316 publications from this institution
Propoe-se neste trabalho uma formulacao inelastica de 2a ordem da Teoria Generalizada de Vigas (GBT), baseada na teoria de escoamento J2, e que constitui uma promissora alternativa ao MEF de casca. A sua aplicacao e ilustrada na analise de uma viga em I e de uma coluna em C-reforcado. Sao validados resultados da GBT por comparacao com os obtidos atraves do Abaqus (trajectorias de equilibrio, configuracoes deformadas e perfis de deslocamentos), tendo-se concluido que a natureza modal da GBT permite (i) obter resultados precisos com apenas 22% do numero de graus de liberdade usado no Abaqus, bem como (ii) compreender a mecânica comportamental do elemento em qualquer regime elasto-plastico (atraves da analise dos diagramas de participacao modal).
This two-part paper presents a numerical study about the fire resistance behaviour of pultruded GFRP profiles with square tubular cross-section subjected to bending and the ISO 834 time-temperature curve. The first paper deals with the development of numerical and analytical models and the discussion of relevant kinematic issues, including the evolution of beam deflection and position of neutral axis with the fire exposure time. The second part [1] reports an in-depth investigation on the side of static issues, which include the evolution of stress distributions and failure initiation with the fire exposure time. In the present paper, three–dimensional finite element models were developed to simulate fire resistance tests previously conducted by the authors on GFRP beams, in which different degradation curves were considered for compressive, tensile and shear moduli, based on experimental data. In these numerical simulations, both effects of varying the assignment of material properties (depending on the position of neutral axis) and of considering different thermal expansion coefficients were taken into account, and some conclusions were drawn on their influence on mid-span deflection evolutions. Since no failure criterion was implemented, both models were not able to reproduce the failure of the beams, but the overall tendency of the numerical results was consistent with the experimental data. Alongside the numerical study, analytical models based on Timoshenko beam theory were also developed and allowed obtaining accurate predictions of the mid-span deflection evolution of the GFRP beams; the analytical results were in close agreement with the numerical ones and also with the experimental data.
This work aims at contributing towards extending the domain of application of the available DSM, by making it capable of estimating the ultimate strength of cold-formed steel lipped channel columns affected by local/distortional/global interaction. The current DSM expressions, as well as those applicable to interactive buckling design, are first introduced and an extensive numerical (shell finite element) study involving fixed columns is reported − the column geometries are selected in order to have nearly coincident local, distortional and global buckling loads. The numerical ultimate strength values obtained are then used to assess the accuracy of the DSM expressions. As a preliminary recommendation, the current DSM expressions are adequate to estimate the collapse load of lipped channel columns affected by local/distortional/global interaction.
Despite being characterised by a reduction in compressive, tensile and flexural strengths when compared to standard concrete, rubberised concrete (RuC) is being used as a way to conserve natural resources and reduce the amount of tires entering landfills. An excellent solution to limit the RuC's compressive strength drop is to incorporate this type of concrete into steel tubes. However, up-to-date there is a lack of lateral confining pressure formulae for square and circular rubberised concrete-filled steel tubular (RuCFST) columns and rubberised concrete-filled double-skin steel tubular (RuCFDST) columns. Hence, the main aim of this technical note is to provide new design formulae based on lateral confining pressure and calibrated by means of test results available in the literature. These formulae are then used to suggest more accurate design strength models compared to those obtained from European, American and Australian specifications.
This paper presents the derivation of generalised beam theory (GBT)-based fully analytical formulae to provide distortional critical lengths and bifurcation stress resultant estimates in cold-formed steel C and Z-section members (i) subjected to uniform compression (columns), pure bending (beams) or a combination of both (beam–columns), (ii) with arbitrary sloping single-lip stiffeners and (iii) displaying four end support conditions. These formulae incorporate genuine folded-plate theory, a feature which is responsible for their generality and high accuracy. After a brief outline of the GBT fundamentals and linear stability analysis procedure, the main concepts and steps involved in the derivation of the distortional buckling formulae are described and discussed. Moreover, the paper also includes a few remarks concerning novel aspects related to the distortional buckling behaviour of Z-section beams and C-section beam–columns, which were unveiled by the GBT-based approach. Finally, note that, in a companion paper [Thin-Walled Struct., 2004 doi: 10.1016/j.tws.2004.05.002], the formulae derived here are validated and their application, accuracy and capabilities are illustrated. In particular, the GBT-based estimates are compared with exact results and, when possible, also with values yielded by the formulae developed by Lau and Hancock, Hancock, Schafer and Teng et al.
This paper presents a comparative study between Hashin damage criterion and the eXtended Finite Element Method (XFEM) applied to the failure of fiber reinforced polymers (FRP). A brief literature review on failure criteria to predict the failure of FRP is firstly presented. Then, finite element models of square plates with different layer configurations, containing a circular hole with distinct radii and subjected to monotonic uniaxial tension are described within the framework of ABAQUS package. The models are validated by comparison between the numerical results and those of a benchmark model. Finally, the influence of (i) stacking sequence, (ii) hole radii and (iii) failure criteria (Hashin and XFEM) on the load vs. elongation paths, stresses distributions and collapse configurations of the plates is shown and discussed and some conclusions are drawn.
No abstract is provided for this article.
A formulation of the Generalised Beam Theory (GBT) is presented for the 1st order inelastic analysis of thin-walled steel bars subjected to arbitrary loading and boundary conditions. Five illustrative examples are shown to validate the theory for cases involving global deformation only, namely uniform bending, non-uniform bending, combined bending and axial compression, and non-uniform torsion. Lastly, the results are validated against ABAQUS using beam and shell finite element models. The correlation is typically great concerning equilibrium paths, deformed configurations, and stress diagrams. In those cases where results do not compare so well, possible causes are pointed out.
This paper reports research work concerning the use of Generalised Beam Theory (GBT) to analyse the global buckling behaviour of plane and space thin-walled frames. Following a brief overview of the main concepts and procedures involved in the performance of a GBT buckling analysis, one presents in detail the formulation and numerical implementation of a GBT-based beam finite element that includes only the first four (rigid-body) deformation modes — namely, one describes (i) the kinematical models developed to simulate the warping transmission at frame joints connecting two or more non-aligned U- and I-section members, (ii) the procedures adopted to handle the effects stemming from the non-coincidence of the member centroidal and shear centre axes (cross-sections without double symmetry), and (iii) the definition of joint elements, which involves providing a relation between the connected member GBT degrees of freedom and the joint generalised displacements. Finally, one presents and discusses numerical results that make it possible to illustrate the application and show the capabilities of the above GBT-based finite-element formulation and implementation. For validation purposes, the GBT-based results (critical buckling loads and mode shapes) are also compared with values yielded by shell (mostly) and beam finite element analyses carried out in the code ANSYS.
No abstract is provided for this article.
The objective of this paper is to provide (i) the fundamental derivation details and (ii) a comparison between Generalised Beam Theory (GBT) and the constrained Finite Strip Method (cFSM), two alternative modal approaches to analyse the elastic buckling behaviour of unbranched thin-walled members.Thin-walled members may generally buckle in three families (or types) of modes: global, distortional and local (or local-plate) modes.The distinguishing feature of the GBT and cFSM methodologies to obtain buckling solutions is that they can formally separate these three types of buckling modes.An overall comparison of the two methods is provided, including practical aspects, such as the different notations, and theoretical points related to how the displacement fields are either constructed or decomposed into deformation modes akin to the above families.Specific derivation details are provided for both GBT and cFSM, along with numerical examples concerning the buckling behaviour of cold-formed steel lipped channel members under compression and bending.The numerical examples (i) show the power of both GBT and cFSM to separate general stability solutions into pure solutions related to the buckling mode types, (ii) illustrate the use of the identified deformation fields to examine the modal contributions to a buckling solution, and (iii) demonstrate that, in spite of their quite distinct developments, GBT and cFSM modal approaches provide essentially the same extended capabilities for examining and understanding thin-walled member stability.Moreover, considerable attention is also paid to the different handling of the membrane deformations by the two methods, which is responsible for the minor (but fully explainable) discrepancies existing between the results yielded by the two methods.
This work deals with the numerical evaluation of the structural response of simply supported (transversally loaded at mid-span) and cantilever (subjected to tip point loads) beams built from a commercial pultruded I-section GFRP profile. In particular, the paper addresses the beam (i) geometrically linear behaviour in service conditions, (ii) local and lateral-torsional buckling behaviour, and (iii) lateral-torsional post-buckling behaviour, including the effect of the load point of application location. The numerical results are obtained by means of (i) novel Generalised Beam Theory (GBT) beam finite element formulations, able to capture the influence of the load point of application, and (ii) shell finite element analyses carried out in the code Abaqus. These numerical results are compared with (i) the experimental values reported and discussed in the companion paper (Part 1) and (ii) values provided by analytical formulae available in the literature.
This paper presents a study on transverse tensile fracture properties of a pultruded glass fibre reinforced polymer (GFRP) profile, based on compact tension (CT) and wide compact tension (WCT) tests. Through these tests, both the critical energy release rate and the laminate cohesive law were determined. The test specimens presented a variety of initial notch lengths to assess the specimen geometry dependency. As these tests require the completion of some post-processing tasks to determine the fracture properties of the material, the following methods were applied: ASTM E399, J-integral, Compliance Calibration (CC) and Modified Compliance Calibration (MCC). Test results have shown that the CT tests were unsuccessful in reaching the stable propagation stage and provided overestimations of the critical energy release rate. On the other hand, the WCT tests were effective in achieving a stable propagation stage and results presented a good agreement across visually based methods, both in respect to critical energy release rate and laminate level cohesive laws. An overall value of 20 N/mm was determined for the transverse tensile critical energy release rate of the GFRP profile tested. Some variations were documented across initial notch lengths, but these were considered negligible in face of the intrinsic variability typically found in pultruded GFRP materials. Finally, modified compliance calibration results were found to be significantly lower in respect to other methods, with an average critical energy release rate of about 16 N/mm.
The paper presents the formulation of a second-order Generalised Beam Theory (GBT) developed to analyse the buckling behaviour of composite thin-walled members made of laminated plates and displaying arbitrary orthotropy. The derived second-order GBT equations are compared with the Vlassov-type ones obtained by Bauld and Tzeng and a few remarks are made concerning the cross-section mechanical properties appearing in the non-linear terms. Next, a few ambiguous aspects related to the structural meaning of the results yielded by member linear stability analyses are discussed and clarified. Finally, the application and capabilities of the formulated second-order GBT are illustrated by means of an investigation of the buckling behaviour of thin-walled orthotropic columns and beams, which takes into account both local and global deformation modes.
Carbyne chains are the thinnest structures found in nature. The synthesis of long and stable inside carbon nanotubes has recently drawn renewed attention to this linear one-dimensional carbon allotrope. Carbyne’s mechanical properties have been predicted to exceed that of carbon nanotubes and graphene, making it a very suitable structural component for many nanoscale applications. While carbyne’s mechanical behavior under tensile loading is superlative, this linear chain of bonded carbon atoms readily buckles due to thermal fluctuations, showing minimal strength under compressive loading. Here we present a detailed study on the enhancement of carbyne’s mechanical properties under compression by confinement in small diameter carbon nanotubes. First, we develop a new classical empirical forcefield based on ab-initio calculations. Molecular dynamics (MD) simulations are then carried, using this forcefield, to determine the mechanical properties of carbyne under tensile loading, namely to assess their dependence on chain length and temperature. The bending rigidity of carbyne and its persistence length are also calculated. After the validation of the new forcefield, MD simulations are employed to study the mechanical behavior of carbyne under compressive loading, when confined inside (5,5), (6,6) (7,7) and (8,8) CNTs. It is found that the mechanical behavior of confined carbyne chains depends on the confinement radius and the chain length, and can be described in three stages. For “tight” confinements, carbyne chains assume a rigid rod behavior, allowing the calculation of compressive mechanical properties and attesting an effective “bracing” effect. For “looser” confinements, a spring like behavior arises which can be modeled by Hooke’s law. In the third stage confined carbyne chains tend to buckle into a large local bend, losing the elastic flexibility that characterizes the previous stage.
Structural systems made of high-strength and/or high-ductility metals are usually also rather slender, which means that their structural behavior and ultimate strength are often governed by a combination of plasticity and instability effects. Currently, the rigorous numerical analysis of such systems can only be achieved by resorting to complex and computationally costly shell finite element simulations. This work aims at supplying to designers/researchers an efficient and structurally clarifying alternative to assess the geometrically and/or materially non-linear behavior (up to and beyond the ultimate load) of prismatic thin-walled members, such as those built from cold-formed steel. The proposed approach is based on Generalized Beam Theory (GBT) and is suitable for members exhibiting arbitrary deformation patterns (e.g., global, local, distortional, shear) and made of non-linear isotropic materials (e.g., carbon/stainless steel grades or aluminum alloys). The paper begins by providing a critical overview of the physically and geometrically non-linear GBT formulation recently developed and validated by the authors (Abambres et al. 2012a), which is followed by the presentation and thorough discussion of several illustrative numerical results concerning the structural responses of 4 members (beams and columns) made of distinct (linear, bi-linear or highly non-linear) materials. The GBT results consist of equilibrium paths, modal participation diagrams and amplitude functions, stress contours, displacement profiles and collapse mechanisms some of them are compared with values obtained from ABAQUS shell finite element analyses. It is shown that the GBT modal nature makes it possible (i) to acquire in-depth knowledge on the member behavioral mechanics at any given equilibrium state (elastic or elastic-plastic), as well as (ii) to provide evidence of the GBT computational efficiency, which is achieved by excluding from the analyses all the deformation modes that do not play any role in a particular member structural response.
