Elevated concrete slab with a hole in the center

[bustaone]

Does anyone know if the code has stipulations for a concrete slab with a signifigant void in the center with reguards to steel reinforcement? for example: I am trying to design the reinforcement for a 31'x25'x8" elevated slab with a 5'x5' opening in the center supported by concrete beams on all sides with poured concrete columns at the corners. Could i do double or triple rebar on the perimeter of the void? I'm at a loss for what to do about this.
Thanks in advance for any advice.

 

[Bagman2524]

The cases I've seen like this have both rebar around the perimeter and in a square shape around the hole. I would suggest as many layers of rebar as you can get in comfortably. 3 sounds about right for an 8" slab.

 

[JAE]

Many engineers that don't use finite element analysis will assume that a strip of slab on either side of the opening is supporting its own share of the load plus that half portion lost by the hole. In your case, you'd take maybe 36" of slab on one side of the opening and add the loads (both dead and applicable live) from the 2.5' half-strip.

I've seen this also do this twice - once for each direction, which gives you a little redundancy in your design.

Diagonal bars are also sometimes place more to control cracking than anything else.

 

[bustaone]

thanks a lot guys, i think that's what i'm going to do. triple strand both directions adjacent to the opening with some additional bars for the accompanying 3' or so near the opening. Would it be reasonable to cap the perimeter of the opening with some C9x15's? Thanks for the replies.

 

[JAE]

I don't think the C9's would do much for you.

 

[bustaone]

C9's just as a facade, the opening is a skylight.
Thanks!