ABSTRACT
A definite though small size effect has been established, and in order to compare the strength of a timber sample with recorded data, it is necessary to adopt the sizes set out in the standards. Current testing standards still permit the derivation of structural stresses from small clear test pieces, but it is better to use structural-size test pieces as the effects of strength limiting characteristics are easier to quantify with structural-size pieces. The sensitivity of strength to grain angle in clear straight-grained timber is identical with that for fibre orientation in glass-fibre– and carbon-fibre–reinforced plastics. Ring width of itself does not affect the strength of the timber; nevertheless, it has a most important indirect effect working through density. In terms of the practical use of timber, the duration of time over which the load is applied is perhaps the single most important variable. Classical and practical approaches are two approaches to the concept of strength and failure in timber.
