Stress Concentration in Timber Beams

Stress concentrations in timber beams are of practical importance now that the glulam industry is producing large beams of diverse shapes. The finite element technique is reviewed to consider an orthotropic material and then used to calculate the stress distribution near the end of a simply supported beam and over the area of a shear test block. It is then shown that the non linear stress strain behavior of wood must be considered if the results are to be realistic in regions of high stress gradient. The finite element technique is further modified to include this non linear behavior but numerical calculations are almost meaningless because of the lack of stress-strain curves beyond the proportional limit. The technique thus points out what research is necessary in the laboratory before a realistic evaluation of beam strength is possible. It is shown how failure may be predicted with the finite element approach but again the analyst is confronted with a lack of experimental data on failure under combined stress. Finally, an experimental technique is outlined which may provide the required date and the results of several laboratory tests on the shear modulus are presented.