Experimental verification of a weak zone model for timber in bending

In order to verify a stochastic model for the variation of bending strength within and between structural timber members, tests with long members subjected to constant bending moment have been performed. The span with constant moment contained between five and nine weak zones, i.e. zones with a cluster of knots. In a previous investigation test specimens, each containing one weak zone, have been tested in bending separately. Based on these tests a hierarchical model with two levels was formulated. The test results show that the bending strength of the long timber members on the average is 5 to 15% lower than is predicted by the proposed hierarchical model. Energy considerations show that the reduction in strength of long beams may not be solely a statistical effect caused by an increased number of possible failure modes in the long beams as compared to the short test specimens. The large elastic energy released in a long highly bend beam at the onset of failure may cause that a later higher external load level cannot be realised such as it can in a controlled slowly progressing failure.