Abstract
Since the beginning of the XX century it has been recognized that the bending strength of timber is affected by the size of the specimen. In the present work the influence of depth (h) and thickness (t) on the characteristic bending strength (f k ) of both Scot (Pinus sylvestris) and Laricio (Pinus nigra) pine Spanish grown timber is assessed. 1.733 pieces of both species ranging in size from 100x40x2500 mm to 200x70x4.500 mm and especially sampled for this work, have been tested for bending in accordance with UNE EN 408 standard. In both species, the influence of depth and thickness on the characteristic bending strength of timber is highly significative but different between them and lead to conclude that the effect of the species is also significative and thus that the proposition of general values should be done cautiously, avoiding extrapolations. It is also concluded that, for both species, the effect of thickness seems to be highly significative and thus shouldn’t be ignored by the European standards. Finally, the convenience of revising the present depth factor considered in UNE EN 384 and UNE ENV 1995-1-1 standards is also suggested.
Highlights
Since Newlin and Trayer (J) (cited by Barrett and Fewell (2)) it has been recognized that the bending strength of timber is affected by the size of the specimen
Newlin and Trayer (J) suggested what has become known as the ''support theory" under which the fibres in compression zone of a beam working in bending act individually as small colunms, being the highly stressed fibres, located near the edge of the beam, restrained by the action of those relatively unstressed fibres located near the neutral axis
For a correct detection of the effect of the size on the strength of a piece of timber and avoid, as far as possible, the influence of other effects, such as the variation of quality and wood properties induced by the zone of provenance of timber specific samplings for each species were designed, and the sample for each species came from an unique and selected area, chosen to have averaged properties within the species
Summary
Since Newlin and Trayer (J) (cited by Barrett and Fewell (2)) it has been recognized that the bending strength of timber is affected by the size of the specimen.Many theories have been put forward to explain this loss of strength, which occurs as the size of the beam increases.Newlin and Trayer (J) suggested what has become known as the ''support theory" under which the fibres in compression zone of a beam working in bending act individually as small colunms, being the highly stressed fibres, located near the edge of the beam, restrained by the action of those relatively unstressed fibres located near the neutral axis. Pierce (4) and Tucker (5) and, Weibull (6), who studied brittle materials including concrete, developed the theoiy known as "the weakest link theory" which says that ''when subjected to tension, a chain is as strong as its weakest link". This theoty, developed mathematically by Weibull (6), has been successfully applied to explain the behaviour of timber, under tension parallel and perpendicular to grain (Barrett (7), Colling (8)), but under shear as well (Foschi and Barrett (9), Foschi (JO), Colling (8)). The use of this theory in the case of compression and, in bending is highly debatable (Rouger and Fewell (11))
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