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Diagonal Bar at Opening or Not

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LearningAlways

LearningAlways

Structural
Aug 17, 2014
68
US
I worked for a company that did NOT put diagonal bars at corners of openings. They were precasters but as a standard they used only one horizontal and one vertical bar.

I'm checking shops for a tilt-up company and they add these bars AND the vertical and horizontal bars as well.

I don't think they're necessary. If your reinforcing is developed at corners of openings and there is one vertical and one horizontal bar at that location then you would get equal cracking resistance (bar tension) from the resolved components.

Just curious what is common outside of my logic and what I've seen.

They are a waste of money, material, labor, and time in my mind.

Diagonal bar or not to diagonal bar... that is the question...
 
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I've been down this rabbit hole. My first EOR jobs were in the US where the diagonals were always installed. When I returned to Canada, the firms that I've worked with here do not do the diagonal bars. And, as far as I can tell, there are no consequences to that. And, like you, I've had the same experience with precast.

Mechanically, I'm sure that diagonal bars are more efficient. But, then, it's not as though those bars ever get calc'd out. And congestion is reduced by eliminating them. So my personal position is that the diagonals are superfluous.
 
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I use them 100% of the time for crack control. I use them in two layers, even if I only have one layer of steel in the wall. I believe you keep cracks tighter this way.
 
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It's typically shown on many legacy details. I have questioned its efficacy when it's typically placed in a 3rd layer far from the crack you are trying to stop. However, I continue to show it because there's likely going to be lawyers that will point to that missing bar when something cracks and say, "See, you didn't provide this cure-all crack control bar".
 
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I don't use them... too much congestion in an area of high stress... horizontal and vertical bars are adequate... got into a huge discussion about 5 years back on the blog the company I worked for on this same subject... common practice, but not good practice, IMHO.

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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I use them at re-entrant corners in lightly reinforced slabs on grade - that's about it.
 
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Yup... I do that too... only place

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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I've got one more question. What is the history of these bars in walls?

Are they only there to prevent cracking at re-entrant corners?

As a general rule, I feel comfortable shifting a #5 bar no more than 6" from the edge of an opening. Usually the shifts are due to plate studs or whatnot but I'm trying to determine how far away I can shift these away.

But in order to do that I need to gauge the intent of the code. ACI provides no commentary on this requirement.
 
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In my experience:

- 99% of the time it's just the cracking.

- Once in a while it's to improve opening joint capacity for something like a view wall shear wall in a parking garage.
 
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LearningAlways said:
As a general rule, I feel comfortable shifting a #5 bar no more than 6" from the edge of an opening. Usually the shifts are due to plate studs or whatnot but I'm trying to determine how far away I can shift these away.
Click to expand...
Interesting question.
I think I have read somewhere that it should not be beyond slab effective depth, but no reference though.
 
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LearningAlways -

Thank you for starting the thread. I love it when I see a thread like this.... on a topic that I've wondered about, but never done a "deep dive" on.
 
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ACI continues to show the diagonal bar. The snip below is from their latest MNL-66. For what it's worth, ACI thanks Burns & McDonnell for the details (so it's really theirs that ACI may have adopted)
Wall_Opening_oxnifb.png
 
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I wouldn't install any of the diagonal bars in that elevation.

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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When I worked on nuclear power projects, a lot of attention was paid to diagonal bars at the corners of openings. I spent a lot of time developing various bar bend details to cover every possible wall opening scenario and make sure diagonals were developed on both sides of the corner. The attitude wasn’t that they were a waste of money, but that they were cheap insurance. That being said, cost isn’t really a design consideration for nuclear plants and rebar congestion is the norm. Burns and Mac is in the nuclear arena so maybe that’s part of the legacy of those ACI details.
 
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