Reinforcement of Openings in Existing Residential Basement Concrete Walls

[Craig_H]

Hi all,

First off, thanks to everyone on this forum for making it a fantastic place to bounce ideas off of each other, and gain knowledge through past threads. I have searched the forum for this topic, but cannot seem to find a good discussion of the specific situation that I am currently facing. I've been lurking around eng-tips for the better part of a year, but have not been very active in posting.

I work in a region with a severe crunch on housing. This has spurred many home owners to develop their basements into additional accommodation. In concert with that comes the need for means of egress from bedrooms, and the requests for creation of window openings in existing concrete foundation walls. These walls are generally constructed with the minimum reinforcement required by Part 9 of the Alberta Building Code at the time of construction. Let's take a typical wall as being 200 mm [8"] thick, with 10M reinforcement 300 mm [12"] on-center both vertically and horizontally. As I am sure many know, there is often less than this amount of bar in the walls.

With the hand that I am often dealt, the concrete does not contain sufficient reinforcement to simply cut an opening and design the existing concrete above as a reinforced lintel/header. CSA A23.3 also does not appear to allow the use of plain concrete in this situation. Cutting up to the top of concrete and installing a wooden header is often not an attractive proposition for a suite of reasons (grade elevation, depth of basement and wanting the window low enough to climb out of, etc).

What suggested solutions does the community have for retrofitting reinforcement around window openings in plain concrete walls? I have been toying with the idea of carbon fiber reinforcement on the exterior of the wall, and would be interested in hearing field-tested feedback on how that product has turned out. Also, suggestions on how to protect the fiber from future damage would be very welcome.

Thanks all!

 

[bones206]

First off, I'm a residential design newbie, so take this with a grain of salt. But I think it would be possible to retrofit a header beam within the depth of the rim joist (i.e., a new header bearing on top of the wall).

 

[Canuck65]

The 'typical' basement, below grade, residential foundation wall in my area (Ontario) would be a 200 mm thick wall, unreinforced. The Ontario Building Code (and likely the National Building Code) has provisions for window openings in the unreinforced walls. There are size limitations to the windows. It would be a good idea for you to check the prescriptive requirements of Part 9 as that would be a starting point.

However, I do like the idea of reinforcing these openings, even when they are small. The lateral forces on the wall are from soil pressure on the exterior. In order to support the wall we typically rely on a wood framed stud wall on the interior. The wood framed stud wall is built tight to the interior face of the concrete wall and designed to support the lateral pressure. A horizontal wood beam can be placed between the vertical posts either side of the window opening, just beneath the window opening. The connections of the wood framed members, top and bottom, are critical components. Essentially, soil pushes on concrete wall, concrete wall pushes on wood framing, wood framing transfers forces to slab-on-grade at bottom and wood framed floor at the top.

 

[Craig_H]

Thanks for the input thus far.

I think it would be possible to retrofit a header beam within the depth of the rim joist

Indeed, this is often an attractive solution. In this specific instance, we have the floor hung inside the foundation wall, which makes cutting the window opening to the top of concrete difficult at best (pretty much impossible, in my humble opinion). Please see the below image. For this specific discussion, I am trying to come up with a good solution to creating an opening with a concrete lintel above the window.

In order to support the wall we typically rely on a wood framed stud wall on the interior [...] Essentially, soil pushes on concrete wall, concrete wall pushes on wood framing, wood framing transfers forces to slab-on-grade at bottom and wood framed floor at the top

Interesting... I generally design my own foundation walls to stand alone, and have not designed a composite wall as described. Seems as though you are mainly using the concrete as sheathing to a wood foundation.

I see a couple of solutions to reinforcing a concrete opening in an existing plain concrete wall:
[ol 1]
[li]Cut the initial concrete opening higher than desired, dowel in flexural reinforcement into the sides (joined in the middle with a coupler) and stirrups into the top, and pour a new concrete beam. See the below rough sketch of the basic idea. This seems pretty labour intensive, would require reshoring and sealing of the window opening during concrete cure, and would overall be something that I can see a contractor & client giving me grief about.

[/li]
[li]Reinforce around the opening with fiber & epoxy product, cut the opening, and install window. This may be pricey and require training of the contractor, but seems rather slick. An added benefit is that the reinforcement can be installed prior to cutting, which would help it pick up flexural stress when the cut is made. I would really like to hear from anyone with experience with this type of product, and thoughts on this use of it.

[/li]
[/ol]

 

[KootK]

1) I don't see a residential contractor being willing to do the fiber reinforcing thing. Who knows though, I've been surprised before.

2) For vertical load, I think that you could get by with some arching on this. See below for some ideas.

3) For earth pressures, much depends on how much solid concrete you have on either side of the opening. If there's lots, you might be able to justify doing nothing.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[KootK]

Actually, upon reexamining your section, I don't think that you'll have any trouble with the ledger bolts. They appear to be well above the top of the opening.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[bones206]

One other knock on fiber reinforcement is that it lacks fire resistance. But I suppose that could be mitigated if it’s protected by a gypsum wall.

Another header option is to bolt a channel across the opening with adhesive anchors.

For the lateral pressures, you could check the wall by assigning half the pressure load tributary to the window to vertical strips on each side of the window, then check the stresses. If reinforcement is needed maybe you could place a vertical angles/channels on each side, fixed at the floor and ceiling. All the steel members could be installed prior to cutting the opening.

Using wood studs as reinforcement seems dubious to me because they could be removed for a number of reasons down the road and the homeowner or some other contractor would not suspect that they were being used to reinforce the concrete wall. Steel members would be more obvious that they are not to be messed with.

 

[BridgeSmith]

For out of plane bending of the wall, could you just use flat steel plates as a frame around the opening, with drilled anchors (Tapcons?) through the plate into the wall?

 

[Craig_H]

Thanks for all the input, everyone! KootK, your suggestion or arching action is great. Good call on the anchorage for the ledger system, I had not considered that!

 

[Settingsun]

I don't think I've ever been in a residential basement, just underground car parks in big buildings, so bear with me as I ask a silly question to satisfy my curiosity. Where do egressing people go after crawling through the window? Are there stairs and one of these hatches?

https://i.stack.imgur.com/I0how.jpg

Is the wall spanning primarily horizontally?

 

[Craig_H]

steveh49: the windows are sufficiently large for a (small) person to get through, and generally open either above ground, or into a window well that has sufficient room for a person to climb out. The window wells are small retaining walls that create a depressed area around the window, and generally have a small french drain to keep them free of water. Residential basement walls are generally supported at the top and bottom by floor structures; exceptions include stairways and other floor openings against the wall. Does that make sense & clarify things for you?

 

[Settingsun]

Thanks, Craig. In your specific case, the window looks almost 2m above the basement floor judging by the images in your 29 August post. Is there a ladder on the wall to get up there?

Regarding lateral strength, is it feasible to enlarge the well to reduce the earth pressure sufficiently, or does the excavation become excessive before the numbers work?