I've got several porches/patios which will project from the main
structure of my house. The side walls will each be roughly 6-7 block
courses high. The floors will be concrete slab with some sort of
tile/stone topping. I'd rather not try to fill these rather large
boxes with dirt in order to pour a slab on top. Is there an
alternative method of supporting the concrete floor that won't break
Why not use dirt? Find yourself an excavator that needs to dump some
He'll have to pay a place to dump it or give it to you for free. If
you do this method
make sure you use rebar in the slab because as hard as you try, you
compact the dirt enough to keep the slab from cracking. An alternitive
that you will not find for free. How ever you may be able to find a
local black top
contractor who has some left over that he will give you a deal on.
As Michael & tbasc both suggest (you need to combine both msgs') you could
use pan deck(Q-Deck) with rebar to span the 12'. With respect to that, you'd
need an end wall bearing support (more block?)against/beside the building.
Depending on the slab depth, you 'may' need temp support for the midspan of
the deck until it cures. In essence, it's a deformed metal deck with 1.5"
deformations in it and comes in 36" widths and is at least 20 gauge in
thickness. Common use is industrial construction. An example, albeit not
exact is here;
Essentially you place the deck and install #4 rebar (1/2" dia) midway
between the top face of the deck & the exposed surface of the slab. 100%
ties on the rebar @ 8~12" on center both ways. 6-6/10-10 WWF [WWF= welded
wire fabric/commonly called wire mesh] [6-6= 6" squares/10-10gauge wire
both directions] would substitute but the rebar would be better. IMHO
I suggest to do the slab on grade system. It will cost you less and much
easier to construct. You can use welded wire fabric for reinforcing in the
slab on grade over the compacted fill. You should also think about
reinforcing the concrete block walls even if they are 48 in high. They will
become retaining walls for the compacted soil pressure. The wall foundation
will be the simplest.
The other way will be either composite concrete steel deck with the 12 foot
span it might be 3 " deck with 3" concrete . If not the next most expensive
way is to form a structural one way concrete slab to span the 12 feet span
around 5 to 8 inches thick you will then have extensive reinforcing in the
slab. You will also have to provide some sort of drainage with the space
under the structural slab or steel deck/concrete slab. You might even have
to have an access door under the slab. Your foundation for the walls will
also be different.
Oh don't forget a railing if you have more than 2'-6" height from the slab
floor to the finish grade on the outside of the slab.
Hope this helps.
In a previous post firstname.lastname@example.org wrote...
I recently learned that an engineering firm that I respect recommended the
use of stacked Styrofoam blocks to be used as the support structure for
concrete stadium style seating. They simply stacked blocks to make the
rough shape then formed and poured the steps, seating pads and access
ramps to make the seating. No shoring required. No fill required.
In your case, level out the ground, stack Styrofoam up to the bottom of
your "slab on grade" (say 4"-5" below finished surface). Pour concrete on
top of the Styrofoam. Leave the foam in place. It won't rot.
This method will be more expensive than placing and compacting dirt, but
probably much easier for a small area. One other option is to fill the
area with controlled density fill (CDF). This is a low strength concrete
often used for filling utility trenches. You will have to build side
forms to contain this material.
Or you could do as few others have suggested: pour on top of a steel
corrugated form deck. Leave the deck in place. In this case your slab
will be an elevated slab and would have to be properly engineered to span
the distance between support points after the form deck corrodes away.
Bob Morrison, PE, SE
R L Morrison Engineering Co
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