Slab on grade directly over grade beams

[molibden]

I'm designing foundations for an office building. System is steel moment frames in short direction and braces in long direction.
Steel columns are rigidly connected to foundation beams with cast in place anchor rods.
Plan is to first pour the main foundation system, then compact gravel between the beams and then pour slab on grade directly over foundation beams. Slab will be reinforced top and bottom.
My question is, do I need control joints in lines of tie beams? I would rather not do that because contractors hate them and slab will not be visible (covered with screed). I would rather provide additional reinforcement to control cracks but the slab is quite long.
Would you leave the slab and foundation system disconnected? Should I put another layer between the slab and foundation to separate them completely?
On the other hand connecting them with rebar seems complicated and unnecessary. What do you think?

Please see my sketch. Any help or insight is highly appreciated.

 

[hokie66]

Hard to read the slab thickness, but I think it says 12cm. If that is the case, I would put all the reinforcement in one layer, centrally located as far as possible. Without joints, and possibly with them, you will get cracking. The reinforcement just controls the widths of cracks, rather than preventing them. I think that I would sawcut joints, not on the lines of the tie beams, but midway between them. That way, most of the cracks should be straight ones, and you don't have to decide what to do with the joints at the steel columns.

 

[dik]

As per Hokie... and min 0.4% to 0.5% reinforcing.

Dik

 

[molibden]

Thank you both. In my area (EU) it is common to put reinforcement in two layers. Cover 2.5 cm on both sides, mesh with 0.6 cm diameter bars in both directions, so I have my reinforcement spaced approx. 5.5 cm center-center. Well that's theory.. I am worried about bending in both directions: if foundations settle, and if gravel settles. Both cases will induce bending moment in my slab.

Please note that I have my foundation beams also on the perimeter in the long direction. There the slab will be poured over and will be additionally restrained when shrinkage occurs.

How deep is the sawcut? Do I cut/terminate my top reinforcement? Is there any other easier method for control joints?

Where would you predict construction/cold joints? Probably there is no problem to pour everything in one phase.

 

[dik]

not for a slab that thin, single layer only and usually use 2" (50mm) cover with 1-1/2" (40mm) dp sawcut. If concerned, then thicken the last 2m and put a double layer of rfg. Advise client that potential for cracking without thickening. and sawcut at 2m location.

Dik