RESUMEThe paper reviews some advances in the understanding and modelling of the effects of moisture conditions and moisture changes on the long-term performance of structural timber. Particularly it describes recent progress in constitutive modelling of the moisture change dependent creep, the mechano-sorptive effect, and in the understanding of the strength reduction of timber structures due to simultaneous load and moisture changes, the 'mechano-sorptive duration-of-load effect'. The importance to analyse the effect of natural humidity changes on stress and strain state is emphasised.
Cet article passe en revue les progr& accomplis clans la compr(hension et la mod(lisation de l'impact des conditions d'humidit( et de leurs modifications sur les peoCormances h long terme des constructions en bois. Sont plus sp(cifiquement analys&s les rdcentes avanc&s en matibre de moddlisation constitutive du fluage lid aux changements d'humiditd, les effets m&ano-sorptifs, et dans la comprdhension de la diminution de la r&istance des structures en bois en raison des modifications simultan&s du chargement et de l'humiditd (l'effet 'mdcano-sorptif de la dur& de chargement'). Ce papier met l'accent sur l'importance de l'analyse des effets des modifications naturelles de l'humiditd sur le diagramme contrainte-ddformation.i!iiiiiiiiiiiiii
A B S T R A C T R !~ S U M I ~The paper describes a method for computational assessment of the long-term performance of timber beams in variable climates. The method is based on a cross-section analysis, in which the moisture content and longitudinal stress-strain distributions are calculated as function of time and ambient conditions assuming asymmetrical mechanical behaviour in the tension and compression sides of the beam. The method is implemented as a non-linear FE-program, which combines the moisture transport and structural analysis, which is based on a non-linear model of longitudinal creep in wood. The calculated stress-strain state is monitored against a failure criterion, which is based on a definition of a local damage variable and on its integration over the volume in the fashion of the Weibull theory of weakest link to give a global damage parameter. Computational results include long-term deformation (deflection) and time-to-failure (long-term capacity) predictions. Examples of time-tofailure calculation are given.
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