-compression and no dilatancy. The cracking model proposed can be implemented in two ways: directly as the constitutive law of an interface element in the context of discrete crack analysis, or as the law of a generic cracking plane in a multicrack formulation in the context of smeared crack analysis. In this paper, the first approach is presented and examples are given of numerical constitutive testing and verification with experimental data.
Barbat, A. H.; Rodellar, J.; P. Ryan, Eugene; Molinares, N. Journal of engineering mechanics Vol. 121, num. 6, p. 676-684 DOI: 10.1061/(ASCE)0733-9399(1995)121:6(676) Data de publicació: 1995-06 Article en revista
A hybrid seismic control system for building structures is considered, which combines a class of passive nonlinear base isolator with an active control system. The active control forces are applied to the structural base with the objective of reducing its absolute displacements. An adaptive control law is formulated to compute the control forces to assure a form of stable behavior of the system under seismic excitation and in the presence of uncertainties in the characteristics of the building and the base isolator. Numerical simulations are performed to assess the effectiveness of the hybrid control system. The global behavior of the structure-base-isolator system is such that the absolute base displacement is significantly reduced, the price paid being a slight increase of the response of the structure.
Lopez Almansa, F.; Andrade, R.; Rodellar, J.; Reinhorn, A. Journal of engineering mechanics Vol. 120, num. 8, p. 1743-1760 DOI: 10.1061/(ASCE)0733-9399(1994)120:8(1743) Data de publicació: 1994-08 Article en revista
In this and a companion paper, a methodology for implementation of modal predictive control of structures is described. Predictive control is formulated, within an independent modal space control (IMSC) approach, for the control of each individual mode. In this paper, the ability of the predictive control strategy to provide stable and efficient control actions for every individual mode is numerically assessed. Two kinds of analyses have been carried out. The first analysis is a “plain assessment” that assumes all the elements in the control loop operate under ideal conditions and that there are no discrepancies between the parameters of the control loop and their values considered in the control algorithm. The second analysis is a “robustness assessment” without such assumptions. As a conclusion, criteria are given to choose the values of the parameters of the predictive control algorithm according to the modal characteristics of the structure and other parameters involved in the control loop. The results are used in the companion paper for implementation on multi-degrees-of-freedom systems.
Lopez Almansa, F.; Andrade, R.; Rodellar, J.; Reinhorn, A. Journal of engineering mechanics Vol. 120, num. 8, p. 1761-1772 DOI: 10.1061/(ASCE)0733-9399(1994)120:8(1761) Data de publicació: 1994-08 Article en revista
This paper is continuation of a companion paper in which a numerical assessment about the efficiency and the robustness of predictive control of structures was performed on single-degree-of-freedom systems. In this paper a formulation for predictive control of multi-degrees-of-freedom systems using a reduced number of sensors and actuators is presented. Control law is formulated, into an independent modal space control approach, for controlling individually some modes and the values of the parameters defining the control algorithm are chosen according to the recommendations derived in the companion paper. The efficiency and the stability of the control action are numerically assessed on a full-size six-story experimental building placed in Tokyo and subject to horizontal unidimensional seismic excitation. Three control cases have been considered using one, two, and three sensors and actuators, respectively. Presented results allow one to conclude that the proposed strategy seems to be able to generate stable and efficient control actions for active aseismic protection of building structures.
In materials with a strain-softening characteristic behavior, classical continuum mechanics favors uncontrolled strain localization in numerical analyses. Several methods have been proposed to regularize the problem. Two such localization limiters developed to overcome spurious instabilities in computational failure analysis are examined and compared. A disturbance analysis, on both models, around an initially homogeneous state of strain is performed to obtain the closed-form solution of propagating wave velocities as well as the velocities at which the energy travels. It also shows that in spite of forcing the same stress-strain response on both models, the wave equation does not yield similar results. Both propagations of waves are dispersive, but the internal length of each model is different when equivalent behavior is desired. In fact, the previously suggested derivations of gradient models from nonlocal integral models were not completely rigorous. The localization modes and the influence of the internal length should be different in each limiter. The perturbation analysis is pursued in the discrete space where computations are done, and the closed form solutions for the dispersion equations are also obtained. The finite-element discretization introduces an added dispersion: the usual dispersion introduced by elliptic operators and another associated to the regularization technique. Therefore, the influence of the discretization on the localization limiters can be evaluated. The element size must be in the order of, or smaller than, the internal length of the models in order to obtain sufficient accuracy on the phase velocities of the propagating waves in transient analysis.
Rodellar, J.; Barbat, A. H.; Martín-Sánchez, M. Journal of engineering mechanics Vol. 113, num. 6, p. 797-812 DOI: 10.1061/(ASCE)0733-9399(1987)113:6(797) Data de publicació: 1987-06 Article en revista
Different continuous-time approaches have been proposed in recent years to formulate active control algorithms to reduce the response of civil engineering structures under dynamic excitations. In this paper, a general formulation of a new discrete-time control methodology is presented and applied to structural control. This methodology, based on the concept of predictive control, is discussed and compared to the optimal control methodology by means of numerical examples.