Egress Window in Concrete Basement - Plain Concrete Header?

[Sokka10]

Hi all. I've got a potential project where the owner wants to replace an existing 24"x24" window in a basement bedroom with a 48"x48" window. It is an 8" thick concrete wall and the load coming down from roof and floor above will be approximately 1 kip/ft.

Obviously they would need to cut the foundation wall for the wider and taller opening. My question is regarding the concrete above the opening which is currently 12" in depth. The home is only 6 years old so I assume there's some reinforcement in that 12" piece over the opening, but I can't count on it. My question is if I could treat the concrete as a plain concrete header? ACI 318-14 states in Chapter 14 (Plain Concrete) that plain concrete is permitted in a) members continuously supported by soil, b) members where arch action provides compression under all loading conditions, c) walls, and d) pedestals. To my understanding my "plain concrete header" would not fit in any of those cases. However, I have seen in other threads the mention of "unreinforced beams" so I think some of you are doing this when in dire straits.

In a new project I wouldn't even think about a plain concrete beam, but with this project I think it could be the way to go. I'm not sure what I would do if the plain concrete route isn't kosher. Would appreciate any feedback you all have.

 

[dhengr]

Sokka10:
You don’t know the conc. strength or what rebar is in the wall. You have to pay special attention to how the wall is attached and supported up at its top, to the fl. framing and fl. diaphragm, at each of the opening jambs, added A.B’s. blocking, etc., and hope there is some vert. rebar in the wall near these jambs. Study these things and make an engineering judgement on what to do. Then, take a look at adding a couple 2x’s or LVL’s to the back of the rim joist (rim board) so that that built-up beam acts as a header over the opening, properly connected, of course. Then, you’ll put a rough 2x wooden buck in the opening, and you might cut the opening full height.

 

[Rabbit12]

What happens when your plain concrete beam cracks?

 

[MotorCity]

Check it as a plain concrete beam. If it works by a large enough margin, run with it.
Concrete can take tension up to its modulus of rupture. If you only have 12" of concrete above the opening, I would be suspicious of the numbers checking out.
Obviously it would be different if you have 5' of concrete above the opening. Let the numbers and your judgement guide you.

 

[OldDawgNewTricks]

You have already established that plain concrete is not allowed for your situation. I would not intentionally violate the code unless I had a damn good reason and there is no other option. In your case, I think you have plenty of other options.

 

[Sokka10]

I appreciate the responses. I was able to go see the home in person yesterday and wanted to post my findings here. The basement is unfinished so I was able to see a lot of good stuff.

The existing floor joists are 12" TJI and run perpendicular to the opening in question. Anchor bolts are spaced at 32". There is 2x sill plate and rim joist over the opening.

Take a look at adding a couple 2x’s or LVL’s to the back of the rim joist (rim board) so that that built-up beam acts as a header over the opening

So the idea with this is to cut the existing floor joists back, add some additional plys to the existing rim, and then provide hangers from the joists to the new "beam", correct?

What happens when your plain concrete beam cracks?

I'm just spitballing with the plain concrete beam since I've seen it thrown around here on Eng-tips before (see MotorCity's comment above). With a super light load and a rather small opening I was wondering if 12" of concrete would work pretty easily, especially since the original plans show that there should be a couple #4 bars above the opening. Obviously I can't verify that those bars are actually there, but if a plain concrete beam calc outs fairly easily it was maybe worth a shot.

I think you have plenty of other options

dhengr suggested cutting back the floor joists and beefing up the rim so that's an option. This thread here (Link) is where XR250 mentioned treating it as a plain concrete beam, which I agreed would be a good course of action if it is allowed. In another thread, KootK mentioned that some arching action from the concrete could work (Link). If it's not allowed, other ideas mentioned were cutting all of the concrete out and putting in a new wood beam, or putting in a steel angle.

So a lot of options, but not sure which route to take. Just wanted to see if any of you had some good/bad experiences with any of these methods. For example, to me the option of cutting back the joists and beefing up the rim sounds like a much more difficult process than say, sliding a steel angle in, but maybe I'm wrong.

 

[TLHS]

If you have documentation of what sized bars would have been used, you can do a bar scan to confirm that they're actually there

 

[OldDawgNewTricks]

This thread here (Link) is where XR250 mentioned treating it as a plain concrete beam, which I agreed would be a good course of action if it is allowed. In another thread, KootK mentioned that some arching action from the concrete could work (Link). If it's not allowed, other ideas mentioned were cutting all of the concrete out and putting in a new wood beam, or putting in a steel angle.

I read through the links you posted and I do not agree that the specific members you mentioned said that you could use a plain concrete beam in contravention of building code requirements. In the first link, XR250 just said to check the capacity of the band spanning across the opening. In the second link, KootK said that arching action might be possible, not to treat it as a plain concrete flexural member. Some other codes might allow plain concrete flexural members for your situation, but if ACI 318 is your governing standard then you probably don't want to violate the code if there are any other options available to you. If you can calc out the capacity you need from some alternative load path like arching, then you are good. If not, then put in some structural members or reinforcement.

 

[JungleJoe]

Here is a detail that I've used before. You'd need to check the steel angle size and bolts of course

 

[OldDawgNewTricks]

Sokka10, for your additional consideration here's what ACI 332 Code Requirements for Residential Concrete has to say on the matter...

This code may or may not be applicable, but if anything went wrong and you were sued, you might have a hard time arguing that you were following the standard of care by using a plain concrete beam.