The influence of wind-induced compression failures on the mechanical properties of spruce structural timber

Abstract

Compression failures (CF) are defects in the wood structure in the form of buckled fibres. They are a well-known 'natural' phenomenon in softwood trees exposed to frequent or strong winds, but their influence on the utilisation of timber is still debated. While a reduction of the mechanical properties in bending and tension at the fibre level and in small clear specimens is generally acknowledged, the effect is less obvious with structural timber in the presence of other defects such as knots or grain deviations. In the presented case study a statistically significant reduction of the moduli of rupture and elasticity in bending is observed in a sample of 563 squared timber beams, but the characteristic values of the mechanical properties still exceed the limits for the strength classes of visually graded structural timber (according to the Swiss standard SIA 265). Nevertheless, because of the potential safety risk by the more brittle fracture behaviour, it is recommended to exclude timber with detected CF from use in load bearing structures stressed in tension or bending.