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Re: Why specify 3ksi concrete for grade beams?

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Chuck and Joe:

I see the exception that you are referring to (had to look at it as I am
not in UBC/CBC country so don't know it forward and backward...but must
get better at is as I will be using it [most likely] for the Struct III
exam in Washington).  My "response" would be a definite "maybe"!  How's
that for a solid, "wishy-washy" answer? <grin>

I give the "wishy-washy" answer because it is somewhat dependant on where
that exception comes from.  My educated guess is that since the concrete
provisions in the 1997 UBC (and thus the CBC) come from (actually is the
with some modifications) ACI 318-95 this exception might come from
provision 1.1.5 ("This code does not govern design and installation of
portions of concrete piles and drilled piers embedded in ground.") and/or
provision 1.1.6 ("This code does not govern design and construction of
soil-supported slabs, unless the slab transmits vertical loads from other
portions of the structure to the soil.").  There are not parallel
provisions in chapter 19 of the 1997 UBC (likely because it is "meant" to
a general concrete chapter for any and all concrete...thus, they "modify"
the scope of ACI 318), but this is likely the intent behind the exception.
Now, the only problem with this possibility is that there IS a chapter in
ACI 318-95 that deals with the design of footings (i.e. spread footings).

In addition, while not necessarily "enforceable" by way of the 1997 UBC,
the up-to-date "state of the art" ACI 318 (i.e. 318-02) has modified
provision 1.1.5 to now state:

"This code does not govern the design and installation of portions of
concrete pile, drilled piers, and caissons embedded in ground except for
structures in regions of high seismic risks or assigned to high seismic
performance or design categories.  See 21.10.4 for requirements for
concrete piles, drilled piers, and caissons in structures in regions of
high seismic risk or assigned to high seismic performace or design
categories."

Thus, under current ACI 318 provisions (which are used by "modern"
building codes...sorry, but the 1997 UBC is a little "dated") the minimum
of 3000 psi would apply to spread footings (this would appear to still be
true under the 1995 ACI 318 _IF_ the spread footings resisted "earthquake
induced loads" since ACI 318-95 applied to spread footings and mat
foundations) AND deep foundations (i.e. drilled piers, piles, and
caissons).  Now there is a "stated" minimum of 2500 psi in table
2904.2.2(2) in the 2003 IBC code, but it would seem that that table gets
"overridded" by the chapter 21 provisions of ACI 318-02 for "seismic
regions" since the 2003 IBC adopts ACI 318-02 by reference and there are
no exceptions/modifications to the 3000 psi provision in the 2003 IBC.
So, some would argue that since the code is a "minimum" and since the 1997
UBC is some what dated, that structural engineers should be using "more
advanced knowledge" if available, which in this case would be some of the
newer provisions in ACI 318-02.  In other words, while the local code
official may not require it, some engineers would argue that the newer,
"more knowledgable" provision of 3000 psi should be used (or while not
enforcable by code offical, it may be "enforcable" by lawsuit [i.e.
standard of care]).

Now, the other "out" is that most residential structures outside of UBC
land will usually fall under the IRC.  And the IRC might not have the same
requirement.  But, I believe the IRC "dumps" you back into the IBC if you
get into high seismic zones or with structures that don't "fit" the
presciptive requirements/parameters of the IRC (don't know for sure as I
don't have the IRC at home...yet).

The end result is that strict reading of the 1997 UBC code would seem to
imply that the exception would apply to footings and thus 2500 psi would
be permissible.  Now, I will split some sematic hairs.  Note that the
exception specifically applies to "footings".  Section 1915 is titles
"Footings" and section 1915.1.1 states "Provisions of this section shall
apply for design of isolated footings and, where applicable, to combined
footings and mats."  Thus, it would appear that it is possible that the
1997 UBC intends the "footings" in the exception to 1921.2.4.1 to strictly
apply to spread footings (and combined footings and mats).  Thus, one
could argue that it would NOT apply grade beams.

HTH,

Scott
Adrian, MI


On Mon, 11 Oct 2004 Jnapd(--nospam--at)aol.com wrote:

> Scott:
> I agree with Chuck because of the exception in the CBC.
>
> Joe  Venuti
> Johnson & Nielsen Associates
> Palm Springs,  CA
>
>

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