Fluage à long terme du bois fléchi en grandeur structurale

Résumé Cet article présente un modèle permettant de calculer le fluage de poutres fléchies de différentes sections soumises à des environnements variables en ce qui concerne l'humidité relative. Ce modèle est utilisé ici pour interpréter de nouveaux essais à long terme sur des poutres en bois massif et lamellé-collé qui ont été chargées pendant plus de 800 jours en conditions extérieures sous abri à Paris

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Summary Long term bending experiments on full size timber and glulam beams have been carried out under external and sheltered climatic conditions in Paris, and more than two years of test results on the creep behaviour of 75 beams are available. A model for predicting the long term mechanical behaviour of timber under variable humidity and load conditions, previously developed by the author, is described. The model is based on transient moisture transfer analysis linked with deformation analysis using a step-by-step computation scheme, and has been compared previously with experiments carried out under laboratory conditions of controlled variable climates by the author as well as with test results obtained from several other institutions. In general, the model has agreed satisfactorily with the experimental results. In the present paper the model is compared with experiments on full size beams carried out under the changeable conditions of a natural climate. The results show that, for low stress levels and corresponding loads, the model provides a reasonable prediction of the end creep levels after two years of loading.

     

Possible methods of creating a functional neoglottis and of constructing a functionally adequate neolarynx

The intention of the following report is to present a concept for constructing a substitute glottis (neoglottis). The concept evolved over the years on the basis of anatomic studies and in the course of plastic reconstructions on patients. It is based on pedicled grafting of a voluntarily controllable--and hence trainable--musculature that might be transplanted into the larynx attached to a neurovascular pedicle. A rima glottidis might be imitated that could be opened and closed from a neutral position. During sleep the neoglottis would be in a relaxed, spasm-free position. A sensitive and tactile stimulus response ought to be possible, as should variations of the state of tension of the substitute vocal muscles.     

A VISCOSITY CONSTITUTIVE EQUATION FOR PMMA—MMA SOLUTIONS

A viscosity constitutive equation has been developed to describe PMMA—MMA solutions over a wide range of concentration and temperature. The approach used here combines both free-volume and coil-coil interaction considerations. The final expression successfully correlates not only our experimental data on several solutions but also literature melt data. This equation provides an essential step in modeling tubular polymerization reactors, where the residence time distribution depends strongly on the velocity profile development in the tube, and thus the rheological properties of the reacting PMMA — MMA mixture at different conversion levels and temperatures.     

Adsorption des acides humiques sur charbon active en poudre. Influence du triphosphate de sodiumAdsorption of humic acids onto powdered activated carbon. The influence of sodium triphosphate

The use of activated carbon beds for the removal of natural humic and fulvic substances found in water supplies, has recently received considerable attention in water treatment operation (Lee et al., 1980; Le Cloirec et al., 1983). Moreover, the use of carbon adsorption for the reduction of haloform precursors (Anderson et al., 1981) and trihalomethanes produced by chlorination process, has contributed to a comprehensive investigation of adsorption characteristics of natural organic compounds (McCreary and Snoeyink, 1981). Many recent works showed the influence of adsorption system characteristics, such as pH, salt type, salt concentration and ionic heterogeneity in multicomponent adsorption systems, on the removal efficiency of humic and fulvic substances by activated carbon (McCreary and Snoeyink, 1980; Randtke and Jepsen, 1982; Weber et al., 1983). The purpose of this study is to examine the effect of a main component of domestic detergents, sodium triphosphate (STP), on the adsorptive capacities of powdered activated carbon (PAC) for commercially supplied humic acids, at different pH values in distilled water. Also, the effect of STP concentration and pH on the adsorption affinity of the PAC for humic acids, is discussed in relation with electrokinetic properties of carbon particles (zeta potential measurements).A first batch equilibrium study (Figs 1 and 2), showed an effective enhancement of adsorption capacity for humic acids as a function of STP concentration, in a non buffered media (pH of distilled water, close to 5.0). For example, visible absorption analysis of humic acids indicates an increase of 93% (500 mg l^?1 PAC) and 133% (1000 mg l^?1 PAC) in the carbon adsorption efficiency for a STP concentration from 0.2 to 1.0mM. A second batch equilibrium study (Figs 3 and 4) led to adsorption isotherms for humic acids in distilled water, as a function of STP concentration and initial pH value of the non buffered multicomponent system. Freundlich isotherms showed an increase in the adsorption capacity of the PAC for humic acids, with a decrease in pH and an increase in STP concentration. However, the adsorption capacity for humic acids is quite reduced at high pH values in presence of STP, in comparison with results obtained with distilled water.Electrokinetic measurements on PAC suspensions (Fig. 5) indicates that both humic acids and STP induce a negative variation of the zeta potential of carbon particles. In such a binary system, the zeta potential is a linear function of the pH; the negative surface charge of the carbon increasing with an elevation of pH (Fig. 6). Therefore, it appears that some adsorption of triphosphate polyanion from solution could occur, contributing then to the apparent negative surface charge of PAC particles.It has been previously showed that the type of anion in sodium salts, had little effect on the enhancement of adsorptive capacities of activated carbon for humic substances (Lafrance and Mazet, 1985), due to Na⁺ ions. However, adsorption of TP anions on the carbon surface may produce a source of repulsive charges, unfavourable to the co-adsorption of humic acids as the pH of the binary system reach more basic conditions. The influence of possible electrostatic interactions between adsorbates at the carbon surface, on the adsorption efficiency for humic acids, could then be studied by zeta potential measurements of PAC particles during the adsorption process.     

Novel approaches to the discovery of antimicrobial agents

OBJECTIVE: To examine the cost-effectiveness of prenatal carrier screening for cystic fibrosis. METHODS: A cost-benefit equation was developed that was based on the hypothesis that the cost of prenatal diagnosis required to diagnose and prevent one case of cystic fibrosis should be equal to or less than the lifetime cost generated from the birth of a neonate with cystic fibrosis. The formula was adjusted because a woman's positive or negative carrier status remains unchanged, thus eliminating the need for testing in subsequent pregnancies. The formula was manipulated to identify the optimal cost per screening test, as well as the net cost savings per prenatally diagnosed case of cystic fibrosis for various racial or ethnic groups. Sensitivity analyses included some key assumptions regarding the cost per screening test ($50-150), patient screening acceptance rates (25-100%), and therapeutic abortion rates (50-100%). RESULTS: Assuming therapeutic abortion rates of 50-100%, the net savings per prenatally diagnosed case of cystic fibrosis are $58,369-$382,369 among whites. Given the previously reported patient screening acceptance rates of 50-78%, the overall annual cost savings in the United States for whites are $161-251 million. However, the screening program was not found to be cost-effective for blacks, Asians, or Hispanics. CONCLUSION: Under most assumptions and sensitivity analyses, a prenatal cystic fibrosis-carrier screening program appears to be cost-effective.