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RE: Raised concrete floor slab

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For an interior deck, would there be any cost savings with using metal deck instead of the 1-1/8" plywood???  It may help.  Then, I would just check what is needed using one way slab bending, a simple span of 16", and, to be conservative, ignore the strength of the plywood and design the concrete to hold all of the load.  Your floor joists should be sized to carry the dead and live load above.
 
On the other hand, we are currently doing an apartment complex here in Gillette, Wyoming.  The architect wants, again, BCI's at 16" o.c., subflooring, and 1" of Gyp-Crete.  The bottoms of the TJI's are looking to have 5/8" Type X GWB.  The Gyp-Crete is unreinforced, and therefore, we did not account for it as any type of a load carrying member.  Therefore, we designed the subfloor, which is only 3/4" thick plywood, to be able to carry all of the load to the floor joists.  And the floor joists to carry all of the load above.  Thus, as stated earlier, we are not relying on the gyp-crete for any strength.  It's simply a topping and dampening assistant to help decrease floor vibration.  You could do the same thing with your situation.  Our spans were 24' clear for 40 psf live load and 6'  clear for 100 psf live load.  The gyp-crete was approximately 9 psf per inch thick.
 
Another option you could do is use your TJI's only as temporary support of the floor slab.  You could do a thickened edge of concrete at your load bearing walls, and spec. out temporary cribbing during pouring and curing of the interior floor slab.  The floor slab is reinforced and poured monolithically.  By doing it this way, you can possibly cut down on the size of your TJI's and plywood based on your temporary cribbing.  The materials for the cribbing and plywood would be waste, however, so there is a possibility of money wasting.
 
There are many ways you can go about this, all depending on if you want your floor slab structural or not.  It sounds like you have a hefty plywood sheathing in there for floor shear.  You also have a good size for your floor joists.  If those things are staying structural, I don't see why you would need to go very thick or require reinforcing for your concrete.  You may want to consider a gyp-crete overlay, as I think it may be lighter than LWC.
 
Those are my thoughts.  If I am wrong, I am sure someone will point out the errors in my ways.  This is not a standard practice for me, so I am only telling you what I have seen done.  The Gyp-Crete was an architect spec, and was not labeled to be designed by the engineer, so I can't speak for reinforcement in that, however, with only one inch thick, I doubt you could fit much.  We have also used lite-deck forms, but I won't go into that.
 
Good luck, and I hope I didn't confuse you too much.
 
Dave Maynard, PE
Gillette, Wyoming
-----Original Message-----
From: Mark Pemberton [mailto:markpemberton(--nospam--at)sbcglobal.net]
Sent: Wednesday, June 22, 2005 12:58 PM
To: SEAOC
Subject: Raised concrete floor slab

A residential client would like a concrete floor at the second story.  I've done this with periodic supporting walls below, but this basement area is open with a 29 foot short direction span.  With TJIs at 16" oc and a 1-1/8" plywood deck, what would be an appropriate light weight concrete fill depth and reinforcement?  Any pointers on this subject?  Thanks.


Mark Pemberton, S.E.

Pemberton Engineering
502 Mace Blvd., Suite 2B
Davis, CA 95616

Ph:  (530) 792-0511
Fax: (530) 792-7109