The effects of openings on the strength characteristics of glued laminated timber beams

Abstract

The recent resurgence of mass timber for use in buildings brings many issues regarding detail design and structural optimisation. An important design consideration is the overall depth of timber structures. To reduce the depth of the structural zone, one must be able to pass services through the web of the timber beam, as is similarly done in steel beams. Eurocode 1995-1-1 (2004) gives no guidance on quantifying the effects of web openings in timber beams. This dissertation investigates the difference in the flexural performance of glued laminated timber beams with and without openings. Component testing was carried out on the timber and the adhesive to understand the failure modes associated with timber structures and determine the characteristic and average strength and stiffness property values of both. The strength values obtained were then compared to the design values published in available data tables and the formulation provided in EN1995-1-1 (2004). This design data is normally used to predict glued lamination timber beams’ failure load. If an opening of diameter 0.67D, where D is the depth of the beam, is placed in the centre of the span of a beam subjected to four-point bending, with point loads applied at third points along the beam, then it can be shown that the predicted failure load is reduced by 16.7%. If an opening of the same size is located adjacent to the applied load, that is, at the position of greatest shear and bending moment, it can be shown that the beam’s load-bearing capacity is reduced by 31.3%. The experimental results showed that adhesive failure led to the premature rapture of the full beam along the glue lines. This failure mode occurred before the timber reached its ultimate tensile stress.