Flexural properties of glued-laminated Southern pine beams with laminations positioned by visual-stiffness criteria

Abstract

To establish the effect of using modulus elasticity in addition to visual grade as criteria for the positioning of laminations in laminated beams, an experimental study on southern pine members was conducted. The beams were manufactured in accordance with current specifications for glued-laminated southern pine timber, except that (a) minimum-quality tension laminations meeting AITC 301-67were used and (b) restrictions were placed on the modulus of elasticity of all laminations. Current specifications require that boards lie positioned in laminated members on the basis of their visual grades. The use of modulus of plasticity in addition to visual grading in selecting quality laminations for highly stressed areas should provide for better assessment of lamination quality and result in stronger and stiffer laminated timber members. Five glued-laminated southern pine beams, 5-1/4 inches 24-3/8inches deep, and 40 feet long, were manufactured using the dual criteria for positioning of laminations. Results of bending tests were compared to data reported in USDA Forest Service Research Paper FPL 113 for comparable beams made using only visual grading requirements. In both sets of beams, the tension laminations were selected to represent near minimum visual quality for the AITC 301-67 grade. The average modulus of elasticity of the beams manufactured using both visual and stiffness criteria was 12 percent higher than that for beams using only the visual criteria. This agreed with the theoretically expected increase in modulus of elasticity. The average modulus of rupture of the visual-stiffness beams was 14 percent higher than that for the visual-only beams. However, the minimum strength for each group was approximately equal.

Citation

Moody, R. C.; Bohannan, Billy. 1970. Flexural properties of glued-laminated Southern pine beams with laminations positioned by visual-stiffness criteria. Research paper FPL no. 127. Madison, WI : USDA, Forest Service, Forest Products Laboratory. 20 pages.