RC detailing question

[mar2805]

People hi!
I was wondering how you design an RC wall to RC slab conection if you design an slab as "pinned" to walls, not fixed!
The slab is on top floor
I asking this couse I had an debate with another engineer and we couldnt agree on this matter at all.
Heres my ilustration and explanation.

In red is the wall reinforcement (meshes)
In blue is the slab reinforcement (meshes)
The wall width is 25cm wich is more then you need for anchorage lenght of the slab reinforcement (diametar of bars is 8mm)
Since the slab is designed as (pinned) supported ( no negative moment above the support) I dont see any need to insert adittional horizontal U bars that tie slab bottom and top reinforcement.
The top reinforcement in the slab is just above the design minimum reinforcement (just to provide some ductility for negative moment) but not enough to develop an negative moment.
The provided slab bottom reinforcement is design to positive span moment.

For me this is a clean pinned, RC wall to RC slab connection.

Your opinion.

 

[JAE]

That is what I've typically assumed....used a pinned end but then provide some amount of top reinforcement to contain cracks. Usually above the minimum level of reinforcement - perhaps 60% of the bottom reinforcement amount.



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[YuleMsee]

I have seen a case where a slab was assumed to be simply supported hence no top rebars were provided but a reinforcement mesh which technically met the required minimum steel area. The slab still cracked along that line.
Is the project budget that tight not to allow you U bars? You can alternate the u bars, one u, the next a bend
Secondly, although calculations might show anchorage is enough for the straight bottom bars, will a bend cost too much?

 

[molibden]

I always use U bars in walls and slabs. Seems most logical and robust.

 

[mar2805]

@Yule
"The slab still cracked along that line."
Can you please draw tle place, where the cracked formed.
If your talking aboth top portion of the slab above the support (wall), this is normal since youve designed the slab as pinned (so no fixed connection to the walls)....BUT to restrain the top crack to some amount, Ive provided minimum area of reinforcement in the top part of the slab just above the support.

@Yule
"secondly, although calculations might show anchorage is enough for the straight bottom bars, will a bend cost too much?"
My question is, why? What are you going to achive with this detail?


@molibden
"I always use U bars in walls and slabs. Seems most logical and robust."
Do you always design your RC slab as fixed to walls or as pinned to walls?

I dont see the logic of using the horizontal U bars in the slab egde that rests on the support width that more then anchorage lenght.
If I had a situation where my wall width is less then slabs reinfrcement anchorage lenght, I would certainly provide the horizontal U bars.

 

[molibden]

In most cases I design RC slab as pinned to wall.
Some negative moment will be there anyway, so providing steel will prevent excesive cracking.
For full anchorage length of 8mm diameter bars you need more than 25cm, right?

 

[mar2805]

8mm bar with concrete C25/30 gives you basic anchorage lenght of 32cm.
When using meshes, wich have an purpendicula bars welded to them, you use 0,7 factor, wich gives you lenght of 22cm.

 

[mar2805]

The engineer I talked to, use this detail for slab to wall joint (top level)
I think this is too much....

 

[molibden]

How do you anchor mesh from the wall? I use U bars so for them to put bottom mesh on top of wall they need to cut transverse bars so anchorage length should be 32 cm. I also use the same detail as you posted because of cracks, robustness, seismic forces etc. I find it good practice.

 

[mar2805]

In this egsample, since this is a top storey, the mesh goes over the vertical bars that "peep" from the wall.
Same goes for any other storey....you can always cut horizontal bars from the mesh, that make wall reinforcement. You get two things with this....clean overlap with wall reinforcement form the next storey + possibility of puting the slab reinforcment over the wall reinforcement.