Creep behavior of structural timber under varying humidity conditions

The effects on wood of simultaneous mechanical and moisture loading are studied. A constitutive model is given for the case of uniaxial stress in the longitudinal direction. The model has been quantified on the basis of test results found in literature. The validity of the model was checked independently against other tests and a number of practical load situations were studied. Finally the serviceablity requirements for design are discussed. The calculations show that the model is capable of describing the response of wood with reasonable accuracy. Some differences in behavior are found to depend on moisture cycling. The results of the simulations show that there is a significant influence of strain on the shrinkage and swelling response. An influence of beam dimensions has been found. The relative deflection for a small beam is larger than the deflection for a larger beam. 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 small in comparison with the case when the characteristic load is assumed to be constant over time.