Influence of the angle between the force and the wood’s grain on its tension strength

Abstract

Wood is an anisotropic material and its strength varies with the angle between the force and the grain direction (a). The Brazilian Code, NBR 7190 - Timber Structures Design, from Brazilian Association of Technical Codes - ABNT (1997), foresees the strength correction to the normal stress whenever a ³ 6º, and recommends applying Hankinson's expression to obtain design values. However, there is no experimental evidence on the validity of using this expression on design values. The objectives of this study were: to research the literature for other expressions used; and, to verify the validity of these expressions, including to characteristic values, to represent the variation of the tension strength with the a angle. From a sample, formed by twelve boards, tension specimens were sawed and tested in agreement with NBR 7190, with different grain directions. Four models (Hankinson, Sin, Karlsen and Keylwerth) were evaluated, using the pairing and residues tests. The model that best represented the phenomenon was Keylwerth’s.