5 concrete garage floor over wood frame

huangyhg

5" concrete garage floor over wood frame?
many times i saw 1.5" concrete topping over plywood and wood frame called out for townhome or condo.  i never saw 5" concrete of (2) cars garage floor over a plywood and wood frame floor.  have anyone ever call out 5" concrete on top plywood and wood frame before?
tia

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i can't say that i have.
i trust that the plywood is acting as formwork only.
in the past most building codes prohibited wood framing from supporting the permanent dead load of concrete.  there were some exceptions, one of which was the use of light weight concrete fill over plywood floors.  although some codes today no longer have this restriction, i still would avoid supporting concrete on wood framing.
one of the major problems is that wood creeps, so over time the deflection due to dead load will almost double.   
you might want to review the apa research report 139 which addresses plywood garage floors.  it has some recommendations regarding concrete topping over plywood.
i have used 1.125" t&g ply over 2x12 joists with a 3" normal weight topping.  have to mind the deflections, which i restrict to l/1000 in order to not crack the concrete.  i add extra beams as needed to achieve this, depending on the layout of the garage floor.  i also specify a marine grade for the ply.  expensive, but it works.
  
mike mccann
mmc engineering
the apa has technical topics
tt-071a dated nov 2005