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ACI Wall Corner Diagonal Bar

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cpb213

cpb213

Structural
Apr 22, 2019
7
US
Hi All,

I have a detailing question about the diagonal bars shown in the ACI Detailing Manual's details for Reinforcement at wall corners. I do have a water retaining structure and are including them in the details but I can't find any good description of how these bars should be detailed.

Does this bar just need to be developed on each side of the corner or does it need enough length to be considered a splice to the exterior rebar? Does the tail after the bend in the bar need to be a certain length (like a standard hook does)? Any help would be appreciated.

Capture_kyjnco.jpg
 
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I have seen this type of bar used in retaining walls, but not in a wall corner (I have learned something new today[smile]).

My understanding is the bar extension is simply a standard hook, not a lap splice.

DaveAtkins
 
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I'm with Dave. I do not lap the bar extensions into the wall bars. I just have a 12" extension.

I use these diagonals on water retaining structures with walls thicker than ~14 inches(EDITED). I've found that the corners get very congested when you try and put hem in on thinner walls. (although, I have seen details from other engineers where they have them on smaller walls)

In my mind, the bar's purpose is just to prevent a crack from forming right at the corner. Thus, I select #4 bars; even if the regular wall bars are significantly larger.

Below is the math that I use to figure out the bar dimensions......... No one has ever back-checked it; so feel free to let me know if I screwed up the trig. :)

Corner_Bar_Math_gizlka.jpg
 
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Check out the concrete international article "Corner detail for wall horizontal bars" Also I would keep it simple if you try to get fancy the contractor will misplace the bars

image_jfikbx.png

image_vairhw.png
 
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No idea why 'a)' is no good... and I would never use 'd)' or 'e)'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Dik - 'd' and 'e' are the CRSI recommended ones lol

'a' is bad for constructability because you have the long hooked bars, which I think can be difficult to align and get tied up. (or you have to do a series of laps immediately after the corner)

On thinner walls, I use 'd' and I like 'e' on thicker walls. The rodbusters can assemble the cage and then just slide in the U-Bars at the corners. You need enough wall thickness to make the 180 bend in the bar though.
 
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Agreed, its all about constructability. Imagine threading the corner bars on a huge mat with #8 bars.

image_jumhzm.png
 
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I've not found it to be a constructability issue... they are easy to place and can be tied to the horizontal bars in the opposite wall. What you want to avoid is having bars with two hooked ends, then there is not adjustment for slightly off forms. I think it's bad terminating bars at corners because if any shrinkage, due to the restaint, it is likely to initiate a crack. Just my $.02...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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The efficiency ratings in GC Hopi's post are based on testing by Nilsson and others. The double U bars are simple to place. The corner bars are belt and braces in opening corners.
 
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Hokie66, you beat me to it...good work.

Title: Reinforced Concrete Corners and Joints Subjected to Bending Moment
Author(s): Nilsson, I. H. E., and Losberg, A.
 
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thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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To answer the OP too: I make sure the rebar is developed across the critical crack location, with usually enough to tie it around the horizontals for placement. Typically I specify 12” on the bent leg overlapping the horizontal bars. This isn’t a fail safe but works in most cases. I just check it quick before it gets issued to make sure the typical detail works.
 
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The MacGregor text also repeats what Hopi posted.
U-bars are easy (in relative terms) and reduce the number of bar planes vs individual hooked bars. However, unlike what e) shows, the "main" wall bars should not, and do not need to extend into the corner - the horiz bar lap should be away from the joint.
For water retaining boxes, the diagonal bar not only minimizes cracking at opening corners but also helps with the combined tension-shear problem, where the "bursting" tension is seriously derating your one-way shear capacity.
 
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