Assessment of strength and stiffness variation within old timber beams

Abstract

The mechanical properties of timber elements vary significantly along its extent due to natural variability of wood cellules and presence of defects. In this work, the variability of stiffness and strength properties of old chestnut (Castanea sativa Mill.) floor beams with more than 100 years, was experimentally assessed and mechanical properties were correlated. Visual inspection, non-destructive and destructive tests (bending, compression and tension parallel to grain) were considered in different scales aiming to obtain reliable information for predicting the mechanical properties of old timber elements by use of small clear samples and defect concentration. For that purpose, the timber elements were systematically cut to smaller samples, to minimize the presence of defects, and tested in each phase. Visual inspection evidenced large variability in old timber elements' properties and proved to be qualitatively useful in predicting its performance, although conservative for higher visual strength classes. Strong correlations between moduli of elasticity in bending were found in sawn beams and boards (r2 = 0.82-0.89), however with weak correlations to non-destructive tests except for ultra-sound tests (r2 = 0.51-0.87). The bending results in sawn boards revealed that stiffness properties vary both in height and length of each beam in different proportions. Also variation within an element was lower compared to the variation within the full sample. Density was found to be an important parameter for properties correlations determination. Finally, knot area ratio was used to correlate the influence of defects with stiffness properties of the existing timber elements.