RC Detailing for Wall Opening Near Wall Edge

[llocou]

I'm looking at a concrete upstand that I've been treating as a wall. It's about 1100mm tall, 200mm thick, and there are some openings of 1050x220 mm. Some of the openings are very close to a wall edge - leaving around 115mm of concrete from edge of opening to edge of wall. How would you go about detailing this? The 'standard' practice of proving half of the bars that have been broken on either side of the opening is a bit unrealistic (the wall has 2 layers of 16mm bars @ 150mm centres both ways, so would mean fitting in 2 layers of 4 bars into the 115mm). On top of this, 14.3.7 of ACI 318 dictates that two additional 16mm bars are required around the opening in a wall with 2 layers of reinforcement. On top of this there's the good practice of providing diagonal bars, but being so close to the edge of the wall means there's no space for a diagonal bar, and simply bending it to be vertical would be an additional vertical bar on top of all the vertical bars I've already mentioned! I don't know if I'm overthinking this for a 'wall' that's so short, and isn't carrying any significant load above it. Any views would be appreciated, thanks.

 

[TehMightyEngineer]

My approach would be to design it as a discrete column. The wall will be stiffer and should take the majority of any lateral loads and will take the majority of the tributary loads; you'll likely be left with a column that only needs minimum code reinforcement. Continue the stirrups with the longitudinal bar development length into the wall above and below the opening (hooks help reduce this length to a minimum) and call it a day.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

115 mm is nothing to work with structurally. If the comdition is unavoidable:

1) Ignore all the detailing protocols as they wont help you.

2) Keep two wall vertical bars in the edge bit.

3) Accept that the edge bit will probably crack a fair bit under shrinkage strain.

4) If this is a beam to any extent, design it as though there's no concrete above the opening at all as that concert will be useless.

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]

Yep, ignore my recommendation as I missed the width you had to work with. I was thinking you had at least 200 mm or something.

I'd probably reinforce it as KootK indicated and design it as a plain concrete column to support the tributary load. I'd do everything I could to push load away from this "post" as I could. I'd also be worried about cracking at the corners so I'd bend one of the 16 mm bars as shown below:

If you actually need this to take some load then I'd either try to find some more concrete somewhere or look at more exciting ways to reinforce this.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[llocou]

Thanks both, very helpful. I've managed find some more space after speaking to M&E and the minimum concrete I now have is ~215mm. I've attached a sketch of what I think I'll do. I can now fit 2 vertical bars in. Unfortunately I don't have space between openings for a diagonal bar so it'll have to be 90 degree bends at each corner, which I'll continue ld beyond the opening. I've only showed the wall bars at extremities for clarity. The U-BAR detail is one that's been used throughout the project and was shown on a previous revision, so I'd rather just leave that in.