Duct hole through support beam
Acknowledging that it’s foolish to rely on internet advice about structural matters, I would appreciate some advice on a structural matter. I’m installing a concealed duct minisplit underneath a floor, trying to keep ducts… well, concealed. I want to avoid a mess of exposed elbows by running one 4″ branch through a solid wood support beam. I assume the 1/3-of-depth maximum hole size applies only to joists. This is a quadruple 2×10 beam. What are the chances that I can make a 4″ hole through this beam…if it’s occurring directly above a steel support post?
It’s a 1960 ranch house. Floor joists are 2×8, 16″ OC. At the center of the 24′ span is the carrying beam, four 2×10’s laminated. It’s not underneath, but framed into the floor such that the joists are hung on its sides. (Load is only the floor above, and some interior walls that support the attic floor–ZERO roof load.) My duct runs need to be concealed in the joist bays, and in only this one spot is it critical to get to the other half of the house, on the other side of this obstruction. As it happens, there is a steel lolly column supporting the beam in exactly the center of a joist bay. There is no point loading coming from above the floor.
Intuitively it seems that a 4″ hole through the center of the beam at this point would not significantly compromise the carrying capacity of the beam. Can anyone confirm this, point me to an online reference, or briefly explain why this is probably NOT a good idea?
Thank you!
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Replies
The d/3 rule applies to both beams and joists. Spend the money - usually not a great deal - and hire a local structural engineer who can provide you with an acceptable method (if any) for reinforcing the area where the hole will be located. Expect this method to involve some amount of steel.
Alternatively, you may be interested in the following Web site: http://www.joistrepair.com.
Look at https://joistrepair.com/collections/featured-products/products/2810hr for a product that allows a hole up to 6" dia through a 2x10. They can probably recommend a solution.
"The d/3 rule applies to both beams and joists." I don't see that phrasing in the code, but you make it sound unequivocal from a physics standpoint; that the concept is valid regardless of member size or application. Yes?
I would spend the money but I'm in a rural area and the only PE I know is so many months behind on jobs for friends I don't want to bother him.
The joistrepair.com products look like a great solution, but they're all 15" or longer, I presume to make them useful only in the midspan of joists and impossible to use in my 14.5" beam location.
ohioandy,
Assuming you are under the IRC, the relevant section is R502.8.
https://codes.iccsafe.org/content/IRC2018P4/chapter-5-floors
IRC seems to only cover holes in joists. Doesn't mention holes in supporting beams, not to mention holes in beams/girders that are directly over a supporting post.
ohioandy,
If you look at the wording of the section I linked to it covers "solid lumber joists, rafters and beams".
Beams need certain amount of bearing. So if you have two beams over a post and want to run a 4" duct, the support underneath needs to be 2x bearing width + 4". If your existing post top plate has it, you are good, if not, you need to widen it or add another support post right beside it.
I don't understand your formula... I have only one beam, although it is a quadrupled 2x10. Are you suggesting a scenario where two beams butt together on top of a post? Adding a wider structural plinth, or support of some sort, between post top and beam, seems like a great way to spread the support over a length of the beam wider than the hole.
ohioandy read Akos reply again. He answered your question with the correct solution.
"Intuitively it seems that a 4″ hole through the center of the beam at this point would not significantly compromise the carrying capacity of the beam. "
You're intuition has failed you. At the ends, it's the center of the beam that carries the weight. At the ends you're allowed to notch up to 1/4" of the depth. If you notch the bottom it has to bear on something, but at the top it's not supporting anything other than what's directly above it. In the center of a beam it's the top and bottom that provide all the strength, holes should go in the middle.
With that said, I don't think this is the way to go. Are there any partition walls? For a 4" duct, you can flatten it slightly and run it through a 2x4 wall. Or you can use a 3-1/4x10 or 12 duct that is meant to run through a 2x4 stud bay. Even if there aren't walls, the soffit you would need to hide a duct like that is quite small, there isn't an out-of-the way spot it could go?
Tremendously grateful for everyone's patient responses. So DC, the 4" hole in the center of a 9.25" deep beam that I proposed is not only too large by code (R502.8.1), but weakens the beam even more if located directly over a support post. I'm getting from the code language (thanks, Maclcolm!) that a smaller 3" hole would, in fact, be allowed in my supporting beam, and that you would suggest it should be placed somewhat AWAY from the support post to avoid the weakening effect.
You're not going to be able to make the hole you need in that beam.
Start thinking about alternate routes.
Ohioandy, in this case I don't think there is a prescriptive solution but an engineer MIGHT give you a pass. The ends of any beam only have shear force acting on them; if the beam were made of jello you can imagine how it would fail, more or less along a vertical line at the edge of the supporting post or wall.
At the center of most spans the only force on the beam is bending; the top is in compression, the bottom is in tension and the middle (height-wise) of the beam is only keeping the top and bottom where they are supposed to be, with no shear force trying to slice the beam vertically. That's why it's ok to drill holes near the center of the span, near the center of the beam, because it's the top and bottom that are doing the work. And that's why you can't notch the top or bottom of a beam near the middle of the span, because the bending strength is determined by the height of the beam.
Prescriptively, we don't know how much shear load is on the ends of the beam; it varies greatly with the span and load on the beam. There need to be a certain number of square inches of material at the end of the beam to prevent it from shear failure. An engineer could quickly assess it and tell you how large a hole is safe.
There are a few additional variables. When the loading is not uniform, such as when there is a point load on the beam, it changes where it's safe to make cuts and holes.
Are you sure there isn’t a way not to go through your support beam? What about a thin rectangular duct and just slide it underneath beam. Couldn’t be that much to cover in ceiling even if you build one. Just my opinion, I know going through the beam is best asthetically but I would avoid the trouble.
"Are you sure there isn’t a way not to go through your support beam?" I guess if there was a way to not do it, I would not have asked the question. There are no partition walls or closets that could serve as a chase, above or below. The client placed a high value on no bumpouts, and I engaged this forum because it usually produces a lot of solid advice and basic education. Success on that, I now know how to go back to the client and speak with a little more authority on what's possible and what's not. We'll probably go with some version of Akos' advice in #3.
Yeah, I retract my post #11. If you had a saddle on the top of that column that was 8" or more wide you could put a 4" hole in the built-up beam without compromising it. Presumably if the post is supporting 4 2x10's it has some sort of saddle already.
I might be late to the party on this one but I am an engineer so I'll give you my two cents.
A lot of other replies talk about the ends of a beam only having shear stress. That is true in a simply supported beam. However, based on you description it sounds like this column is somewhere inside the house and the beam is continuous over it? If so, then this multi-span beam is in negative bending over the column. If you exaggerate the beams deflection in your mind it looks like a frown, while on the other hand positive bending looks like a smile.
If this is a multi-span beam, I would be warry of notching the top of it as those fibers are handling the bulk of the tensile stress due to the negative bending.
At the end of the day there are local engineers that will make a house call and sketch up a solution for a relatively low fee. There are too many "what ifs" and when you consider how much of the house this beam is likely supporting the fee for a proper design is peanuts. Get a local engineer to get eyes on it or find a different route.