There is a need of more advanced analysis for studying how the long-term behaviour of glued laminated timber structures is affected by creep and by cyclic variations in climate. A beam theory is presented able to simulate the overall hygro-mechanical and visco-elastic behaviour of (inhomogeneous) glulam structures. Two frame structures subjected to both mechanical and cyclic environmental loading are analysed to illustrate the advantages the model involved can provide. The results indicate clearly both the (discontinuous) inhomogeneity of the glulam products and the variable moisture-load action that occurs to have a significant effect on deformations, section forces and stress distributions within the frame structures that were studied.
Wood is a hygro-mechanical, non-isotropic and inhomogeneous material concerning both modulus of elasticity (MOE) and shrinkage properties. In stress calculations associated with ordinary timber design, these matters are often not dealt with properly. The main reason for this is that stress distributions in inhomogeneous glued laminated members (glulams) and in composite beams exposed to combined mechanical action and variable climate conditions are extremely difficult to predict by hand. Several experimental studies of Norway spruce have shown that the longitudinal modulus of elasticity and the longitudinal shrinkage coefficient vary considerably from pith to bark.
The question is how much these variations affect the stress distribution in wooden structures exposed to variable moisture climate. The paper presents a finite element implementation of a beam element with the aim of studying how wooden composites behave during both mechanical and environmental load action. The beam element is exposed to both axial and lateral deformation. The material model employed concerns the elastic, shrinkage, mechano-sorption and visco-elastic behaviour of the wood material. It is used here to simulate the behaviour of several simplysupported and continuous composite beams subjected to both mechanical and environmental loading to illustrate the advantages this can provide. The results indicate clearly both the inhomogeneity of the material and the variable moisture action occurring to have had a significant effect on the stress distribution within the cross-section of the products that were studied.
Long-term serviceability is an important aspect in the implication of wood as a construction material. In this study, a comprehensive experimental program aims to address all the required parameters in long-term constitutive models of wood available in the literature which was taken from inconsistent sources earlier. The experimental program considers the effect of viscoelastic and mechano-sorptive creep, shrinkage and swelling, thermal and moisture inelastic deformation, and deformation due to Young’s modulus changes. The tests include tensile loading of wood specimens invariable outdoor climatic conditions in different applied stress levels. Sustained tensile loads were applied in parallel to the grain direction to specimens of Splash Pine (Pinus elliottii), Pacific Teak (Tectona grandis), and Laminated Lumber Veneer (LVL) of Radiata Pine (Pinus radiata). Tests were conducted at three different stress levels simultaneously and environmental parameters viz. temperature and relative humidity were monitored continuously throughout the loading period. Complementary data for diffusion coefficient, shrinkage, and swelling were measured in three orthogonal directions. In addition, sorption-desorption isotherm of the sample in the range of 0-100% relative humidity is presented.