Combination Layup: Wednesday's Word(s) Of The Week

Glulam is produced using standard lumber as the raw material, which is supplied in nominal sizes between 1x4 and 2x12 and up to 20' long. In all glulam beams, the wide faces of lumber are glued together to build up the beam depth, for example 16 layers of nominal 2x lumber are used to make a 22" deep glulam. And, in almost all glulam members, lumber is finger jointed to create glulam beams which are longer than the raw material or to efficiently use the material when creating odd lengths. 

However, in the third dimension (glulam beam width), it is not typically necessary to add to the raw material. The width of a 2x6 is used for a 5" wide glulam, and a 2x12 is used for a 10½" glulam. Beyond 10½" (or sometimes 10¾") wide, it does become necessary to build up the glulam beam in all three dimensions. There are two ways to do this. 

The first, and simplest, method is the Combination Layup. Here, two different sizes of lumber are used side by side to increase the beam width (a 2x6 & 2x12 are combined to make a 16" wide glulam), and their locations alternate to stagger the joint between them. This joint between the edges of lumber is not glued, so there are continuous voids inside the glulam and on the face. For architectural appearance, the void on the face is routed and plugged. The internal voids do reduce the weak axis shear strength of the member, which is accounted for in footnote 3 of NDS Table 5A (and footnote 2 of NDS Table 5B.)

*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.