Expert: Dan Griffin - 6/20/2008

Question
I am a small builder/remodeler in Nebraska and need some help on the design of a self-supporting concrete deck that is approx 36" above grade.  My concrete sub does commercial flatwork and suggested a pouring a 4" slab reinforced with #4 rebar onto a temporary metal deck supported by wooden joists.  The deck would be 20'x16' with one end of the 20' side attached to a house for lateral support.  I envision one set of footings approx 5 or 6' feet from the house so that the house doesn't bear the weight and the other set of footings on the opposite end of the deck.  I would also form an integral beam into the pour above each set of footings as support for the deck.  My questions are #1.  What type of rebar layout would I need within the slab and the beams?  2. What type and psi of concrete should I specify for the pour? 3.  How thick should the concrete be within the beam to support the slab (ie- what dimension should the form for the beam be)?  I have a 3D images of the design if that will help with your answer.  Thank you for your help and consideration and I apologize if these questions are not appropriate for your forum.

Answer
Jon. I really think you are way under on concrete dimension.  YOu never indicate what the suspended slab will be used for, but spanning 16 feet will need more than a 4" slab, especially outdoors.  A typical concrete floor with columns around 20 feet apart is 7" thick with two mats of steel, one for tension and one for compression.  Another common method  places bar joists about 2' apart with welded steel decking creating a composite deck often with no additional steel.  #4 bar each way is 1" and exterior concrete requires 2" of cover which would make the slab 5" or more.  I agree that the slab cannot bear on the house framing, but it can share the house footing.  A 16x20 fully supported slab on grade would need control joints to reduce cracking, hanging it up in the air would be worse.

I would be uncomfortable designing anything below these typical details, I really think you need a structural engineer to determine the best approach.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Jon, I'd really appreciate hearing what the engineer details.  My personal email is dgriff23@7cox.net (remove the number seven - done to reduce spam)