Glue Laminated Lumber Blocking/Rim Board – Something To Think About
by Heinz Mueller, Design Center Leader (for RedBuilt’s Hillsboro office) and P.E. at RedBuilt™
When RedLam LVL is not viable, glue laminated lumber (glulam) is a wonderful option for use as large section main carrying members in structures. It is made of wood (a renewable resource) and is easy to design, just like RedLam LVL.
That being said, I have started seeing some applications with glulam material specified that I would encourage the design community to think about a bit more. The applications that can be of concern are blocking between joist members (prefabricated I-joists, open web trusses, or structural composite lumber) or continuous rim board at the ends of joist members; especially when there are load bearing and/or shear walls above.
Allow me to expand on this concern.
This blocking/rim board is the primary mechanism to transfer stacking vertical loads from above to the supporting structure below (more critical on floors then roofs). It is also the primary mechanism to transfer lateral loads from the diaphragm above to the supporting structure below. Because the joists are typically not designed to transfer the stacking vertical load at perpendicular bearing walls, the blocking/rim board is a pretty important part of the structural system.
Now to the part that causes concern. Differences in the type of lumber being used (dimensional in glulam; veneers in LVL and I-joist), as well as manufacturing processes, creates a potential for differences in moisture contents and dimensional changes between glulam and joists/LVL. In other words, if everything else were equal, glulam material can tend to shrink and joists/LVL can tend to swell.
Below are a couple of actual jobsite photos illustrating this potential issue of dimensional changes in glulam blocking/rim board in stacking walls and the negative affect it can have.
Because of this potential issue and the negative impacts it would have on building performance, RedBuilt discourages the use of glulam material for blocking/rim board in a pre-fabricated I-joist, open web truss, or LVL joist systems.
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