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RE: minimum fc for Residential slab on grade

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Dennis,

You have found a reason why I don't like the UBC...when it takes the
provisions from ACI 318 for concrete, ASD and LRFD from AISC, etc, the
provisions tend to get "smushed" together which makes them more difficult
to use and also can sometimes cause the "intent" to be lost.

While you like having all those things reproduced in the UBC code so that
you don't have to buy several different publications, I like it.  It tends
to allow the true "intent" to sometimes be more readily apparent.  Plus, I
have to admit that I am used to having the various publications be
seperate, so it is rather easy for me to find stuff in them as opposed to
the UBC (which was part of my problem when I took the Struct III exam in
WA...I wasn't able to find stuff in the UBC code as easily).

But, then it also helps that I worked for ACI for a period of time, which
meant that I worked with those that crafted the ACI 318 and was there
through the final approval and publication of a new edition.  This tends
to give one some serious insight into the backgroud and intent of some
provisions.

As far as concrete strengths being below the strength specified, this is
permitted within the ACI 318 code.  In the ACI 318 code, this is dealt
with in section 5.6.  The companion section in the 1997 UBC is 1905.6.  It
is basically the same as what is in the ACI 318.  This section outline
code "required" minimum acceptance requirements (1. every arithmetic
average of any three consecutive strength tests meets or exceeds f'c AND
2. No individual strength test [average of two cylinders] falls below f'c
by more than 500 psi).  If these requirements are not met, then there are
provisions that oultine how to deal with low strength results, such
possible acceptance by the engineer of record if the low strength does not
jeopardize the load-carrying capacity of the structure.  Now, admittedly
those provisions don't address things like contractual obligations (i.e.
the client presumably paid for 3000 psi concrete but really got 2000 psi
concrete...thus, the structural integrity may not be jeopardized but the
client could have ended up paying more than s/he should have).

HTH,

Scott
Ypsilanti, MI


On Thu, 10 Jul 2003, Dennis Wish wrote:

> 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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