Ties for wall vertical reinforcement 9

[WARose]

I have a feeling what the answer is on this one......but I'll ask anyway ()......Does anyone know of anyway (via a code anywhere) that allows you to consider a single layer/curtain of vertical wall reinforcement as "tied' via the horizontal steel? The way I've always read ACI code.....you need to have 2 layers/curtains of vertical steel, with ties between them in order to call it tied as per code.

Basically what I have is: a wall with a single layer of vertical reinforcement (and a horizontal layer as well). I'd like to use that steel as compression steel (for a vertical load on the wall).....but it's not tied. It's a pretty short wall (4' high, 10" thick).

 

[bones206]

I want to circle back to the original question regarding the horizontal bars acting as ties to restrain buckling of the verticals. The first images that came to mind were all the wall failures in the 2010 Chile earthquake. Check out some of those reports and photos and you can see that the horizontals did not provide much in the way of lateral restraint.

Link

 

[CURVEB]

Koot - great post. I second bones on copying this into the ACI commentary

 

[Settingsun]

JAE, I understand the frustration when these apparent 180s slip in without any commentary. Leaves you wondering whether they mean what's printed or

 

[JAE]

I received the following reply from ACI.

It confirms that the language in 318-14 was in error (“or” should have been an “and”).

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[JAE]

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[slickdeals]

This seems to be an interesting departure from previous editions of the code where the language seemed to imply that if you have reinforcement of 1% or more at wall ends, then you would have to tie them regardless of whether it was being used for compression or not.

I don't think current software have an ability to turn off reinforcement in the compression zone in strain compatibility calcs. I have a wall design spreadsheet that can do it, but I don't think commercial programs do it. The difference in some of the test cases I have run for shear walls of normal proportions has been around 3-5% (lower capacity without compression zone reinforcement).

 

[slickdeals]

ACI seems to be struggling to make up their mind on this. This is from the Q&A (January 2019)
"Regardless of whether the reinforcement is required for tensile capacity, compression capacity or both, note that lateral ties are required if the area of longitudinal reinforcement exceeds 0.01 times gross concrete area".

The clarification in ACI 318-19 seems to imply that you can have 4% reinforcement in the boundary zone of a wall but as long as you are not counting on it for compression, it doesn't need to be tied.

 

[JAE]

slickdeals - yes, tension rebar doesn't need transverse ties. Or am I misunderstanding you?

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[slickdeals]

JAE - I agree from a behavior/mechanics standpoint that tension rebar doesn't need to be tied and I think the modified 318-19 language clarifies that in a much better way.

The way 318-11 and earlier were written (IMO and based on other articles/literature) was that 1% was the trigger for transverse ties regardless of the following:
1. The reinforcement was in tension
2. Reinforcement is not counted on the compression side

 

[JAE]

I think you are correct.

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[KootK]

@slick: your primary interest here seems to be the shearwall aspect. You're not actually hoping to NOT tie tension only boundary elements in practice are you? Just an academic inquiry? For something with reversible demand, I live in fear of buckling the bars when they see compression, even when they're not needed for compression. It's not as though they'll refuse to absorb compression just because they've been told to. In a ULS sense, I suppose that you could buckle the bars and then just straighten them out with the load reversal. But then that raises all kinds of questions about aesthetics, lap splice performance, and overall integrity. As I mentioned above, I think that one could make a theoretical argument for not tying any zones, compression or tension, other than in seismic hinges etc. That said, I wouldn't be willing to rely on that in practice myself.

I'd like to chase down what level of bar strain we feel is appropriate for the expected bar buckling strain. I know that I've seen this before but I can't recall where. As with piles, I suspect that it takes shockingly little to brace a bar in practice and that a very high level of strain is required to produce a buckle/blow out. It would be informative to know how that buckling strain compares to, say 0.0035 etc. That way, one can gauge whether a tension bar falling within a concrete compression block can be expected to buckle or remain stable in the absence of ties.

 

[bones206]

Some research: Link

 

[KootK]

@Bones/CURVEB: thanks for your kind words regarding my previous post. When I dive deep like that I always fear that I've wasted 45 minutes of billable time -- and my life -- to produce something that's too oppressively long for anybody to bother reading (internet age etc). It's a comfort to know that the effort was not for naught.

 

[WARose]

[blue](Kootk)[/blue]

It would be informative to know how that buckling strain compares to, say 0.0035 etc. That way, one can gauge whether a tension bar falling within a concrete compression block can be expected to buckle or remain stable in the absence of ties.

New Zealand code (NZS 3101; which I think is available on-line for free) seemingly addressed this with some of their codes in the past. And that has been reproduced in some texts I have. In the NZS 2006 equation it is Eq. 10-40 & 10-41 for plastic hinge regions. If you'll notice, 10-40 includes axial load as a variable.

You might could back out of that with a strain.

 

[haynewp]

So why wouldn't this same 1% rule apply to masonry?

 

[Celt83]

you'd probably be violating the bar clearance requirements before getting to 1% steel in a masonry wall.

Open Source Structural Applications:

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[haynewp]

I mean if the reinforcing is less than 1% then why can it be counted on for compression in concrete without ties but not in masonry? ACI 530 requires ties for rebar anytime it is being counted on for compression regardless of percentage.

 

[JAE]

haynewp - ACI 530-13, section 5.3.2 allows for "Lightly Loaded Columns" where lateral ties are not required.

No mention of lateral ties in walls that I can see - except for pilasters. Am I missing it?


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[haynewp]

JAE

I am out of office now, the version I have at the moment is 2005. It has the below:

2.3.2.2 Compression
2.3.2.2.1 The compressive resistance of
steel reinforcement shall be neglected unless lateral reinforcement is provided in compliance with the requirements of Section 2.1.6.5.

 

[EZBuilding]

JAE

ACI 530-13 9.3.2 e

Axial capacity of reinforced masonry walls ignore any contribution which would be provided by vertical reinforcement.