The strength of glulam beams with holes - A probabilistic fracture mechanics method and experimental tests

Abstract: This thesis deals with experimental tests and methods for strength analysis of glulam beams with holes. Test results and methods for strength analysis available in literature are compiled and discussed. The methods considered comprise both code strength design methods and more general methods for strength analysis. New strength tests of beams with quadratic holes with rounded corners are presented. The test programme included investigations of four important design para\-meters: material strength class, bending moment to shear force ratio, beam size and hole placement with respect to beam height. One important finding from these tests is the strong beam size influence on the strength. This finding is in line with previous test results found in literature but the beam size effect is however not accounted for in all European timber engineering codes. A probabilistic fracture mechanics method for strength analysis is presented. The method is based on a combination of Weibull weakest link theory and the mean stress method which is a generalization of linear elastic fracture mechanics. Combining these two methods means that the fracture energy and the stochastic nature of the material properties are taken into account. The probabilistic fracture mechanics method is consistent with Weibull weakest link theory in the sense that the same strength predictions are given by these two methods for an ideally brittle material. The probabilistic fracture mechanics method is also consistent with the mean stress method in the sense that the same strength predictions are given by these two methods for a material with deterministic material properties. A parameter study of the influence of bending moment to shear force ratio, beam size, hole placement with respect to beam height and relative hole size with respect to beam height is presented for the probabilistic fracture mechanics method. Strength predictions according to the probabilistic fracture mechanics method is also compared to the present and previous test results found in literature and also to other methods for strength analysis including code design methods. The probabilistic fracture mechanics method shows a good ability to predict strength, with the exception of very small beams.