Uniform Layups: Wednesday's Word Of The Week

The second type of glulam layup and associated Combination Symbols are the "uniform layups" listed in Table 5B of the NDS. These are used for axially loaded members or parts loaded in weak axis bending. In this case all plies are equally stressed, so there is no reason to vary the material grades used. A single material grade is used for lumber throughout member depth.

Bending (left) vs. Uniform (right) glulam layup

Three physical properties are used to judge the strength of southern yellow pine (SYP) lumber:
The presence of knots (N1 has fewest knots, N2 has more knots)
The spacing of growth rings (Dense/D lumber has tighter spaced growth rings than Medium/M)
The slope of grain (1:16 slope of grain is very close to straight, 1:8 is more angled)

An example SYP Combination Symbol is #49/1:12.
The first number represents the knots and density and the second half indicates the maximum allowable slope of grain.
47/48/49/50 indicate N2M/N2D/N1M/N1D material, respectively.
1:12 would require grain to be at no more than a 1:12 slope compared to the ply axis.

Common uniform layups are:
#47/1:8 for members which are not highly stressed.
#49/1:12 for members requiring higher allowable stresses.
#50/1:12 for maximum capacity (and maximum cost)

*Wednesday's Word Of The Week is a feature on Unalam's Wood Times Blog. Each Wednesday our structural engineer, Rik Vandermeulen, will discuss a new term associated with glulam manufacturing. He will do this until we run out of words. If there is a timber or glulam term that you have heard of and want to know more about, let us know in the comments.