Coring/cutting holes in reinforced concrete wall - max diameter? 2

[psychedomination]

Hi there,

I have a client that would like to create two 6” holes in an existing reinforced concrete basement foundation wall. I asked for the existing drawings and the only drawing in archives they were able to find is showing the reinforcement layout in the slab above, but in this same drawing it states the thickness of the wall as 10” and that it is reinforced concrete. However, the reinforcement size and layout is not known.

A 6” hole seems small to me in a reinforced concrete wall.

I am wondering if there is any guidance/rule of thumb for the maximum hole size in a wall, where considerations for additional structural framing/beams above are not necessary.

Logically I am thinking for a 6” hole, the load will just redistribute around it, however, I am unsure of the spacing requirements/best practice for this (what the minimum/maximum spacing should be between the holes). The client is digging a trench outside the building and running the two 6” pipe duct side by side. They said it didn’t matter if they were side by side horizontal or side by side vertical. I am thinking from a structural point of view it would be better to have them side by side vertical, as the interaction zone of the load above the hole would be smaller? Any comments on this would be helpful.

I was thinking they could just core through the wall without any temporary supports. Or perhaps they would need to do a 6" wide saw cut rectangle going down like 14" and drypack/fill with mortar when the pipe ducts are in position?

A photo of the site is below with a sketch of what I am thinking. I would like to get any advice or general guidance for situations like this and the associated literature link if possible that I can read for my own knowledge/confirmation.

 

[miningman]

well if it was me , I would ensure that the pillar between the holes was twice the diameter of the hole ... is 12 inches in this case

 

[DayRooster]

It is common place to have the concrete scanned for rebar to avoid drilling through the rebar or something even worse like an embedded electrical conduit; even when drawings are available. In this case it justifies rebar scanning even more since no information is known. The cost of rebar scanning by a third party has come down in price significantly.

If you were looking for a stud in a house wall would you just smash your hammer through the wall until you found a stud? Or would you use a stud finder?

 

[psychedomination]

@miningman when you say pillar, I'm assuming you mean the vertical space between the holes right?

@DayRooster That's true and a good point. I did have in the back of my mind that I will instruct the contractor to avoid hitting any existing rebar. **I guess more from a purely theoretical point of view though, for a hole this small, would it necessarily matter if a bar is cut? It is unlikely the bar spacing would be less than 6"; if one bar was cut vertical and horizontal, the load would just redistribute to the surrounding bars, wouldn't it?

There is still the question, in a general sense: when is a hole too big for just coring through a reinforced concrete wall? Is this just dependant on the existing rebar spacing e.g. if spacing is 8" max hole would be 6", if spacing is 12" max hole would be 10"? What about mass concrete walls?

 

[bones206]

If there is significant axial load on the wall, it will have to arch or flow around the openings. Arching action creates a horizontal thrust, which needs to be resisted somehow. If the hole is in the middle of a wall, you can use the wall itself as a buttress. If near the edge, you can’t necessarily count on buttress action and so a tension tie would be needed to resist the thrust. That’s one good reason not to cut the horizontal bars around the holes. Especially since you plan to core near the edge of the wall.

If there is out of plane bending on the wall due to earth pressure, that pressure load will also arch around the opening. You could check the “jamb” wall strips on each side of the opening for the added tributary pressure load they will be seeing. I’m assuming the wall spans vertically here.

I think the approach you take in these situations depends on the relative magnitude of the loads and the opening sizes. With nominal loads and small openings you can usually just use judgement to say it’s ok. Other times may warrant analysis. Either way it’s good practice to avoid cutting rebar so the horizontal tension tie paths and vertical bending bars can remain intact as much as possible. Having a good idea where the loads are coming from and visualizing the flow of the load around the openings is the first step. If you don’t have a good feel for that then you’re kind of flying blind.

 

[DayRooster]

psychedomination - there is not a maximum hole size that you can cut. ACI has some general guidance for two slabs but even then it’s not always completely clear. It is up to the engineer to determine if it’s acceptable. Also rebar may be cut as long as the engineer determines that there will not be stress concentration problems. I traditional lean towards rebar scanning because even if the wall could lose one rebar I don’t feel the strength reduction needs to happen. I prefer the best practice route than to play a game of jenga. Also, keep in mind if anything bad happens and you are sitting in court trying to explain your actions then you don’t want to be saying “well I didn’t even know what was in the wall” and “I assumed it would be okay”. Unless you feel with your engineering judgment that this will never happen since you have the ultimate decision. Best of luck.

 

[XR250]

Honestly, based on the aspect ratio of the wall segment, I would not worry about it.

 

[MIStructE_IRE]

I agree with XR250 and wouldn’t even give this a second thought at only 6” diam.

 

[DayRooster]

I should add that I don’t give this stuff a second thought either. I call out concrete scanning to ensure nothing is cut (like some hidden embedded conduit) and just move forward. I have seen way too much conduit and pipe embedded into concrete. I’m not getting into trouble if I give the okay for the contractor to drill without checking first…just my opinion but I guess my years as a field engineer might have made me too cautious. Also the cost of concrete scanning is beyond cheap now.

 

[bones206]

Well, you should give it some thought. A 6” pipe going to the exterior of the building will likely need a larger diameter hole for a linkseal. So say 8” core centered 12” from edge of existing opening. That leaves just 8” of meat between the openings. The existing opening jamb would likely be compromised, since large openings such as the one in the photo would typically have additional closely spaced jamb bars. Again, depending on the wall loading there could be an issue. We don’t really know how hard that wall is working based on the information given.

 

[psychedomination]

@XR250/@MIStructE IRE thanks and I was kind of leaning toward this view as well. I am somewhat early in my career, so I am still developing my engineering judgement and trying to determine when something can be a quick answer vs the need to charge a client for an in-depth analysis. For a hole this small though, I was leaning more to the quick answer, especially if the holes are laid vertical instead of horizontal (resulting in a 6" to 8" max horizontal span)

@Dayrooster/bones206 thanks for your input it is greatly appreciated and I agree that some thought will always need to be given for scenarios like this; although I tend to overthink some of these very small things, which I am trying to work on. The red marks on the original photo aren't to scale - it will be quite easy to get a 12" clear distance from the existing opening to the edge of the cut out, which I will call out as a minimum. The building is framed with concrete-encased steel beams (you can see a portion of one in the far left of the image), which I believe are doing the majority of the work for the structure.

 

[bones206]

I was playing devil’s advocate a bit, but if it’s essentially a non-bearing partition wall it’s hard to see any way the cores would cause a structural issue.