Multiple-knot failure mechanisms in structural lumber

Knots are the most severe defects present in wood, causing the largest reduction in lumber's tensile load-carrying capacity. Considerable amounts of lumber stock now possess the pith and as a result many small and closely-spaced knots commonly occur in individual pieces. While some progress has been accomplished in strength predictions of single-knot-containing lumber, little is known about the combined effect of multiple knots on lumber strength and stiffness. This dissertation reports on an essential investigation to determine the failure mechanisms of structural lumber as influenced by multiple knots.; The investigation was targeted on answering the following questions: To what degree do closely-spaced multiple knots interact in a lumber board subject to tension? What is the effect of the interaction between knots on lumber tensile strength? What factors govern the ultimate tensile strength of multiple-knot lumber?; A finite element and fracture mechanics model, named GASPPMK, was constructed for the multiple-knot strength simulations. This model extends the previously developed stepwise fracture accumulation method for single-knot strength analysis to the multiple-knot situations. It includes many functions for the multiple-knot analysis as well as a new input system. The new input is established on using grain angle scanning, x-ray density scanning, and dive angle simulation data together with experimentally-developed property equations for Southern Pine lumber. To serve as input of the new computer model, a data base was established involving over 50 actual multiple-knot situations.; Using the GASPPMK model as a tool, the tensile strength failures of the 50 specimens were numerically simulated and physically tested for verifications. Results were used to quantitatively analyze the strength interactions between knots of each individual situation. A parametric study was also performed to reveal the isolated influence of various key multiple-knot parameters. Results were employed to derive a set of formulations for calculating the strength lost by individual multiple-knot factors. Failure principles of multiple-knot lumber and a multiple-knot strength points method are proposed.