Assessment of wood degradation produced by insects in order to quantify the loss of mechanical strength of load-bearing elements

Biological degradation due to insects is very frequent on load-bearing wooden elements in historic buildings. Degraded areas are usually excluded from the resistant section, although they may still make a mechanical contribution to the structure. An assessment of the depth and degradation level of the insect attack is therefore crucial for a better knowledge of the residual strength of the wooden elements. Specimens degraded by insect were evaluated in terms of percentage of surface occupied by bores, a procedure that an expert operator could apply easily during the on-site assessment. Following, resistance drilling measurements and destructive mechanical tests were performed to determine density, compression, shear and bending strength and bending modulus of elasticity. The results showed moderate but strongly significant negative correlations between the degradation level and the mechanical properties. On the contrary, density was unrelated, as well as drilling resistance. The latter, however, correlated strongly with density and mechanical properties, and, combined with the visual assessment of degradation, made the mechanical property prediction as effective as for sound wood.