Concrete shear transfert between new and existing

[Waldoa]

I would like to know a reference or a method to calculate the shear transfert of a new concrete beam to a concrete existing column.
News rebars will be anchored on the side of the column, I don’t know if I can use the steel area of the rebars to transfert the shear?

Thanks
Waldo

 

[par060]

try shear friction...and absolutly you can use some reinforcing to engage the shear transsfer

 

[csd72]

Be wary when using friction to transfer shear. Check that the new concrete is not going to shrink away from the concrete face and therefore void any friction.

Best to use dowel bars to transfer the shear if this is possible.

 

[Taro]

csd72, the new concrete cannot shrink away when reinforcing bars cross the interface. That's the whole point of the shear friction concept.

 

[mitchelon]

The provisions of ACI 11.7 are to be applied where it is appropriate to consider shear transfer across a given plane, such as: an existing or potential crack, an interface between dissimilar materials, or in your case, an interface between two concretes cast at different times.

For concrete cast against hardened concrete not roughened to a full amplitude of approximately 1/4", shear resistance is primarily due to dowel action of the reinforcement.

Waldoa, I would only use the shear capacity obtained by the reinforcement. If I were placing my stamp on this design, I would not count on the new-old concrete friction shear capacity.

 

[AggieYank]

I'd use shear friction if I could. It is critical that you can get your developement length for the new rebar into the old column. I'm not sure how you'll get it.

Another option may be to attach some kind of steel saddle seat to the face of the existing column, where the vertical leg of the plate is plumb against the existing column and the horizontal leg sticks out from the face of the existing column into the bottom of the new beam. I'd use "stiffeners" at each edge of the plate. Use epoxy or other anchors to attach the bent plate seat to the existing column. Make sure the plate sits at the underside of the new beam.

Another option may be to chip out a keyway from the existing column, probably no more than 1 1/2" to avoid rebar.

No matter what you do, be conservative.

 

[swivel63]

shear friction and roughen the surface