URGENT: question about rebar detail around windows in basement wall

[DavidButler]

I'm in the process of building the basement for my own new home. As an engineer myself, I understand each engineer develops their own prescriptive or 'template' designs for certain common details. In that context, there's a detail in my structural engineer's wall rebar design for which I'd like a 2nd opinion. Not that it matters but my structural engineer is acting only in an advisory role, as I don't need a stamped design.

The basement is fully below grade with 6" poured-in-place walls just under 10' in height (9' below grade). The rebar mat is #5 in an 18x18 grid, with closely spaced vertical members on either side of the 60x48 windows, which will have deep window wells. My question is about the terminus of the lateral rebar members that abut the windows. The engineer's detail calls for 180-degree hooks wrapped around the twin vertical members (see attached diagram). My concrete crew somehow missed that detail and say they can't do this bend in the field (should have been done by the rebar supplier). The rebar matt is installed and the forms go up tomorrow.

I wasn't able to reach the engineer today so I contacted a couple of colleagues (builders) who have lots of basement experience and neither had ever seen that detail before, casting doubt on its necessity. I obviously want to avoid sending the forms crew home tomorrow and halting construction to have these members fabricated per spec. The site is rural so logistics is an issue (1-1/2 hr drive to rebar supplier). On the other hand, I don't want to ignore the detail. I would be grateful for any informed opinions.

 

[hokie66]

I doubt that with the information provided, any of us here can give you a definitive answer. However, I would want the hooks. You don't have to take out all the reinforcing. Just get lapping bars with the hooks on them. I wouldn't give credibility to what your builder friends have seen or done.

 

[KootK]

My experience has been in the US and Canada for the most part. I've seen a lot of typical "opening in wall" details and most that I've seen have not included the 180 hooks on the horizontal bars. And, frankly, I don't see a clear need for the hooks. Were it my project, I'd let it go. Engineering's really a game of "good enough". The hooks probably are "better" but I'd certainly consider no hooks to be good enough, especially if allowing no hooks is going to save money.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Agree with hokie, not enough information to know whether they're required or not. However, if I were to make a complete guess in the dark I would suspect you can omit them. The better way is as hokie suggested, to bend hooks with a straight lap splice adjacent to each bar that was supposed to be hooked. Should be cheap and quick.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[spieng89]

Also I would add it'd be pretty tough to bend #5 bar in that way within a 6" wall. Never seen it done in my short time as an engineer. Doesn't seem constructable in the first place.

 

[KootK]

Yeah, I was noodlin' on the constructability piece too. 6" isn't a thick wall and a the width of a 180 fiver will be in the 4-5" range. That said, I imagine that you could swing the hooks vertical-ish and get it done. As others have alluded, at this point, the hook would probably have to be a second piece rather than something existing bent in place somehow. All that said, the hook violating cover and becoming a vector for corrosion is probably the worst possible outcome here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I might add it is pretty tough to get rebar located accurately in a cast-in-place 6" wall - especially with 9 ft. of backfill on it. There ain't a whole lot of wiggle room. You might want to try to move the forms out 2" to turn this into 8" wall.

 

[jayrod12]

My own spec for openings in residential walls is just additional straight rebar on all sides that extends a minimum distance past the opening. No issues to date that I know of.

I will add however, that I would never allow for a 6" basement wall. Especially not at the height you're talking about.

 

[JoshPlumSE]

Per ACI it is possible to bend a horizontal #5 bar like that for a 6" wall. You need 4db for the inside bend diameter = 2.5", plus 1db on either side for a total of 3.75". But, that only leaves 1.25" of cover on either side (assuming verticals are centered).

I agree with others that the hooks are probably better. But, I don't know that they are 100% necessary. Though it is tough to tell.

 

[dik]

In these environs, 6" walls are too thin for 9' of backfill. At openings I run horizontal and vertical wall steel past the opening by 2'.

Dik

 

[BridgeSmith]

There are 2 reasons that a hook would be required - 1) To develop the bar for a high moment applied within the straight bar development length. 2) To provide confinement for the vertical bars so they can be counted for compression capacity.

Neither of those seems likely for a basement wall.

 

[DavidButler]

Wow, I appreciate all the replies!

There were a couple of logistical reasons that would have been much more consequential than concrete cost that drove my desire to go with 6" if at all possible. The engineer's first pass at 6" thickness had two #4 rebar matts, which he agreed would be difficult to build (my concrete crew went further on that point!). He was able to get 6" to pass with a single matt by going with #5, but with the caveat that there would never be any vehicular loads within 5 feet of the wall. This was an easy trade for me given the site design, and given the challenges of going with 8". Also, the floor trusses will be set inside the wall instead of on top and will be heavily anchored to the wall. As I understand it, this creates a stronger diaphragm. I had other reasons for wanting the floor to be flush with top of wall, although I originally planned for trusses to bear on the furred out framed walls.

BTW, the engineer had the benefit of geotech testing so he didn't have to assume worst-case soil conditions. His original 8" design had the same hook detail, suggesting that's a template detail he uses rather than dictated by the loads on a project basis. Also, I erred in my original post... the wall height is 9'4" and the below-grade depth varies from 7' to 8.5'.

Thanks again for your opinions!

 

[DavidButler]

@dik wrote: > At openings I run horizontal and vertical wall steel past the opening by 2'
@jayrod wrote: > My own spec for openings in residential walls is just additional straight rebar on all sides that extends a minimum distance past the opening.

My design already has the extra full height vertical rebar on each side. The openings are 21" below top of wall so the design has continuous horizontal rebar at 2.5" above opening and 2.5" below top of wall. It would be easy to lap an extra horizontal member onto that first rebar above the opening and extend 24" beyond. Is that what you guys are talking about? I wouldn't think it would help to double the horizontal rebar below the opening, but I could add that as well.

 

[DenverStruct]

Does not seem necessary but it is not impossible. The hook does not have to be perpendicular to the wall. It can be installed in an angle to fit the wall better.

 

[NS4U]

It’s a good detail. I wouldn’t be shocked if it was necessary too... a 6” wall is pretty thin for 8.5 ft of retained soil. Bars may be working hard. I’m used to seeing 10” residential basement foundation walls.

In the final condition you may have no trucks near the foundation. But what about the temporary condition? Does the contractor know about this requirement that vehicles be 5 ft away?

Also just an FYI- When does the contractor plan on backfilling? Especially important to get concrete up to strength (probably 75%) with such a thin wall. Probably will take 2 weeks.

 

[DavidButler]

@NS4U, I'm overseeing the project myself. The only impact will be during backfill. I'm guessing the bobcat can position 90% of the dirt without violating the 5 ft restriction. I'll have a couple of extra men on hand to assist with bringing backfill up to final grade adjacent to the house.

 

[dik]

If you only have 21" then provide a hook at that end... Found a sketch... don't ususally use diagonal bars, only for large round openings.

Dik

 

[DavidButler]

@Dik, Section A on your attachment answers my question about positioning double perimeter rebar. Thx. The 21" distance I mentioned is to top of wall (beginning of above-grade framing). I think those hook prescriptions are where there's a nearby adjacent surface that's perpendicular to the opening.

 

[XR250]

I'm overseeing the project myself. The only impact will be during backfill. I'm guessing the bobcat can position 90% of the dirt without violating the 5 ft restriction

Make sure you have the floor system in place before you start backfilling as that is what provides the bracing for the wall.

 

[dik]

xr250... in particular with a wall that thin...

Dik