Glulam design based on lamination grades and the use of mill shorts

The strength of a glulam beam depends on factors such as the number of and wood qualities of the laminating timbers that are glued on top of each other to form the beam; however, glulam beam strength is also affected by the length of and the number of joins (finger joints) in each laminating timber.

This project investigated the strength of glulam beams made of short lengths (from 0.3 to 2.1 metres) of laminating timber (radiata pine, slash pine and Victorian ash) with larger numbers of finger joints.

The current standards for manufacturing glulam beams link finger joint bending and tension (i.e. energy required to pull the joint apart) strengths to the bending and tension strengths of glulam beams. There is an upper limit on glulam strengths where they cannot exceed the strengths of the laminating timber. Unfortunately this penalises lower grade material, such as MGP10, which benefits most as laminating reinforces the greater number of knots and other defects.

The project quantified the effect of finger joint spacing on laminating timber tension strengths, and determined that when joints are one or more metres apart glulam strengths are not affected. It found that glulam tension strengths were stated conservatively in previous AS/NZS Standards and recommended an increase of these strengths in the glulam (GL) grade values.

In addition the report developed a lamination factor that allows glulam bending and tension strengths to be determined from finger joint tension strengths.

The report suggests changes to the AS/NZS Standards for determining the strength of laminating timbers based on the spacing and quality of joins (so that good quality joints are rewarded and poor ones are penalized), and proposed a framework for future glulam standards, which would give designers greater flexibility in design, enhancing the appeal of structural glulam.

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