Concrete detailing -- slab bars at cantilever ends -- why turned down?

Many times the perimeter downturn serves a more productive purpose, providing a stiff supporting edge for your wall framing. It's not that the slab can't support the wall, but between columns the slab deflection may be a bit too much and so having a nice stiffened edge works wonders.

Whether you have a thickened edge or not, assuring good cover on the reinforcement at the edge of buildings is paramount. "Concrete cancer" due to inadequate cover is a curse on many coastal buildings. Making the end of cantilevers too thin makes it more likely for these issues to occur. Another reason for a thicker edge is to give adequate robustness to balustrade or railing connections.

OP said:

why orient those hooks in a vertical plane? Couldn't you do it in the horizontal plane (e.g. the slab edge) instead, and simplify the formwork?

Click to expand...

I agree, that sounds anathema to the whole flat plate formwork economy thing. Can you provide a pic that might help us to parse out an alternative motivation if one exists? Certainly, I wouldn't downturn the slab just to provide cover to the hooks. I'd sooner go 180 or tilted 90 even if those have their own issues.

I'm with jayrod and hokie, the edge beam likely serves a more important purpose than development of reinforcing. Unless the reinforcing consists of very large bars, 1m of development should be enough without a hook. The downturn of the reinforcement is likely there to reinforce the face of the edge beam.

These "inverted parapets"/"edge beams" are typically about 100mm wide and 250-350mm deep. Containing the slab bar hook, plus two 8-10mm bars running parallel to the slab edge. So I wondered about the stiffening effect that Jayrod mentions, but these would be way past their cracking moment. Maybe the fully cracked stiffness is still enough to provide some benefit?

From below

From above (pre-pour)


As you'll see, slab construction here is not flat plate.. labor & formwork is still cheaper than concrete.

----
The name is a long story -- just call me Lo.

Looks like a slab support edge beam to me.

A reinforced, 250mm deep edge beam is much stiffer (and stronger) than the edge of a slab without it, even if fully cracked.

I'm guessing a designer/architect/owner wants some definition at the slab edge, rather than just a fine line. Based on the depth of the "beams" which are in the top photo, the slab must be very thin.

- That there is a pretty narrow beam. Still, it appears that it was meant for greater things than bar cover.

- In your jurisdiction it's quite common to stack masonry walls on these slab edges too, yes?

- With all of the other beams flying around, that mostly neuters the simplified formwork argument I think.