Some aspects of load duration behaviour in wood

Present design stresses for structural timber include a reduction factor to account for the duration of loading. Recent studies have shown however that this factor does not apply to commercial grade timber containing knots and other irregularities of grain. It has also been shown that the apparent Young's modulus perpendicular to grain in clear material decreases greatly with longer durations of loading. It was therefore hypothesized that in commercial grade wood, stress concentrations are made less severe in long term loadings because of stress redistribution, made possible by straining perpendicular to grain. This thesis found experimentally that the amount of tensile straining perpendicular to grain around a knot in timber beams subjected to bending increased substantially with long duration loadings. In conjunction with this, a computer simulation of the material around a knot showed that a decreasing apparent Young's modulus perpendicular to grain reduces the stresses perpendicular to grain.Applied Science, Faculty ofCivil Engineering, Department ofGraduat