Reliability of Reused Sawn Lumber: Effect of Fastener Holes

Abstract

The results of a comprehensive research study on the effect of fastener holes on the reliability of structural timbers, specifically No. 2 Douglas fir 102 × 303 mm (nominal 4 × 8 in.) structural joists and planks, is reported. Through this study, adjustment factors are developed to account for the presence of a hole as a function of its size and location within the member. Such adjustment factors could be used in design by the engineer, or perhaps more appropriately, grading agencies to develop appropriate grading procedures and design values for reused or recycled timbers to ensure acceptable levels of safety. The specific objectives of this paper are, therefore, to (1) quantify and determine the effect of a fastener hole on the reliability of wood beams when various diameter holes are located at various positions throughout the depth; and (2) develop and recommend possible adjustment factors to account for the reduction in reliability created by the presence of a fastener hole. The reduction in reliability caused by a 44.5 mm hole was found not to be significantly different from that caused by a 25.4 mm hole. The reliability, however, is significantly affected by hole location. If the hole is located at the point of maximum moment, the reduction in reliability is over twice as much as when the hole is located randomly within the central third of the length, which is similar to the grading criteria for knots. For a flexural member containing a 25.4–44.5 mm hole anywhere within the central third of the length, the recommended adjustment factor is ω′ = 0.90. In other words, a 10% reduction in design capacity is recommended when such a fastener hole is present in the central third of the member's length.