Residental building - large shear forces in the RC slab 1

[n3jc]

Hello,

I'm dealing with the small residental building with RC slab that is 260 mm thick. The slab can't be any thicker.
The slab is heavily loaded on the cantilever part (overhangs in both directions) because the upper storey has different plan view (see 3d image of the structure), so it has loaded cantilever parts on both sides/edges of the slab.

The blue line is the load from upper storey on the slab:

The shorter cantilever might be able to transfer loads directly to support – the wall bellow since its only 200 mm away. So it can be used as compression strut right? Is this the right approach?

The FEM analysis shows large spikes (hot spots) of shear forces at the corner of the supporting wall as a result of both overhangs of the slab.

According to my calculation the shear capacity of the slab is around 140 kN/m which means I need a shear reinforcement. How would you reinforce this slab – any suggestion?
I'm aware of the shear perimeters around the corners but my shear forces are kinda large and perimeter is small.

The Eurocodes (EC2) says that shear reinforcement in the slab is allowed if the slab thickness is at least 200 mm.

I was thinking about using shear links or bent bars. What is the largest bar diameter for such links?
Are the links even a good idea in slab that is only 260 mm thick?

 

[KootK]

Your walls will be concrete, right?

A direct strut approach on the short end probably does make sense. If your walls are concrete, there might be an argument for something similar on the long side too.

I've historically stayed away from rebar shear reinforcement in slabs less than 400 thick. At 260, I'd want stud rails which would probably be an unpopular choice.

 

[n3jc]

Thank you for an answer. Yes, walls are concrete...

I've historically stayed away from rebar shear reinforcement in slabs less than 400 thick. At 260, I'd want stud rails which would probably be an unpopular choice.

What is the main reason behind it?

 

[KootK]

What is the main reason behind it?

Space. Too much rebar in a tight space = hot mess. Particularly when the impact of bar radii is considered. If you're confident that you can do it cleanly, getting your shear reinforcement pretty close to the flexural compression block, go nuts.

 

[JAE]

Agree with KootK.
I might be inclined to embed wide flange beams inside the slab at discrete locations for the long cantilever and rely on reinforcement on the short side but not how you show it. Afraid that the slab would crack behind your shear ties (hot mess per KootK).

How the embedded WF beams and the perpendicular rebar interact would be a problem - the beam could be perhaps 80% of the slab thickness and located on the bottom of the slab so top rebar could extend over it. Bottom rebar could be extended through pre-cut holes in the WF webs.

 

[KootK]

This was why I asked if the walls were concrete. I'm not sure if this makes things better or worse computationally.

 

[n3jc]

Space. Too much rebar in a tight space = hot mess.

Makes sense. I agree.

I might be inclined to embed wide flange beams inside the slab at discrete locations for the long cantilever

Well, sounds good but since I have never done anything like that I don't think I can afford to experiment.
I'm not even sure if I will be able to find anything about it in Eurocodes.

I think I'll go with stud rails.

 

[KootK]

Most codes have (or had) guidance on the use of steel beams as "shear heads" in their punching shear sections. But, yeah, the rails are probably a cleaner solution on balance if that's a viable option.