ACI 352 1

[WARose]

I am doing a reinforced concrete joint. As I understand this, the (capacity) values for joint shear given in ACI 352 are pretty much the maximum values you can use......right? I ask because I have never been sure if the addition of more confinement reinforcement/ties would be a acceptable way to increase joint shear strength.

In a normal beam......you can add shear reinforcement up to a point......I'm just not sure where that point is in ACI 352. It may be that the limit is already there based on confinement steel equations you have to comply with.

 

[KootK]

Yes and no.

- if you're using the ACI 352 methods, I take those to be the maximum values.

- I don't know that the use of ACI 352 methods prevents you from using other methods if you wish.

- were I to go beyond ACI 352 capacities, I'd want to be able to hang my hat on some supporting documentation beyond "design by fundamentals". One assumes that the ACI 352 stuff is an good reflection of available capacity and relatively state of the art. Strut and tie would be the obvious choice but I suspect that a) you've not got time/fee for that and b) the capacity might actually calc out worse.

Good on you for designing your joint in the first place.

 

[WARose]

were I to go beyond ACI 352 capacities, I'd want to be able to hang my hat on some supporting documentation beyond "design by fundamentals". One assumes that the ACI 352 stuff is an good reflection of available capacity and relatively state of the art. Strut and tie would be the obvious choice but I suspect that a) you've not got time/fee for that and b) the capacity might actually calc out worse.

It would have to figure worse (than anything I am aware of). ACI 352 gives the joint strength as Vn=γ√f'c(b_j)h.

γ can equal 20, 15, or 12 (for Type 2 joints) depending on if it is a interior, exterior, or corner joint. That's more strength than a typical RC beam is allowed period (concrete+steel; effectively, γ maxes out at 10).

 

[WARose]

Kootk, let me ask you this (get your opinion on something).....I was looking at the joint detail this morning and I had a idea about the beam/girder skin reinforcement passing through the joint.....I was thinking: if it's lightly stressed, why not use that to satisfy the transverse reinforcement requirements (Ash)? (See attached pic.) But then I started thinking: nah. Yes it crosses, but it doesn't have that 135 hook that ACI 352 talks about......but then again, it is developed on both sides of the joint......what do you think? (This is kind of a reach.)

 

[WARose]

I've been all through the ACI 352R-91 & ACI 352.1 R-11 (slab & beam to column joint connections)......and I really cannot justify (in my mind) going with what is in my last post. Thinking about the mechanics of it.....there is nothing stopping that skin bar from popping out laterally (along an edge). That is, outside of stirrups in the other direction. And if you want to say it's providing confinement.....it can't do that.

The 352's all kind of say: either you have confinement by a beam/slab on all sides.....or you need ties.

In any case....look forward to your opinion Kootk.

 

[KootK]

Let's start with this: why is it that your four beam cannot provide confinement? Me no understando.

 

[WARose]

Let's start with this: why is it that your four beam cannot provide confinement? Me no understando.

It's a very long and complicated story.....but basically: it's not just one I am looking at here. It's a bunch. To ask about every condition (i.e. cases where the slab is on one side and nothing on the other, beams on all 4, narrow beams on 2 sides, etc ,etc)....we'd be here forever. So this is just kind of fundamental with an idealized case.

I just haven't come across any R&D that calls this rebar arrangement "confinement".....and I have a sinking suspicion it's because of what I was talking about in my last post.

 

[KootK]

Like shown below? If so:

1) I definitely think that you get some measure of confinement but;

2) For the reason that you mentioned, I've no idea how to prove that it's enough confinement.

Maybe you could make your standard detailing be #14 skin bars. Raise a few eyebrows.

 

[Lomarandil]

We could justify this idea (as shown in KootK's sketch) for very small columns.. certainly we say that column bars can be confined by a straight stirrup leg if within 6" each side of a stirrup hook/corner. So for columns less than say 12-15" in width -- I'd give it a pass.

But I'm guessing you all are working with columns and bars bigger than mine (300x300mm and 12mm diameter).

----
just call me Lo.

 

[WARose]

Thanks Kootk. That's what I suspect as well.