Elevated RC slab extension

[molibden]

I have a case of extending existing elevated one way slab. Don't ask.

Old wall will be removed. New wall was built to support the added slab. See attached sketch. Units = meters.

There are bars top and bottom at the connection, but let's say there is not enough data to account for them. Maybe for shear (dowel action), but definitely not for bending. Surface between old and new slab was flat and not roughned in any way.

How would you design this connection? Adding lamellas on the bottom? Can you recommend a good design reference (Eurocode)?
What about shear?

If I'm not comfortable with the solution I can still add some beams below the slab.

Thank you.

 

[KootK]

You'll have to restore underside flexual tension continuity somehow (frp/epoxy bars/steel strapping). With that done the joint may be self solving as shear friction will likely suffice. I'd throw in some top towels anyhow for good measure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[molibden]

Thank you KootK. So you don't have any second thoughts for this type of extension? I just never seen something like this so I was a bit skeptical.
Do you know of any design guide for frp or steel strapping? By steel strapping you mean steel lamellas glued or bolted to the underside of the slab?
Also how do you check shear friction by your code? I don't know if there are any guidelines in EC 2.

 

[KootK]

Oh, you're most welcome molibden.

So you don't have any second thoughts for this type of extension?

I definitely have second thoughts. They just aren't centered around shear transfer at the joint as I think that's the easy part. My concerns are:

1) If you're really going to have to go into this not knowing anything about the existing rebar, then then you're basically going to have to reinforce the underside of the entire slab. That's a lot of work and won't come cheap.

2) With the flexural reinforcing route, I feel that you'll like likely get an instantaneous and permanent bit of sag in the slab as everything settles into its new equilibrium. That's tough to predict and could cause serviceability issues depending on the nature of the occupancy here.

Do you know of any design guide for frp or steel strapping?

I've seen several over the years but I'd have to ferret them out on google in order to share them with you. I'll leave that exercise in your capable hands.

By steel strapping you mean steel lamellas glued or bolted to the underside of the slab?

Yes. I've done a version where holes are drilled through the slab so that stud projecting up from the strapping can be grouted in place. This helps our with #2 above as it limits slip. It would be a TON of work here though.

Also how do you check shear friction by your code? I don't know if there are any guidelines in EC 2.

1) obtain a coefficient of friction from the code. For a roughened surface in my jurisdiction, this will be 1.0 I think.
2) calculate available clamping force based on reinforcement and reliable compression available. In my jurisdiction you could take another 0.5 MPa for adhesion but I normally ignore that.
3) Multiply 1 x 2 x Safety factor.

I'd be very surprised if EC didn't have their own version of this someplace. Shear friction has it's haters but they rarely comprise entire continents of research saavy foks.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.