Development of poplar glued–laminated timber. I: Tensile strength and stiffness of poplar laminating stock

Abstract

The tensile strength and stiffness of 263 full-size pieces of 38 × 80 mm in-grade poplar lumber from two mills in Ontario were determined. Prior to testing, complete maps of all grade-related defects in each piece were made; these defects included general slope of grain, centre knots, and edge knots. Specific gravity and moisture content were also recorded for each piece. After testing, the defects that triggered each piece's failure were noted. This information led, through regression analyses and the assumption of a Weibull distribution, to models for tensile strength and stiffness. The models were developed for the pieces from one mill and were tested against the results obtained from pieces from the other mill; agreement was very good. The knot frequency and slope frequency data were used in a modified form of the Foschi and Barrett glulam beam simulation model to predict the fifth fractile value of a population of poplar glulam beams. Taking the volume effect into account, the predicted beam strengths and stiffnesses are close to those of glulam beams of stress grade 20f–E.