Confinement Ties in Beams of moment frames

[e104909]

Hi,

I would like seek your opinion on when is it required to apply special confinement in beams for ordinary moment frames where you have to include inner links to support every other bar like what you have in columns.

My understanding is ACI detailing manual recommends special detailing practices for special moment frames and for structures in Seismic Design Category C, D & E. Its not very clear to me, or at least I cannot find the provision where it is suggested to be used in Ordinary moment frame in Seismic Design Category B.

I am confused whether I need to put special confinement ties in my beams, columns and shearwalls (where you have to include inner links to support every other bar)

Can you give some literature or code provisions that would support requiring confinement in beams for Ordinary Frame and Seismic Design Category B.

I attached two detail, detail A is the one I was thinking of doing based on shear and moment requirements. But detail B is where I added ties in the middle to make sure rebars are supported by vertical ties at least 6".

Thank you

 

[HTURKAK]

Still you should provide minimum ductility and toughness so, provide confinement.

I have ACI 318-14

Pls look Section 18.3—Ordinary moment frames for the requirements.

The following doc. will be useful to see the concept.

 

[e104909]

HTURKAK,

Thank you for the reference. I was looking at your response and I have to edit my post to make it clear.

I understand the confinement having a tie with 135 degree hooks. What I am trying to find out is if I need to put extra inner links at every other bars.

Lets say I have 7 top bar and 7 bottom bar. 1 single ties is enough to carry the design shear. BUT, do I need to add another inner link like the attached?

 

[HTURKAK]

No !!!!!! You do not need another inner link.. One link covering all rebars is OK.

 

[Agent666]

I'd disagree with hturkuk, and say it depends on the width of your beam relative to the effective depth. You need links for shear to be relatively evenly spaced across the cross section in wider shallower beams otherwise you risk longitudinal cracks along the section due to the internal compression struts acting across the cross section being very shallow to get to the edge of the members, resulting in tension across the tension face and increasing the liklihood of the beam face cracking along the beam axis.

Codes also have requirements regarding spacing across the section for main bars and every second bar requiring restraint for confinement and antibuckling in beams. I'm unsure what ACI318 exactly has to say on the matter (which I know is what you're asking). But it's common practice and a code requirement round these parts, especially if you're actually utilising compression reinforcement for strength (for example if it is actually in compression when you analyse the cross section) for non seismic cases, and always for seismic where loss of cover concrete might occur under cyclic loading where you really need the confinement.

I've seen some people say, we'll I don't technically need the compression reinforcement (for strength), so I don't need to restrain it. But in my opinion if you've provided it, it may be in compression so needs the restraint required by codes if it actually is.

https://engineervsheep.com

 

[Agent666]

Clause 9.7.6.2.2 in ACI318-19 is what I'm referring to at the start of previous reply. You need to satisfy these spacings across the section width.

https://engineervsheep.com

 

[HTURKAK]

I agree with you.. it depends on the width of beam relative to the depth .. Doubling the stirrup is also necessary when the stirrup spacing along the length is less than 3 in. for placing the concrete. The sketch that OP provided implies the aspect ratio b/h around 1.5 ..

I have ACI 318 -14 and i copy and pasted the relevant section at below;

Apparently this item revised at ACI 318 -19.

If the OP can clarify the size of the beam , nominal shear strength provided by shear reinforcement (Vs), we can see the max. spacing requirement across the width acc. to ACI 318 -19.

 

[e104909]

Thanks guys. I think this is very helpful. I wasn't even looking at the latest ACI 318-19 and it provides a better clarity on this matter.



Regards,
E104909
Civ-Str P.E.

 

[e104909]

@HTURKAK,

is there a clause in ACI that talks about the width depth ratio?

@Agent666,

is this only applicable for hanger reinforcement for shear transfer?

I have 24 wide beam and the height is 40". There were 6-#10 bottom bars.
Av/s required from etabs is say 1. Vs = Av x fy x d / s = 1 x 60 x 37.24" = 2220kips. vs 4SRQT(6000) x 24 x 37 / 1000 275 kips

Regards,
E104909
Civ-Str P.E.

 

[Agent666]

Those ACI provisions I posted are for all beams and apply throughout the entire length of the beam. Hanger reinforcement requirements/detailing where more onerous may govern locally at intersecting beams and so forth.

The table I posted is the minimum spacing requirements only, other requirements such as shear strength, confinement, antibuckling and minimum shear reinforcement provisions may all govern depending on the situation. As a designer, you should obviously be checking all provisions as part of the design.

https://engineervsheep.com

 

[Agent666]

In NZ (my local concrete code), there is a limit for spacing across the cross section as noted below for applying what is essentially the same intent as the ACI provision.

You will note these provisions are more onerous than ACI, requiring d/2 spacing across the section, not d as for ACI low shear requirement. Under higher shear >>0.33*b_w*d*(f'c)^0.5, half the spacing applies, so d/4, but no less than 200mm as noted below.

https://engineervsheep.com

 

[e104909]

Agent666,

Also, is this only required in ACI 318-19 version or it is also in ACI 318-14. It is not very clear in ACI 318-14. (I mean the ties across the width)

Regards,
E104909
Civ-Str P.E.

 

[HTURKAK]

I did not see any clause at ACI 318-14 but, the commentary R 9.7.6.2.2 at the excerpt which Mr.AGENT666 posted provides the concept..My personnel experience , i provided double stirrups for wide beams ( ribbed slab beams etc ) and when the stirrup spacing along the length is less than 3 in. for placing the concrete.



It is not reasonable to say Av/s =1.0 .. In this case you are expected to increase the depth !!

Pls look 22.5.1.2

Cross-sectional dimensions shall be selected to
satisfy Eq. (22.5.1.2).

Vu ≤(Vc + 8 √fc bwd)

 

[Agent666]

Also, is this only required in ACI 318-19 version or it is also in ACI 318-14. It is not very clear in ACI 318-14. (I mean the ties across the width)

Not sure to be honest, we've had it in our standard since 2006, and it's the first time I've noticed it in ACI318-19. There's quite a lot of recommendations that have flowed through from NZ code into ACI318 based on our recent learnings in major seismic events and associated research projects, but these particular requirements predated the changes following 2010/2011 Christchurch earthquake sequences.

I think as a designer you should always be adopting current best practice (being the most recent code). Though I know in the US everyone puts their head in the sand and goes with whatever is approved by their jurisdiction and potentially continues doing outdated practices for many years until a new code is officially adopted.

Things work very differently in this part of the world, as soon as a draft is released some provisions are provisionally adopted on a best practice basis if its solving a particular shortfall in a requirement (this being one in my opinion).

Locally in NZ we've collectively as an engineering profession learnt our lesson regarding poorly detailed concrete structures in a way so this isn't a hard sell.

https://engineervsheep.com