Holes in wood Joists vs TJIs 1

[taharvey]

I'm looking at trying to get a 4" duct through a 4x10 doug fir joist. The IRC only "allows" a hole up to 1/3 the height of the joist, however having looked at the guidelines for TJIs, it makes sense to me that solid lumber should be similar: larger holes should be potentially acceptable the further you are from the bearing edge, since shear is reduced.

Are there similar rules for solid lumber that I haven't found?

 

[Ron]

Not likely. Engineered wood sections and systems are just that...they're engineered to increase properties where they are needed and optimize the efficiency of the section where they are less critical. Solid lumber doesn't do that and that is the reason for greater restriction on cuts, notches and holes.

For solid lumber, stress distribution around holes is influenced by the grain. You can have stresses that concentrate in odd directions because of the grain orientation. In plywood and composite wood sections this is not the case. The stress is more uniformly distributed around holes, thus allowing a greater attenuation of the stress by the member.

An alternative would be be to provide a flitch plate on each side of the joist where the hole is cut to strengthen the member. Some of the wood connector manufacturers make similar reinforcement for smaller sections...check with them (Simpson or USP) and see what they have. If they do not have anything, use a double steel sheet, say 10 gauge or similar, extending 18" or so beyond the hole in both directions.

 

[msquared48]

Or you COULD re-route the plumbing!

Interesting thought...structure BEFORE plumbing.

Mike McCann
MMC Engineering

 

[MiketheEngineer]

Don't do it - you have just turned a perfectly good 2x10 into maybe a 2x6 or 2x5. You didn't mention moment capacity loss....which will be significant.