A direct stiffness analysis of a composite beam with partial interaction

The use of the conventional semi-analytical stiffness method in finite element analysis, in which interpolation polynomials are used to develop the stiffness relationships, leads to problems of curvature locking when beam-type elements are developed for composite members with partial interaction between the materials of which it is comprised. The curvature locking phenomenon that occurs for composite steel–concrete members is quite well reported, and the general approach to minimizing the undesirable ramifications of curvature locking has been to use higher-order polynomials with increasing numbers of internal nodes. This paper presents an alternate formulation based on a direct stiffness approach rather than starting from pre-defined interpolation polynomials, and which does not possess the undesirable locking characteristics. The formulation is based on a more general approach for a bi-material composite flexural member, whose constituent materials are joined by elastic shear connection so as to provide partial interaction. The stiffness relationships are derived, and these are applied to a simply supported and a continuous steel–concrete composite beam to demonstrate the efficacy of the method, and in particular its ability to model accurately both very flexible and very stiff shear connection that causes difficulties when implemented in competitive semi-analytical algorithms. Copyright © 2004 John Wiley & Sons, Ltd.     

Beam tests of composite steel-concrete members: A three-dimensional finite element model

Modern construction makes frequent use of composite steel-concrete beams for bridge and building applications. This paper describes a three-dimensional finite element model in which all components forming the composite member are modelled by means of solid elements. The proposed approach is developed using the commercial software Abaqus and is able to model the composite response without requiring information from push-out tests commonly performed to define the constitutive relationship for the shear connectors. All materials are assumed to behave in a nonlinear fashion. Contact between the elements is simulated using surface-to-surface and embedment techniques. The adequacy and accuracy of the proposed modelling approach are validated against experimental results available in the literature on simply-supported and continuous beam tests with both solid and composite slabs.     

Performance and design of shear connectors in composite beams with parallel profiled sheeting at elevated temperatures

This paper describes an extensive experimental and numerical study conducted to evaluate the thermo-structural response of shear connectors embedded in composite slabs with steel sheeting parallel to the steel beam, with particular focus on opentrapezoidal profiles. For this purpose, eight push-out tests were carried out at different levels of temperature. A threedimensional finite element model was developed in Abaqus and its accuracy was validated against the experimental measurements collected as part of this study. The model was then used to perform a parametric study to gain insight into the structural response at different temperatures. The experimental and numerical results were then used to evaluate the accuracy of available European guidelines for predicting the capacity of shear connectors at elevated temperatures (when embedded in composite slabs with the profiled sheeting parallel to the steel beam). Finally, a new design equation was proposed to calculate the degradation factor defining the resistance of shear connectors for different levels of temperature.     

Detailed kinetic modeling of the thermal degradation of lignins

The aim of this kinetic work is to provide a better understanding of the pyrolysis of lignin and biomasses not only in terms of devolatilazation rate but also of the volatile species released. The complexity of both lignin structure and its degradation mechanism meant that a lumping approach suitable for handling the huge amount of initial, intermediate and final products had to be used. Despite these simplifications, the proposed semi-detailed kinetic scheme involves about 100 molecular and radical species in 500 elementary and lumped reactions. It has already been proved that this lignin devolatilization model correctly predicts the degradation rates and the detail of the released products. This work constitutes an initial yet significant step towards deriving a complete kinetic scheme of biomass devolatilization.     

Alterations of the glucose metabolism in a triose phosphate isomerase-negative Saccharomyces cerevisiae mutant

The absence of triose phosphate isomerase activity causes an accumulation of only one of the two trioses, dihydroxyacetone phosphate, and this produces a shift in the final product of glucose catabolism from ethanol to glycerol (Compagno et al., 1996). Alterations of glucose metabolism imposed by the deletion of the TPI1 gene in Saccharomyces cerevisiae were studied in batch and continuous cultures. The Deltatpi1 null mutant was unable to grow on glucose as the sole carbon source. The addition of ethanol or acetate in media containing glucose, but also raffinose or galactose, relieved this effect in batch cultivation, suggesting that the Crabtree effect is not the primary cause for the mutant's impaired growth on glucose. The addition of an energy source like formic acid restored glucose utilization, suggesting that a NADH/energy shortage in the Deltatpi1 mutant could be a cause of the impaired growth on glucose. The amount of glycerol production in the Deltatpi1 mutant could represent a good indicator of the fraction of carbon source channelled through glycolysis. Data obtained in continuous cultures on mixed substrates indicated that different contributions of glycolysis and gluconeogenesis, as well as of the HMP pathway, to glucose utilization by the Deltatpi1 mutant may occur in relation to the fraction of ethanol present in the media.     

An experimental and kinetic modeling study of cyclopentadiene pyrolysis: First growth of polycyclic aromatic hydrocarbons

The importance of 1,3-cyclopentadiene (CPD) and cyclopentadienyl (CPDyl) moieties in the growth of polycyclic aromatic hydrocarbons (PAHs) was studied using new experimental data and ab initio calculations. The experimental investigation was performed in a tubular continuous flow pyrolysis reactor under both high (24molN₂/molCPD)$(24{\text{mol}}_{{\text{N}}_2}/{\text{mol}}_{\text{CPD}})$ and low (5molN₂/molCPD)$(5{\text{mol}}_{{\text{N}}_2}/{\text{mol}}_{\text{CPD}})$ nitrogen dilutions, covering a temperature range of 873–1123 K, at a fixed pressure of 1.7 bara. At the most severe conditions up to 84% of CPD is converted, and the amount of PAHs is more than 65 wt%. Major products observed during CPD pyrolysis were benzene, indene, methyl-indenes and naphthalene, in line with previous studies. On-line GC × GC-FID/(TOF-MS) also allowed to quantify minor species (methane, toluene, styrene, phenanthrene, anthracene, etc.), never reported before at this level of accuracy. The new experimental data have been used to further analyze the role of the successive interactions of CPD, indene, and naphthalene as well as the recombination and addition reactions of their resonantly stabilized radicals and refine their kinetics. The results of the modeling study are in good agreement with existing and new experimental observations.     

不同优化算法在工程实际中的主要特点及优势

算法取代工程师的人工调试工作一直是一个比较热门的结构设计研究方向,但未大量应用于工程实际。主要介绍了实际工程中,利用结构设计行业内出现的新变量,如更高效的优化算法、编程能力在新一代结构设计从业人员中的普及、国内一线设计软件逐步开放程序控制接口等,针对不同工程实际问题采用不同算法进行设计的实际案例;并介绍了采用算法设计的优势,如在超高层设计中采用了响应面算法优化型钢用量,在大跨度天幕设计中采用模拟退火算法进行曲线找形,在桩基设计中通过对桩布置进行参数化快速设计更新。通过这些案例笔者认为结构设计行业推广算法设计的条件已基本具备。     

A wide range kinetic modeling study of the pyrolysis and combustion of naphthenes

The aim of this paper is to analyze and discuss the kinetics of the pyrolysis and combustion of naphthenes. The primary propagation reactions of cyclohexane and methylcyclohexane are presented to extend the validity of a semi-detailed kinetic model for the pyrolysis and oxidation of hydrocarbons. Naphthenes are relevant species as reference components in liquid fuels and surrogate blends. A lumped approach is used to reduce the complexity of the overall scheme in terms of species and reactions. Particular attention is devoted to the role of the isomerization or internal abstraction of H atoms in competition with β−decomposition ones. Primary oxidation and decomposition reactions of the cyclohexyl radical are discussed to explain and justify this lumping procedure. The modeling predictions are compared with different sets of measurements. The validation of the low temperature oxidation mechanism of cyclohexane is based on the ignition delay times obtained both in the rapid compression machine at Lille and in closed vessels. Jet-stirred reactors at different pressures and stoichiometric ratios also confirm the reliability of the overall mechanism of oxidation. The comparisons between the model’s predictions and the measurements relating to the pyrolysis and oxidation of methylcyclohexane in the Princeton turbulent flow reactor further support this extension of the kinetic scheme to naphthenes. Finally, the agreement with the oxidation experiments using mixtures of toluene + methylcyclohexane is a primary and simple example of the model’s ability to deal with the combustion of real fuels or surrogate blends.     

Time analysis of structural concrete elements using the equivalent displacement method

This paper presents a modelling technique referred to as the equivalent displacement method (EDM) which describes the behaviour in time of structural concrete elements, such as reinforced concrete beams and composite beams with full shear interaction, accounting for time effects, such as creep, of the concrete component. The time-dependent behaviour of the concrete is modelled using the algebraic representations, such as the age-adjusted effective modulus method (AEMM), while the steel joist and reinforcement are assumed to behave in a linear-elastic fashion. The main advantage of the EDM method is that it requires only one analysis to obtain the deformation state of the structural system at one step in time based on the AEMM methods, instead of the two required by available modelling techniques (i. e. one performed at timet ₀ and one performed one step in timet). The EDM method is then applied to the analysis of structural concrete elements using the stiffness analysis and the results obtained based on this modelling technique are validated against other modelling methods. The advantages of using the EDM method in design applications are also illustrated.

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Résumé Cet article présente une méthode appelée méthode de déplacement équivalent (EDM), qui décrit le comportement temporel des éléments de béton, par example les poutres en béton armé et les poutres mixtes à interaction complète, tenant compte des effets temporaux, tel que le fluage du béton. Le comportement temporel du béton est modelé par représentation algébrique, tel que la méthode age ajusté du module effectif (AEMM), tandis que la poutre d'acier et les renforts sont supposés linéaire et élastique. L'avantage majeur de la méthode EDM est qu'elle n'exige qu'une seule analyse pour obtenir l'état de déformation du système à un point temporel (méthode EDM), au lieu des deux points exigés par les méthodes courantes, en temps t₀ et l'autre à t. La méthode EDM est ensuite appliquée à l'analyse des éléments structurels en béton, utilisant l'analyse de raideur, et les résultats obtenus par cette méthode sont vérifiés par d'autres méthodes. Les avantages de l'utilisation de la méthode EDM dans les applications du dessein sont aussi illustrés.

     

Control by nutrients of growth and cell cycle progression in budding yeast, analyzed by double-tag flow cytometry

To gain insight on the interrelationships of the cellular environment, the properties of growth, and cell cycle progression, we analyzed the dynamic reactions of individual Saccharomyces cerevisiae cells to changes and manipulations of their surroundings. We used a new flow cytometric approach which allows, in asynchronous growing S. cerevisiae populations, tagging of both the cell age and the cell protein content of cells belonging to the different cell cycle set points. Since the cell protein content is a good estimation of the cell size, it is possible to follow the kinetics of the cell size increase during cell cycle progression. The analysis of the findings obtained indicates that both during a nutritional shift-up (from ethanol to glucose) and following the addition of cyclic AMP (cAMP), two important delays are induced. The preexisting cells that at the moment of the nutritional shift-up were cycling before the Start phase delay their entrance into S phase, while cells that were cycling after Start are delayed in their exit from the cycle. The combined effects of the two delays allow the cellular population that preexisted the shift-up to quickly adjust to the new growth condition. The effects of a nutritional shift-down were also determined.