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RE: minimum fc for Residential slab on grade

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Thanks Scott - this is the best explanation yet. Unless the slab is
"flatwork" or a slab placed independent of the foundation walls or
footings that carry gravity and/or lateral loads, the concrete must be
2,500 psi. I've changed my notes accordingly. Historically, in this
area, concrete delivered to the site (at least a far as I knew) was 2000
psi concrete for all except moment resisting foundations in which we
specified 3,000 psi to force special inspection for grade beams
resisting bending and shear. The cut-off point of 2,500 psi was
important because I've been questioned on prior plan checks to specify
less than 2,500 psi to avoid special inspection (as you noted the
important term was "greater than" rather "greater than or equal to") and
thus 2,500 psi did not specifically require inspection.
While it might be common to deliver 3,000 psi concrete to the site
(which was the case in my own home where I specified higher but without
special inspection, but just to exceed minimum code requirements) I've
inspected slabs where the cores taken proved to be between 1,900 psi and
2,800 psi concrete. These were issues brought up in law suits where I
represented one side or the other and the deciding factor was to ask
what was specified if known.
I think, however, that what is disturbing and the reason why I missed
this section in my notes is that it appears hidden in a section of the
code that refers to rigid connections between diaphragms. Unless it is a
grade beam or cantilevered column (generally requiring special or deputy
inspection for lateral load design) this section is not clear for box
systems using wood shearwalls that do not represent a rigid connection
but are flexible and have some amount of rigidity that should be
designed. In other words, in the wall is a low capacity shearwall, most
municipalities have ever questioned the f'c=2,000 psi limit in my (and
others) notes. However, one source has, on a high capacity wall
(sheathed both side with considerable uplift and shear) has asked me to
design the foundation of the wall for bending - which can be done using
f'c=2,000 psi. They never wrote me up a correction for 2,500 psi. 
EsGil was the agency here who brought this section to my attention and I
am saying this because they deserve credit for their observation. This
is how we learn and after reading your interpretation of ACI 318, I
understand and agree with the plan checker. This saves a lot of time
trying to justify an interpretation that was not the intent of the code
writer.

Thanks Scott,
Dennis

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Wednesday, July 09, 2003 11:59 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: minimum fc for Residential slab on grade


Dennis,

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.

HTH,

Scott
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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