The effects on wood of simultaneous mechanical and moisture loading ar
e studied. A constitutive model is given for the case of uniaxial stre
ss 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 practica
l load situations were studied. Finally the serviceablity requirements
for design are discussed. The calculations show that the model is cap
able of describing the response of wood with reasonable accuracy. Some
differences in behavior arc 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 i
s 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 sno
w-load variation shows that the long-term deflections are small in com
parison with the case when the characteristic load is assumed to be co
nstant over time.