Evolution of Glulam Strength Criteria

The evolution of glu-lam bending, strength criteria as they are controlled by knots is reviewed. Early research in the 1930’s was the foundation for the glued-laminated industry in the United States. Empirical strength criteria were based on the hypothesis that the strength of glu-lam timbers could be controlled by selective placement of grades of material in the members. Two grade classifications of glu-lam timbers were suggested. However, two strength grades of glu-lam timbers were somewhat restrictive. Thus, in the early 1940’s, a different criterion based on the accepted strength ratio concept for solid-sawn wood was developed. Experience suggested the strength ratio concept was overconservative. Consequently, the strength of beams 12 inches in depth was related to a nondimensional factor commonly called IK/IG, where IK is the moment of inertia of the area occupied by knots within 6 inches of either side of a critical cross section, and IG is the gross moment of inertia of the beam. With wood-laminating industry growth, manufacturing techniques and architectural design developed to the extent that timbers of almost unlimited size and shape were possible. This growth resulted without accompanying advancement in engineering technology. However, an extensive research effort during the 1960’s was directed at the engineering technology for large timbers. This research established the effect of several factors involving strength and design of large timbers– a principal one being the effect of tension lamination quality on beam strength. Such research has resulted in revised specifications for glued-laminated timbers which will insure a more reliable engineering material.

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