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Fire rating of built up lumber columns

Fire rating of built up lumber columns

Fire rating of built up lumber columns

Hello All

I am working on a project with existing columns made of exposed 4 ply 2"x10" (no treatment). We need the fire rating of the column. The only thing I can find are calculations for glulam columns and am wondering if that can be used for the built up lumber posts as the calcs only take into consideration the wood and not the glue. We now the existing posts are fastened together with nails and glue.

Am I off base using the glulam calcs in this case?

RE: Fire rating of built up lumber columns

I've basically been told when it's built-up members they're basically matchsticks. Unless it is a solid piece of wood then they have zero fire rating. and they need to be fairly large to qualify for a reasonable rating at that point.

Basically the theory (as I understand) is once a certain thickness of the wood has burned it creates a barrier which slows the inner wood from burning. With built-up lumber you don't get that barrier as it will quickly burn completely through each 1.5" thick member and leave nothing behind as a barrier.

RE: Fire rating of built up lumber columns

Wow, that was a lot of basicallys in a short period.

RE: Fire rating of built up lumber columns

Basically I think Jayrod provided a well written, yet basic, answer.

RE: Fire rating of built up lumber columns

4" nominal thickness is 1 hour at roof (3.5" solid or 3.125 Glulam OK).

For 1 hour at floors minimum is 6" nominal (5.5" solid or 5.125 Glulam OK).

Mike McCann, PE, SE (WA)

RE: Fire rating of built up lumber columns

Thanks Everyone

Just fr sake of argument what the builder did was take the original solid post (4"x 10") then added two layers of 2"x10" on each of the 10" flat sides (the 2"x10" layers had a 3/4" strip added so that they were a full 10" wide) So the final post is 10"x10". He then filled all cracks and spaces with a fire retardant plaster.

RE: Fire rating of built up lumber columns

Also should add the goal is to have a 1 hour rating and the posts are supporting a floor.

RE: Fire rating of built up lumber columns


If you're in the US section 722.6 of the IBC shows a method of calculating he fire resistance of an assembly. 2x4 wood studs have a rating of 40 minutes (table 722.6.2.2) If you added a 5/8" layer of type X gyp board (20 minutes) you'd have a one hour assembly. DCA-4 CAM for Calculating and Demonstrating Assembly Fire Endurance also discusses this. It's available here:


The plaster is worthless - I'd ignore it, but you have to add the gyp board. Also, check for protection on the connections



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