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.

 

[Celt83]

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Celt83]

With no beam over the windows I could also be tempted into checking punching shear at the smaller center support


Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

Moreso at the outer corners of the piece actually. Kinda like a corner column punching shear situation there.

 

[Celt83]

yeah good point

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

The alt layout is clever too but it definitely suits me better to have my fuax beams 3' long rather than 10' long. There's stacking wall load on these things but, thankfully, it mostly at the other end of the span. It does nothing for my detailing concern but, with the 3' beam spans, one could probably make an argument that load just arches across the louvers so long as the shear issues work.

 

[KootK]

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.

I may have now. I think that most of the opinions here, and my original concern, are predicated upon a model of concrete shear where:

1) there is shear reinforcing.

2) final shear delivery is via a last, diagonal strut.

3) there is a need to move the vertical force component of the diagonal strut up to the top of the beam (fundamental concrete detailing stuff).

But this is not a truss model shear situation. Rather, it's the diagonal tension model. As such, the diagonal tension moves the shear load up to the top of the beam without requiring any stirrup.

Buy that?

 

[jayrod12]

I can buy that, but is there enough concrete tension resistance on that plane to transfer your shear load up?

 

[KootK]

I can buy that, but is there enough concrete tension resistance on that plane to transfer your shear load up?

Is that not automatically the case if reinforced shear works at that location? That's kinda what I'm hanging my hat on.

 

[jayrod12]

Not if you were to require stirrups? isn't that the entire question?

I guess if your slab shear works normally, and the beam shear works normally (as plain concrete Vf<1/2Vc), then I'd be on board.

 

[KootK]

Is that not automatically the case if reinforced unreinforced shear works at that location?

 

[jayrod12]

Agreed