Mechanical Behaviour of Wood Exposed to Humidity Variations

University dissertation from Structural Engineering

Abstract: The effects on wood of simultaneous mechanical and moisture loading are studied. In order to clarify the mechano-sorptive behaviour of wood, a review of different phenomena presented in the literature is included. Compressive tests have been performed on spruce. The tests have been performed with both constant load and varying load. The humidity has been varied during the tests. The properties of the material used have been investigated both before and after the main testing was performed. Based on the review of mechano-sorptive phenomena and the test results presented in this study, a constitutive model is developed. The model was quantified on the basis of test results presented both in this thesis and in the literature. The validity of the model was checked independently against other tests. Finally a number of practical load situations were studied. In these simulations both humidity and load were varied. The calculations show that the model is capable of describing the response of wood with reasonable accuracy. The simulations indicate that the response of small test specimens is more difficult to describe than that of larger beams. Some differences in behaviour are found to depend on loading mode and nature of moisture cycling. Very large and fast moisture cycles seem to give larger mechano©sorption than smaller variations. The results of the simulations show that there is a significant influence of strain on the shrinkage and swelling response. Studies of the effect of varying load show that a temporarily applied load has little effect on the long term deflection of structural timber. A simulation of snow load variation shows that the long term deflections are rather small in comparison with the case when the characteristic load is assumed to be constant over time.

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