Dowels to soffit of existing concrete beam 1

[ajk1]

I want to deepen a series of 2-span existing concrete beams so that their flexural strength is increased, by adding 12" depth of concrete with added rebar in the bottom of the added concrete. The existing beam is 24" x 24". When 12" concrete is added they will become 24" x 36" beams. To be conservative, I will design the deepened concrete for a positive moment of WL/8 (since I cannot add any top steel to increase negative moment resistance).

The maximum factored horizontal shear stress at the interface of the new and existing concrete is 115 psi, based on VQ /(Ib). The vertical shear stress resistance is ok based on the existing 24" x 24" beams and their stirrups.

There are about 100 such beams, each being about 55 feet long. I was thinking of 2 lines of dowels to the soffit of each beam. (Please see that attached sketch. I know the attached sketch shows deepening only to 30" but that is a slip I just noticed. My intention is to deepen it to 36").

Question:

What is the best type of dowel to use for this purpose?
I am reluctant to use epoxy type dowels in overhead applications because of the difficulty of installing overhead (the dowel tends to slip down until the epoxy sets; etc.). But mechanical type dowels generally need to be torqued to set them and there is nothing to be torqued against in this application. Perhaps there is a mechanical dowel that does not need to be torqued to set it?

 

[rapt]

Also, whatever you add externally needs to be fire rated. After the internal unbonded PT is lost, the only fire rated steel you have is the 2 #5's in the bottom, so basically nothing. Anything added externally probably needs to be encased in concrete. This would suggest Ingenuity's encased bonded PT option.