The AITC Technical Note 21, titled “Volume Factor for Structural Glued Laminated Timber,” was published by the American Institute of Timber Construction (AITC) on October 28, 2023.
This technical note documents the history of factors used to adjust for the effect of the size of a timber bending member on its apparent strength and the development of the current factors that are used for structural glued laminated timber design in the United States.
It discusses depth factors first published in 1924, which were used to adjust the allowable bending strength for solid-sawn timber members having depths greater than two inches.
With the introduction of structural glued laminated timbers (glulam) to the U.S. in the mid-1930s, additional research related to the effect of the depth of a timber member as it related to the member’s load-carrying capacity was conducted. This led to the development of a revised depth factor published in 1954.
In 1966, Bohannan presented a derivation of a size factor based on Weibull’s weak link theory. This theory relates the apparent strength of a beam to its volume and assumes beam strength is governed by tension failure. The wood industry adopted a size factor, CF, to replace the depth factor.
For more detailed information, you can refer to the full document here.