Quantitative Analysis And Design For Bending Modulus Of Elasticity Of Wood Beams With Gradient Moisture Content

Quantitative analysis of wood mechanical properties under gradient moisture content state (GMCS) is the base and prerequisite for structural design. The quantitative analysis allows wood structure designers obtain realistic wood mechanical properties under service environments, and it also provides a reliable way to determine the MC reduced coefficients (MCRC) of wood mechanical properties. However, this method is much complex than the same analysis under EMC conditions. Therefore, the "mechanical property-EMC regress analyzing method" has been used in previous studies. But the method cannot precisely reflect the constructive relationship between wood mechanical properties and gradient moisture content. Because of this problem, the Chinese codes and standards for timber structures currently still relies on experiences and massive experimentations.In order to solve the above problems, this research employed Pinus yunnanensis (YNP), Popuhus euramevicana wood (P) and laminated veneer lumber (LVL) of poplar wood bonded with phenol formaldehyde resin as studying materials to complete the following studies. Firstly, the basic physical-mechanical properties of the three materials and their absorption/desorption behavior under different environments were investigated. Secondly, the absorption/desorption models of moisture transfer in the three wood materials were established. In the third step, theoretical models were established to calculate bending moduli of elasticity (Eb) of the three wood materials with gradient moisture content. In the fourth step, the MCRCs of Eb for YNP and LVL under Kunming’s climate condition were presented. Finally, based on the two models established above the design criteria and procedures were demonstrated using examples of wood I-beams and wood rectangular beams.Innovative results of this research are summarized as follows:(1) Sample dimensions, Eb, modulus of elasticity parallel to grain (Et) and shear modulus parallel to grain(G) of YNP, P and LVL were all changed with changing EMC. The changing rates of Eb, Et and G during moisture desorption were all smaller than during moisture absorption, but the modulus of elasticity perpendicular to grain (E) and tensile Poisson ratio were only slightly affected by moisture content.(2) Eb in GMCS during moisture absorption process was smaller than that in EMCS, although they had the same nominal moisture content (NMC), but these differences gradually decreased when the NMC was reached.(3) Rate of moisture absorption/desorption of solid wood was negatively correlated with wood density. The adhesive layers in LVL significantly decreased the moisture sorption rates, but it would not completely prevent moisture migration in the thickness direction.(4) The "diffusion+variable phase" model used in this research could well simulate the process of moisture migration in solid wood and in layered composite materials below fiber saturation point (FSP) with higher accuracy than using the "single diffusion model". But it should be noted that the environment boundary We should be at the actual EMC under the given environment. The diffusivities of bonded-water and vapor increased with increasing moisture content, but they could be regarded as a constant in the special MC range.(5) Very high relative humidity environments could induce larger MC gradient between layers and shorter migration time during moisture absorption, and during desorption the reverse process occurred.(6) The Eb calculation model based on the laminated plates theory could well predict Eb of solid wood and layered composite materials under GMCS. It was also found that the thickness swelling of solid wood and LVL affected calculation result slightly, but the swelling of cross-laminated layered materials significantly affected calculation results.(7) At relative humidity up to65%, the static mechanical indexes of wood beams designed and manufactured in this research according to the Chinese codes for wood structures met the requirements of Chinese and American standards. But bending deflections may exceed standards in short time in a heavy rain climate. This phenomenon proved that the GMC could significantly affect the Eb of wood materials.(8) Results based on the moisture-migration model and Eb calculation model indicated that the MCRCS of YNP service for5,25,50and100years should be0.89,0.84,0.80and0.67, respectively.