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).

 

[KootK]

1) I'm not positive but I suspect that you're right in that you do need a transverse tie to be considered tied from a code perspective. That said, it's pretty hard to see an interior bar buckling out with 5" of cover.

2) You can use untied compression bars if the area of the bars is less than 1% Ag. Unless you've got a lot of reinforcing, this is probably your out.

3) I'm surprised that a central mat would actually benefit you much in a 10" wall system. Is it more for axial resistance or flexure? How much reinforcing are we talking about?

 

[JAE]

2) per KootK is what we almost always do - keep the wall reinforcement below 1% whenever we can.



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[BridgeSmith]

I'd just bump up the concrete strength so you don't need the steel to carry compression.

 

[WARose]

2) You can use untied compression bars if the area of the bars is less than 1% Ag. Unless you've got a lot of reinforcing, this is probably your out.

I've always wondered about that section of ACI 318. (Sect. 14.3.6 in the 2011 code.) It seems to green light it.....but Chapter 14 refers you to Chapter 10 which requires ties....so I've never been 100% sure there.

3) I'm surprised that a central mat would actually benefit you much in a 10" wall system. Is it more for axial resistance or flexure? How much reinforcing are we talking about?

It's got #9's @ 12" (vert.). So that's less than 1%. It had something else (that is gone now) sitting on it at one point.

And to answer your question: yes, it is more for axial resistance than moment......as the new load will be concentric but heavy. Thanks.

 

[KootK]

It's a pretty short wall (4' high, 10" thick).

It's got #9's @ 12" (vert.).

You'll be pretty close to the bottom of the wall before you even develop those bad boys.

 

[jayrod12]

Can you do a 250 wall with a single layer?

I'm certain the CSA code mandates two mats for walls greater than 8"

 

[MrHershey]

2) You can use untied compression bars if the area of the bars is less than 1% Ag. Unless you've got a lot of reinforcing, this is probably your out.

Unfortunately this doesn't appear to be true anymore. Depending on who you ask it may have never been true but code language was fairly confusing.

Screenshot from ACI 318-14 below. This language is different from previous renditions.

 

[BridgeSmith]

Do you really have enough load to need compression steel?

 

[WARose]

I'm certain the CSA code mandates two mats for walls greater than 8"

I am using ACI 318-11. And section 14.3.4 mandates 2 layers when you get more than 10".

Screenshot from ACI 318-14 below. This language is different from previous renditions.

Thanks for the info Mr. H. 318-11 words it this way:

14.3.6 — Vertical reinforcement need not be enclosed
by transverse ties if vertical reinforcement area is not
greater than 0.01 times gross concrete area, or where
vertical reinforcement is not required as compression
reinforcement.

So newer codes are chopping that one off at the knees.

Do you really have enough load to need compression steel?

Yes sir. We are talking Heavy with a capital H here.

 

[BridgeSmith]

Wow! We don't see line loads like that even on our bridge piers.

What about higher concrete strength?

 

[KootK]

Unfortunately this doesn't appear to be true anymore.

Wow... thanks for bringing that to my attention. Some thoughts:

1) If the old interpretation was incorrect, there must be a whole lot of improperly designed bearing and shear walls out there. Lot's of folks have leaned on this.

2) I'm not sure that I understand what reinforcing this provision is aimed at now. Is it out of plane, flexural compression, wall bars that don't need ties? Gosh... thanks. Or is it meant for flexural tension bars assuming that you wouldn't want them to buckle under axial/creep before you get a chance to use them in tension?

 

[WARose]

What about higher concrete strength?

May test to get that. (Remember: this is a existing wall.)

 

[haynewp]

I would have been hesitant to use the vertical bars for compression prior to the rewording at the less than 1% anyway. There is no mention of confining cover required so the bars could have been placed close to the face of one side of the wall without consideration of buckling blowout of the near face.

 

[BridgeSmith]

Oh, sorry, I missed that it was existing. Yeah, can't very well spec. higher strength now...

 

[JAE]

That new section 11.7.4.1 in ACI 318-14 is STUPID. There are millions of square feet of reinforced concrete wall out there that takes axial load and have no lateral ties.
That paragraph, in my view, is a perfect example of how utterly inept some of our code writers are.
I realize that there was public review, etc. but to write this in with NO commentary as to the change is unbelievable.



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[KootK]

but to write this in with NO commentary as to the change is unbelievable.

X2. First thing that I did after Hershey's post was go hunting for that non-existent commentary.

When I get to 1000 stars, I get to meet BAretired for lunch. Current start count = 790. Just sayin'...

 

[WARose]

Agreed JAE. You'd think they would (at least) reference some testing in commentary.

 

[JAE]

I fear that someone re-wrote the paragraph in a reverse order and then forgot to change and "or" to an "and".

I'll post a question to ACI and see what they say.



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[MrHershey]

That paragraph, in my view, is a perfect example of how utterly inept some of our code writers are.
I realize that there was public review, etc. but to write this in with NO commentary as to the change is unbelievable.

Will editorialize a bit here. But in my discussions with a member or two of ACI 318 and the fact that I've not seen this called out as a change in any of the educational material I've reviewed, I get the sense that they believe: a) it was a grammatical change and thus not substantive enough to highlight or comment on, all that was done was fixing the double negative from the previous codes; and b) it has always been required to tie any bars you're using in compression.

I don't think I agree with either point, but that's the sense I've gotten. Probably bolstered by the fact that very few people have noticed the change or made a stink about it (though them not highlighting it certainly doesn't help). The individuals I talked to did acknowledge that the language should probably say 'compression strength' instead of 'axial strength'. I don't believe this was updated in the ACI 318-19 public draft though.

And speaking of ACI 318-19 public draft, I believe they added an exemption for basement and retaining walls to get them out of some of the wall requirements, including this one.