Concrete Detailing Where an Integral Slab Depth Beam Supports an Incoming Slab

[KootK]

The Situation

- Solid precast plank as shown below. Heavy transfer slab loads.

- Normally a one way spanning thing but, at the supports, the thing has to span laterally over some louver openings that extend to the underside of the slab.

- Local gaggle of rebar placed in the ends of the plank to serve as lateral beams of sorts (tiny spans).

The Question

Good detailing practice, as I know it, would have the bottom bars of the supported slab run up over the bottom bars of the supporting "beam". That, so that there is no potential crack such as the one shown in green that does not intercept any reinforcing. I might accomplish such a thing in several ways:

1) Drape the slab rebar.
2) Lap the rebar with an offset piece that comes up and over the beam bottom bars, a bit like a 1:6 column rebar transition.
3) Simply adjust the layering and take the hit with respect to slab flexural depth and crack control.

I could do these thing but the question is this: do I need to? All of the proposals above would be a bit annoying to my precast client for various reasons.

Some Related Logic

4) For a more discrete, serious beam & girder situation, I would definitely do this.

5) In a way, a similar condition exists everywhere within a two way slab and we do not do this.

6) So is this closer to a serious beam situation or closer to a two way slab situation? My gut feel is that it is the latter and I'm inclined to not worry about this here.

 

[KootK]

Note that, for the purpose of this question, one needs to really consider the last sketch above without the bearing reaction as that reaction is not there when the beam is doing it's thing.

 

[phamENG]

I'm leaning toward 2-way slab. Your aspect ratio is nearly 1, and your reinforcement (excluding your edge beams) is identical in both directions. I don't think there's enough value gained to cause the precaster the headache.

 

[jayrod12]

What about a couple of hooked single legged stirrups over the louvers to act like hanger/shear reinforcement?

 

[phamENG]

I say identical - I realize the clear cover will be different resulting in slightly lower capacity in the transverse direction.

 

[KootK]

What about a couple of hooked single legged stirrups over the louvers to act like hanger/shear reinforcement?

I say identical - I realize the clear cover will be different resulting in slightly lower capacity in the transverse direction.

To what end?

- no need for additional shear capacity.
- no localized demand necessitating hanger steel.
- doesn't address the original detailing concern.
- further complicates reinforcing scheme.

This thing's only 8" thick.

 

[dik]

With the small bar size, I suspect that your proposed cracking mechanism will not occur.

Dik

 

[KootK]

It's really something like this that I'm thinking of.

This particular example is a bit unique given that, unlike with CIP, very little negative moment will be developed in the slab at the support.

 

[KootK]

Thanks all for the input.

With the small bar size, I suspect that your proposed cracking mechanism will not occur.

It's kind of a weird thing in that:

a) Yeah, it's good detailing practice.

b) There's really no "check" to be done when you violate it per se.

 

[jayrod12]

The small 10m stirrups would be to avoid the cranked bottom mat. I was thinking it would only exist on the inside of the group of bars so it would cross your potential crack. Probably only need 2 or 3 per louvre opening. Alternately, don't crank the bars, or lap them, just drape them a bit at each end. It's not like at the ends of the slabs they're significantly participating anyway, I'm sure by the time you need the full effective depth, you'd have it.

 

[KootK]

I would view the stirrup solution as being effectively this. As such, I'd consider it more or less the same problem to the extent that there is one. A crack just at the stirrup level isn't so much different from a crack below the stirrup level.

Do you think that, perhaps, there's really no logic in this concept at all in the absence of stirrups? Sure, you've got slab bottom bars hanging over beam bottom bars but, without some kind of vertical stirrup leg, is there a point to that? We don't ostensibly design RC concrete to have things hanging from rebar but that concept informs a fair bit of detailing practice nonetheless.

Something just sort of rubbed the the wrong way when I sketched out the supported member bottom steel running under the supporting member bottom steel. Habit more than anything I suppose.

 

[jayrod12]

I'm thinking the stirrup solution goes from your slab bar to your slab bar, not beam bar to beam bar. So it should prevent the failure you're anticipating or you've got bigger problems.

 

[KootK]

Got it, thanks jayrod.

 

[rapt]

Kootk,

I like U bars at the end. I think they would solve this.

 

[KootK]

I like U bars at the end. I think they would solve this.

Thanks for the feedback. Do you actually feel that there is something here in legitimate need of solving? This still mostly feels akin to a two way slab situation in my mind. It's not like your average trim steel around a slab opening or at a stair landing typically comes with stirrups. And you'd practically need to be a jeweler to fabricate a 5.5" tall stirrup.

 

[jayrod12]

Wall ties are made all the time. seems not a big issue to me.

 

[KootK]

I avoid 8" wall ties like the plague precisely because I think they're annoying to fabricate and install.

 

[KootK]

And whether or not the ties are a big deal to fabricate really isn't a big deal to me. What I want is to not do something that might be extraneous unless there's a legitimate reason to do so. My precaster will wrap my little beam things in 2" pitch 0.5" strand spiral if I insist. It's just my job to keep things limited to the truly necessary. And I very much question the efficacy of a 6" tall stirrup or tie for anything other than bar buckling restraint. I don't even like shear ties in slabs until they get to about 16" thick.

 

[jayrod12]

Fair point. So then, since it was you KootKing a possible issue, have you worked your brain around a solution that satisfies your brain and the precaster's need for ease.

 

[KootK]

Not really. I hate the way that it looks on paper but just can't convince myself that it's a legitimate issue in real life. The current incarnation of me has been forced to become a fair bit more practical than past iterations. And, as is usually the case, [practical = secret engineer code for ass-kissing & somewhat negligent technically].