Anchoring of bars from a cantilever slab to supporting wall 1

[MSUK90]

thread507-424372
Referencing the above thread; I understand the deflection and moment transfer issues at joint. I am just curious about the joint detailing in this case and similar case when a slab cantilevers directly from beam(the beam will be supported by columns obviously). Does the tension rebar from cantilever slab shall be taken 1.5 times the cantilever length into the wall for assuming full moment transfer? What if the slab is supported by a beam?

 

[BAretired]

Does the tension rebar from cantilever slab shall be taken 1.5 times the cantilever length into the wall for assuming full moment transfer?

No. The bars must be adequately anchored into the wall to develop their tensile capacity in the slab. The wall in this case is a 16" wide, 17' high cantilever, so the bending moment in the wall requires tensile reinforcement for its full height. Wall reinforcement would be based on its effective depth in the wall.

What if the slab is supported by a beam?

It would have a large torsional component, so would need to be reinforced for torsion as well as flexure. A 6' cantilevered slab supported by a beam of any significant span is best avoided.


BA

 

[r13]

Fully agree with BA's response. Make sure the column/wall dimension is adequate to provide full development length for tension. I would try my best to avoid the use of beam to support cantilever slab.

 

[KootK]

Does the tension rebar from cantilever slab shall be taken 1.5 times the cantilever length into the wall for assuming full moment transfer?

I'm reading between the lines here but this sounds like the common advice for how far into a backspan a slab cantilever's rebar ought to be extended when there is a backspan. Obviously, this should be verified with a critical look at inflection point locations. This recommendation doesn't translate to wall/slab joints.

 

[MSUK90]

No. The bars must be adequately anchored into the wall to develop their tensile capacity in the slab. The wall in this case is a 16" wide, 17' high cantilever, so the bending moment in the wall requires tensile reinforcement for its full height. Wall reinforcement would be based on its effective depth in the wall.

Thanks for your views.

Make sure the column/wall dimension is adequate to provide full development length for tension.

That would be done for sure.

I'm reading between the lines here but this sounds like the common advice for how far into a backspan a slab cantilever's rebar ought to be extended when there is a backspan. Obviously, this should be verified with a critical look at inflection point locations.

Yes.

This recommendation doesn't translate to wall/slab joints.

So for wall/slab joints, I will conclude that providing proper anchorage of slab rebar into the wall(i.e, developing bars to at least that required for full stress demand in the rebar) will be sufficient?

 

[BAretired]

So for wall/slab joints, I will conclude that providing proper anchorage of slab rebar into the wall(i.e, developing bars to at least that required for full stress demand in the rebar) will be sufficient?

Sufficient for slab reinforcement, but the wall must be reinforced for the cantilever moment which extends from slab down to and into foundation. In a way, this is more critical than the usual backspan practice mentioned by KootK. The moment is constant for the full height of wall and must be developed into the foundation whereas the moment in a backspan varies from maximum moment down to zero.

Your comment above suggests to me that you still don't understand the situation.


BA

 

[KootK]

So for wall/slab joints, I will conclude that providing proper anchorage of slab rebar into the wall(i.e, developing bars to at least that required for full stress demand in the rebar) will be sufficient?

I'd refer you back to the other thread where I discussed this at length. Much abbreviated.

1) With a wall continuing above, development is probably sufficient.

2) Without a wall continuing above, it's really more of a lap splice thing than a development thing.

 

[MSUK90]

Your comment above suggests to me that you still don't understand the situation.

I fully understand what you are saying regarding the design of wall for the moment from slab. But my query was just regarding the development of slab rebar into the wall(joint detailing).

I'd refer you back to the other thread where I discussed this at length.

Can you please send the thread link if possible?

 

[KootK]

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