New concrete slab anchored in existing concrete beams

[greznik91]

Greetings,
slab is 200 mm and anchored in the middle. Slab in one way (I cant use walls perpendic. to beams for slab support)

I have a questions:

- is anchoring with rebars ALONE reliable enough to support new RC slab on existing beams? So everything would be depending on shear capacity of anchors and concrete edge.

- Beams are only 200 mm wide. How long should rebars be? Since they act in shear only I think 150 mm anchored in both sides is okey?

- if my solution is not alright, what would you do?

 

[JAE]

I would not do that - would not depend on a vertical shear plane to support the slab and its live loads.

Reasons not to do it:
1. You would be depending on shear friction across a smooth formed surface of the beam - poor bond.
2. The existing beams would be put into torsion and may not have the proper reinforcement to resist both bending and torsion.
3. You may not be able to drill and epoxy in a bar and get full development of the bar (as required for shear friction to work).
4. The slab is only 200 mm and I'd worry that the bar would spall out of the top of the slab.
5. The resulting slab assembly creates a vertical joint at each end which would let water in if it is not otherwise covered with roofing.

What I would do:
1. Put the slab on top of the beams.

 

[STrctPono]

Agree with JAE. My first choice would be to pour the slab on top of the beams

 

[greznik91]

tnx for reply.

slab on top? Not possible

 

[STrctPono]

Well, in that case, it can work but probably not any Engineer's preferred option.

1. Roughen the side of the beam prior to pouring the slab.
2. Apply an epoxy modified bonding agent to the existing concrete surface prior to pouring the slab.
3. Make sure you have enough dowels. Developing them on the new concrete pour side will not be a problem, but you probably will not be able to develop the full capacity on the existing beam side with epoxy, so you will have to take a reduction on the contribution of your dowels to the shear friction capacity.
4. I would probably want to drill dowels on both the top and bottom. You can lap the dowels on the bottom with your bottom flexural reinforcing. The dowels on the top will help to ensure that the joint doesn't open up too wide.
5. Waterproof the roof

 

[r13]

How about remove the existing beams, then recast with slab.

 

[KootK]

I agree with the concerns expressed above by others. That said, for a small scale roof in a snow-less climate, I might be willing to make a few compromises to make a go of things.

 

[KootK]

so you will have to take a reduction on the contribution of your dowels to the shear friction capacity.

Prorating shear friction reinforcing for partial development length is technically disallowed. That said, we've tried to suss out the logic behind that many times on this forum and have yet to come up with anything definitive beyond "that's how they tested it".

 

[Klitor]

Kootk,OP,
Just to consider.. I would be kinda hesitant to let "small scale residential" contractor do a 45deg "upwards" epoxy instalation.

[URL unfurl="true"]https://ask.hilti.com/questions/overhead-installation-of-adheshive-anchors/dbszw2[/url]

 

[molibden]

I would do timber floor between RC beams. CLT or joists with OSB sheathing.

 

[Blackstar123]

what about bearing the slab on the beam by cutting an L shape at its corner ?

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