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.