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Re: minimum fc for Residential slab on grade

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While I cannot completely speak to what is in the UBC (as I don't really
use it that much), I can provide some light.

I am, however, rather familiar with ACI 318.  And since the concrete
portions of the UBC code are basically ACI 318 with some minor
modifications I can provide some comments about the UBC.

Section 1921 of the UBC code is essentially chapter 21 of ACI 318 (with
some minor modifications).  Section 21.2.4 of the 1999 ACI 318 is
basically the same as section 1921.2.4 of the 1997 UBC.  The main
difference is the exception that is listed in section 1921.2.4.1 of the
UBC does not appear in the 1999 ACI 318.  This provision has been in ACI
318 for awhile (the earliest ACI 318 that I have is 1989 which has a
version of this provision).  Thus, there has been a minimum compressive
strength requirement for STRUCTURAL (more on the emphasis in a moment)
concrete in seismic use for a while.

In the 2002 ACI 318, this provisions still exists, but another provision
has been added in section 1.1.1 of the 2002 ACI 318 code.  This provision
requires that all STRUCTURAL concrete be not less than 2500 psi.  Thus, in
the 2002 ACI 318 code all STRUCTURAL concrete must be 2500 psi or greater
unless it is used in "members resisting eathquake-induced forces", in
which case is must be 3000 psi or greater.

So, the point is that the UBC gets this provision by way of the ACI 318
structural concrete code.

Now for the reason for the emphasis on STRUCTURAL above.  Section 1.1.6 of
the ACI 318 code states:

"This code does not govern design and construction of soil-supported
slabs, unless the slab transmits vertical loads or lateral forces from
other portions of the structure to the soil."

Thus, the ACI 318 only applies to structural concrete.  Or in otherwords,
it DOES NOT apply to slabs-on-grade, UNLESS that slab-on-grade transmits
loads from the rest of the structure (i.e. it then becomes a STRUCTURAL
slab that happens to sit on soil not a slab-on-grade).

Thus, if the UBC is following the intent of the ACI 318 code when it takes
provisions from it, then those minimum compressive strength requirements
would not apply to a slab-on-grade, unless you are transmitting loads
through the slab from the rest of the structure (i.e. having be some sort
of a diaphram to transfer seismic load to other footings).  You footings
and grade beams, on the other hand, DO have to meet such a requirement.
Thus, presumably if your slab-on-grade is being placed monolithically with
grade beams/footings, then it would then have to have meet the same
minimum strength requirements as the footings/grade beams.

Now, on a practical sense, as others have pointed out, even if you specify
2000 psi, you are likely going to get 3000 psi concrete.


Ypsilanti, MI

On Wed, 9 Jul 2003, Dennis Wish wrote:

> I designed a single story residence (spec home but large) using a slab
> on grade which is typical in this area. Historically, we placed
> f'c=2,000 psi concrete for everything but where proprietary frames such
> as Hardy Frames are used. The typical soil in the area is silty sand
> with a bearing pressure of 1,500 psf and with increases allowed for
> depth and width.
> I received a plan check correction based on the 1998 CBC and 1997  UBC.
> He quoted section 1921.2.4 which requires high strength concrete but has
> an exception that allows for concrete not less than 2,500 psi. This is
> generally required when inspection of the concrete is called for and not
> for simple slab on grades. The city has, during the 1997 UBC cycle
> allowed for f'c=2,000 psi concrete, but the contract plan check company
> requires 2,000 psi concrete so as not to force deputy inspection.
> Are there any comments as how I should address the plan check correction
> since deputy inspection can be expensive and there are no problems with
> subsidence or expansive soils in the area.
> Am I wrong on this one????? If so, it comes as a surprise.
> Thanks
> Dennis

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