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Multiple Holes in Floor Joist

Multiple Holes in Floor Joist

Multiple Holes in Floor Joist

I was asked to analyze a series of floor joists that have multiple holes. The code does not address how many holes can be installed next to each other. In this case I think a minimum spacing of 4inches is required to prevent splitting. I am trying to find a reference regarding this concern. Thank you for your help

This is what I got from the client.
1. Joists are DF#2
2. Hole diameter 1-1/2"
3. Distance between two consecutive hole is 2"
4. Joists are 2x10
Replies continue below

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RE: Multiple Holes in Floor Joist

Well, the good news is that the holes are close to the neutral axis. So, the effect on bending strength will probably be minor.

The shear stresses could be an issue close to the supports. But, in my view, the biggest issue is worrying about splitting (which will affect both bending and shear).

Conceptually, I look at this similarly to web openings in wide flange beams (because I'm more of a steel guy) where the preferred reinforcement method would be some sort of horizontal reinforcement above and below the penetrations. Maybe just running a couple of steel straps along that length of the beam where those penetrations are. But, the question is how to do this without risking more splitting..... I think it can be done, but I don't know what precautions should be taken to prevent it. Maybe using higher gauge nails for the straps?

RE: Multiple Holes in Floor Joist

We don't have a lot to go on in the US, but New Zealand has a nice publication. It's geared more toward heavy timber construction, but I've found it useful in the past for this type of thing.

NZ Wood Design Guides: Reinforcement of Timber Members

If you determine that this is a problem (probably is), best bet is likely to remove the conduits, install plywood on both sides (glue it on and then screw/nail it). Do one side at a time, drilling the plywood each time at the existing holes to prevent additional damage.

RE: Multiple Holes in Floor Joist

Thanks this document was helpful

RE: Multiple Holes in Floor Joist


You may find the NJ code is based on the National building code and has holey joist rules that look a lot like the section of the Massachusetts Code in the pdf presentation in the above link.

All those holes in a row, with a bunch of splitting and blown out material of unknown depth is a little disquieting, at least to me.

RE: Multiple Holes in Floor Joist

Tmoose - the issue is that the code is not entirely clear on a) how many you can have or b) how to correct the problem if it's violated. The IRC calls for a maximum size of joist depth/3 and a minimum of 2" between holes and between holes and edges. That's it. You can drill a series of holes 3" in diameter with 2" clear between them for the entire length of a 2x10 joist and still technically meet code. I wouldn't want to stand below or on it, though.

I dug into this a while back. From what I can tell, those guidelines are 100% empirical. I found identical notching and boring guidance in publications back to the early 20s with no evidence of engineering analysis or testing. Just a 'this is what we do, and it works.' Unfortunately, that depends on people not drilling 10 holes in a row. After all, houses in the 20s didn't have 30 electrical circuits, a web of Pex piping, hard wired internet infrastructure, AV cabling, etc. all running over top of the living room.

There's plenty of literature out there for reinforcing Engineered wood products - the manufactures have a vested interest in making sure their product isn't the reason for a headache. But commodity lumber? There isn't really a push for that, especially as its use for floor framing falls further and further out of use. So there's not much out there for a good, US code compliant fix.

RE: Multiple Holes in Floor Joist

Jim, rim board manufacturer catalogs typically recommend 4" minimum nail spacing to prevent splitting, if that's what you're thinking of. But wedging nails in to force wood apart is different than drilling holes. Personally I don't find these diameters/locations too concerning. And I'm not too worried about splitting either. This beam is top loaded, bottom supported, unlike the majority of the splitting scenarios in pham engineer's NZ document. Thnx for sharing btw.

I don't like how it appears the electrician damaged the joists during drilling. I could be wrong here but it kinda looks like a layer of wood busted off. It looks like maybe the back side of the hole spalled off by the pressure of the drill tip like a bullet exit wound. I also don't like how closely spaced holes are. Shear failure will look like splitting and the failure plane will probably travel through all the holes.

That being said, you still might be able to justify this without retrofit. What are your moment and shear ratios at worst case hole locations without considering cores? My guess is the electrician might have taken away 50% of your shear capacity and at most 10% of your moment capacity, just a ballpark estimate. If I really wanted to dig into this I guess I would model the joist as shell elements to get a better handle on stress concentrations around the holes.

RE: Multiple Holes in Floor Joist

Thanks pham... I don't get involved with timber, anymore, but timberframe construction is my favourite.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates


RE: Multiple Holes in Floor Joist

Real late to the party here, but I agree with MikeMike's approach of checking shear with a 50% reduction of allowable vs actual, and a slight reduction in bending moment capacity. In engineered wood, we have some proprietary methods that may or may not transfer to standard dimension lumber timber framing, but I don't think a 50% adjustment to shear strength based on the given photos is unreasonable. Wood is very rarely controlled by shear, so I might expect you would still be within acceptable tolerances for most of the joists, with possible exception to the joist in the back of the photo that appears to have a garage door track hanger attached to it.

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