Edinburgh Napier University

  The share of broadleaved forest in European is growing, but most of the harvested hardwood is burned for energy production. Long-lasting uses, such as building construction, could offer benefits regarding carbon-storage and efficient, circular material use, but a barrier to using more hardwood in structures remains the comparatively low yield in sawn timber and consequently high price. Many hardwood species might, however, offer a competitive edge over softwoods due to their higher mechanical properties, including compression strength perpendicular to grain. Having an accurate description of this property for temperate hardwood species is therefore paramount. However, under the European standards framework, compression strength is not a property that has to be measured as part of a strength grading assignment for structural timber. Instead, it is usually calculated from characteristic density according to EN 384 (European Committee for Standardization & British Standards Institution, 2018). It is unsure how well the given equations predict the compression strength of an ever-growing array of temperate hardwood species coming to market now and in the future. Potentially, other predictors such as side hardness, as suggested by literature, might be better predictors for compression strength. Hardness data for certain resources might already exist, or hardness could be measured with little cost and effort as part of a strength grading assignment.

The study aims to assess the suitability of density and side hardness as a predictor for compression strength. Seventy-five full-size compression tests were performed on a sample of UK-grown sycamore (Acer pseudoplatanus) and characteristic (5th-percentile) values for density and compression strength were calculated according to EN 14358 (European Committee for Standardization & British Standards Institution, 2016). Janka hardness tests were performed on specimens cut from the same battens, and characteristic compression strength was predicted from side hardness using an equation given by Lavers (Lavers, 1983). In the sycamore sample, both density and side hardness underestimate the actual characteristic compression strength, density more so than side hardness. Compression strength and hardness were also tested on small clear specimens of seven hardwood species, to investigate if the relationships given by EN 384 and Lavers are consistent over a wide range of species. The tested hardwoods included paulownia, poplar, sweet chestnut, sycamore, ash, oak and beech. The findings suggest that the currently used equations for calculating compression strength from density might give very conservative results, and that side hardness might have a more consistent relationship with compression strength than density. For a more accurate description of the properties of full-size members, equations for both predictor variables should be adapted.