Moisture distribution in wooden beams under constant bending load

Abstract

Moisture distribution in small yellow poplar beams under two constant load levels was studied in four independent experiments. Beams were subjected to moisture transfer conditions namely; two experiments in adsorption and two experiments in desorption for the duration of ten days each. During this period in each experiment creep deflection readings were taken frequently, and at six occasions, vertical moisture distributions across the grain between the points of load application were determined. Moisture distribution in beams was indicated by five evenly placed positional moisture content measurements. The moisture content determination was based on the oven-drying method. Experimental results indicated that compressive and tensile stresses influenced the moisture distribution in stressed beams to a different extent. Namely, the moisture content of sections in tension were higher than those in compression, regardless of moisture transfer conditions. This moisture content difference was relatively small and seemed to be independent of moisture content level, moisture gradient and load level. The one hald of one percent moisture content difference due to compressive and tensile stresses, was found to be significant and its occurrence was explained suing theories based on the ultrastructure of wood and thermodynamic concepts of elastic gel materials. Moisture content changes were slower in stressed beams than those in stress-free ones. The rate of moisture content changes was high in the first part of the experiments; then, it gradually decreased to a ore or less constant value. Most of the creep deflection occurred while the major moisture content changes took place in the beams under bending. Then an equilibrium was obtained between moisture content of chamber and specimens and creep deflection stabilized in adsorption and desorption. This phenomenon was in good agreement with theories and results of other investigations. The rate of creep deflection was high during the first stage of the experiments; then, gradually decreased to an almost constant value following a close relationship with the rate of moisture content change. Using statistical methods the relationship between the following variables were analysed and discussed: positional moisture content and time, creep deflection and time, creep deflection and positional moisture content. The derived functions representing the relationships were found highly significant.