Expert: Dan Griffin - 6/22/2010

Question
I'm building a root cellar and was looking for advice on design of a cast-in-place, steel reinforced concrete slab for the ceiling/roof.  The unsupported area is 6' x 7'; I'm intending to use #5 bars on a 16", bent into the cores of the block walls poured monolithically with the slab.  I would like to safely support a live load of 150 psf.  For maximum strength, do I want the shorter bars (6' span) on the bottom of the grid?  Is 4" thickness adequate?  How much concrete cover beneath the bottom bars?  Do I need any reinforcement near the top of the slab?

Thanks,
Brian Love

Answer
Brian, I am NOT a structural engineer.  I think the slab needs to be thicker.  I would probably go for 5 1/2 which can be formed with 2x6.  I would hold the steel at center point with 2" of cover.  The prime concern is tension in the bottom of the slab, so if you don't go center, maintain at least 1 1/2" of cover on the bottom.  It would be best to vibrate the concrete to insure intimate contact with the rebar.  Your form work needs to be substantial as the wet concrete will be over 3000 pounda.