Effet des conditions aux limites sur le comportement des assemblages bois boulonnés

Résumé Les assemblages utilisés dans les structures bois sont très variés. Aux dispositions géométriques fort diverses se rajoute le fait que deux matériaux de comportements différents (bois et acier) sont généralement en présence. Dans cette étude, on considère le cas d'un assemblage élémentaire à deux plans de cisaillement avec un seul boulon. On s'intéresse particulièrement à des assemblages de grande épaisseur où l'influence de l'élancement du boulon (longueur/ diamètre) et des cnditions de rotation de ses têtes sur la réponse globale de l'assemblage est analysée. La courbe représentant cette réponse, de type force-glissement, intègre les caractéristiques de rigidité et de résistance de l'assemblage. On considère pour cela un seul diamètre de boulon égal à 8mm et trois élancements égaux à 4, 8 et 12, avec des pièces latérales en métal ou en bois. Le bois est représensé par une loi d'enfoncement du type exponentiel à trois paramètres, et le boulon est supposé à comportement élastoplastique. On constate que les conditions de rotation des têtes de boulon influencent aussi bien la résistance que la rigidité initiale de l'assemblage. Cependant, le comportement du boulon influence seulement la résistance de l'assemblage. En effet, la résistance est conditionée par les rotules plastiques qui se développent, le long du boulon, au cours du chargement. Par ailleurs, les assemblages à pièces latérales métalliques présentent une résistance et une rigidité initiale plus élevées que ceux ayant des pièces latérales en bois. La comparaison des résultats avec diverses études expérimentales et numériques montre la validité du modèle proposé.

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Bolted joints used in wood structures are extremely varied. In addition to presenting quite diverse geometric elements the two materials generally involved (wood and steel) exhibit most distinct behaviors. In the current study, the application of an elementary double-shear wood bolted joint in static short-term loading has been considered. The particular focus herein concerns thick joints, where the impact of the blot's slenderness (length/diameter ratio) as well as the rotating conditions of the bolt's heads on the overall response of the joint has been analysed. The curve representing this response, of the load-slip type, serves to integrate the characteristics of the joint's stiffness and strength. To carry this out, a constant bolt diameter of 8 mm, along with three slenderness values of 4, 8 and 12 with steel or wood side members, was used. The wood behavior in embedment is represented by means of an exponential three-parameter law, and the bolt is assumed to exhibit an elastic-plastic behavior. It has been observed that the rotation of the bolt heads exerts as much of an influence on the strength as the joint's initial stiffness. However, the bolt's behavior only influences the strength of the joint itself. In fact, the strength is actually conditioned by the plastic highes that appear on the bolt sections during loading. In addition, joints with steel side members are stronger and display a higher initial stiffness than those with wood side members. A comparison of results obtained from various experimentalk and numerical studies helps demonstrate the validity of the present model proposed.

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Note de la Rédation Alain Vergne est Membre Effectif de la RILEM.     

Fluorimetric methods for the measurement of intermediate metabolites (lactate,pyruvate, alanine, β-hydroxybutyrate,glycerol) using a COBAS FARA centrifugal analyser

Intermediate products of the metabolism of glucose, fat and amino-acid are important in the evaluation of such metabolic disorders as diabetes mellitus, liver disease and metabolic acidosis. In the present study, methods for the measurement of intermediate metabolites (lactate, pyruvate, alanine, β-hydroxybutyrate and glycerol) have been adapted to a fast centrifugal analyzer: the COBAS FARA. Correlation coeffcients rangedfrom 0.90 to 0.99, compared to established manual spectrophotometric methods. Within-run coeffcients of variation (CVs) ranged between 2.9 and 8.8% at low levels, between 1.5 and 5.7% at medium levels and between 1.2 and 5.6% at high levels. Between-run CVs were between 4.0 and 15.0% at low levels, between 1.7 and 7.0% at medium levels and between 1.3 and 2.7% at high levels. These fluorimetric assays for the determination of intermediate metabolites on COBAS FARA (Roche) have a good sensitivity and precision, are less costly than manual methods and can be used on a routine basis.     

New trends in antineoplastic chemotherapy

The author present some possible ways how to enhance the effectiveness of chemotherapy. In addition to modification of the tactics of chemotherapy it is above all a reduction of the effects of cytostatics and overcoming the resistance to chemotherapy, while the introduction of new drugs is rather rare during the last decade. As to non-traditional ways, a part can be played by more effective prophylaxis of secondaries, induction of cell differentiation, a combination of chemotherapy with biological response modifiers (interferons and cytokines) or treatment with cytokines alone (interleukin-2, tumour necrosis factor).     

Study of etch rate characteristics of SF6/He plasmas byresponse-surface methodology: effects of interelectrode spacing

The characteristics of SF₆/He plasmas which are used to etch Si₃N₄ have been examined with experimental design and modeled empirically by response-surface methodology using a Lam Research Autoetch 480 single-wafer system. The effects of variations of process gas flow rate (20-380 sccm), reactor pressure (300-900 mtorr). RF power (50-450 W at 13.56 MHz), and interelectrode spacing (8-25 mm) on the etch rates of LPCVD (low-pressure chemical vapor deposition) Si₃N₄, thermal SiO₂, and photoresist were examined at 22±2°C. Whereas the etch rate of photoresist increases with interelectrode spacing between 8 and 19 mm and then declines between 19 and 25 mm, the etch rate of Si₃N ₄ increases smoothly from 8 to 25 mm, while the etch rate of thermal SiO₂ shows no dependence on spacing between 8 and 25 mm. The etch rates of all three films decrease with increasing reactor pressure. Contour plots of the response surfaces for etch rate and etch uniformity of Si₃N₄ as a function of spacing and flow rate at constant RF power (250 W) display complex behavior at fixed reactor pressures. A satisfactory balance of etch rate and etch uniformity for Si₃N₄ is predicted at low reactor pressure (~300 mtorr), large electrode spacing (12-25 mm), and moderate process gas flow rates (20-250 sccm)