Bending Layups: Wednesday's Word(s) Of The Week

There are two general types of glulam layups and associated Combination Symbols. The first are "bending layups" listed in Table 5A of the NDS. These are meant for members primarily loaded in strong axis bending (loaded perpendicular to the wide faces of laminations.) This type of loading concentrates tension and compression stresses in the top and bottom laminations. Therefore, the strongest lumber is used at the top and bottom and weaker material can be used in the middle.

Table 5A Expanded can be a little intimidating with all of the different Combination Symbols available. Three steps will help narrow down the choices: Species, Grading Type, and Bending Stress. For example, Unalam does visual grading only, so for Southern Yellow Pine glulam look for SP/SP in the species column and "V" in the Combination Symbol. This leaves:
16F-V2, 16F-V3, 16F-V5
20F-V2, 20F-V3, 20F-V5
24F-V1, 24F-V3, 24F-V4, 24F-V5, 24F-V8
26F-V1, 26F-V2, 26F-V3, 26F-V4, 26F-V5

The first numbers represent the allowable bending stress (1600 psi to 2600 psi), 2400 psi is the most common. Of the five combinations listed, 24F-V4 has a footnote explaining that it contains wane, so that combination should be ignored. So, four combinations left to choose from:
24F-V1, 24F-V3

Unbalanced (left) and Balanced (right) glulam layup 

24F-V5, 24F-V8

The next choice is whether a balanced or unbalanced layup is required. Unbalanced is for simple span beams (high tension stresses only expected at bottom of beam.) Check the allowable Fbx- value, for tension stress at the top of beam. In balanced layups Fbx+ equals Fbx-, which is true for 24F-V5 and 24F-V8. The remaining difference between these two grades is their modulus of elasticity (E); 24F-V8 is slightly more rigid than 24F-V5.

The most common bending layups used for southern yellow pine are 24F-V3 and 24F-V5.

*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.