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RE: 3000 psi concrete and special inspection

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Scott,

I agree with you, but you know that someday some plan reviewer is going
to conclude that stemwalls ARE part of the building's lateral system.
The truth is, of course they are part of the lateral system (they're on
the load path to ground), but here's my final conclusion why they should
not be in ACI chap 21 scope:  In a region of high seismic risk, any
concrete wall must get bumped into the category of "Special reinforced
concrete structural wall" - there is no middle ground.  I believe this
is where any expert will finally conclude that Chap 21 simply doesn't
apply to stemwalls - it is only for taller walls that are detailed and
expected to resist in-plane loads and cyclically degrade but resist
brittle failure or collapse.

Thanks for your input,

Ed Tornberg
Tornberg Consulting, LLC
503-551-4165

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Thursday, February 17, 2005 2:02 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: 3000 psi concrete and special inspection

Ed (and Dennis and others),

I will note that section 1921.2.4.1 of the 1997 UBC is part of section
1921.2.4 and that section is titled "Concrete in members resisting
earthquake-induced forces".  Thus, the requirement in the 1997 UBC that
concrete be a minimum of 3000 psi is ONLY for those concrete members
that
actively resist earthquake forces.

Now, you can certainly "dicker" over what that may or may not include.
To
me, it certainly means any concrete member that is part of the
building's
lateral system (i.e. shearwalls, beams or columns in moment frames,
foundation elements that deal with seismic forces, diaphrams, etc).
But,
that still leaves a lot of concrete in the building that may not require
3000 psi concrete (i.e. typical in-fill beams, non-lateral system
columns
or bearing walls, etc).  It is possible that one could argue that a
column, etc that must still deflection/move with the structure when
earthquake loads are present might still meet this requirement as they
will still likely SEE loads due to seismic even though they are not part
of the system to resist them.

Now, it does become a little more clear when looking at ACI 318.  I will
note that this provision (the 3000 psi minimum requirement for concrete
members resisting seismic loading) has been in place in ACI 318 for
quire
a while.  It has been in ACI 318 since at least the edition prior to the
1989 edition (the oldest edition I have is 1989 and there is no change
bar
in that edition).

The commentary for section 21.2.4 in ACI 318-02 give some insight into
the
intent for this section (not that it is necessarily legally binding...it
is in the commentary).  It states:

"Requirements of this section refer to concrete quality in frames,
trusses, or walls proportioned to resist earthquake-induced forces."

To me, this helps clarify that the intent is that gravity only columns,
beams, trusses, footings, foundations, etc need not be done in 3000 psi
concrete.

So, to me, (this is directed to Ed) the "stem walls" that have been
asked
about would NOT need to be 3000 psi concrete _IF_ they are not part of
the
seismic lateral system.  If for some reason the "stem walls" are part of
the seismic lateral system, then yes, they would need to be 3000 psi
concrete.

I will take this opportunity to point out a new provision in the 2002
ACI
318.  There is now a new part to section 1.1.1 in ACI 318-02.  It
states:

"For structural concrete, the specified compressive strength shall not
be
less than 2500 psi.  No maximum specified compressive strength shall
apply
unless restricted by a specific code provision."

This means that all concrete "governed" by ACI 318 must now be at least
2500 psi and any concrete "governed" by ACI 318 that is part of the
seismic lateral system must be at least 3000 psi.  To me, this further
helps "understand" the scope of section 21.2.4.1.

Now, I say "governed" in the previous paragraph as one also needs to
look
at the scope of ACI 318...look at the various provisions in section 1.1.
In particular, section 1.1.5 states that ACI 318 does not apply to
concrete piles, drilled piers, or cassions UNLESS such elements are in a
building that has a high seismic performance or design category.
Section
1.1.6 states that ACI 318 does not apply to slabs on grade unless that
slab transmits vertical loads or lateral forces from other portions of
the
structure to the soils (i.e. some other structural element loads the
slab
on grade, but loads directly applied [i.e. direct floor loading] don't
count).  So, this means that the previous requirements for concrete
strength would not apply in such situations...unless the model building
code (i.e. IBC) or local jurisdiction/state modifies things.

HTH,

Scott
Adrian, MI


On Wed, 16 Feb 2005, Ed Tornberg wrote:

> Dennis,
>
> Sorry I don't have a copy but doesn't the current CBC (aka 97 UBC) say
> in 1921.2.4.1 that "compressive strength f'c shall not be less than
3000
> psi"?  Note the only exception to this is for "footings".  (Note I
agree
> with Scott's inclusion by reference anyway.)
>
> Doesn't this force you to specify 3000 psi concrete for a stemwall?
>
> Do you draw a distinction between when you "specify" per the code and
> "design" per the code?  I'm referring to the subtle wording difference
> between the above and UBC 1701.5 or IBC 1704.4.
>
> I ask the question again: is everybody requiring special inspection on
> simple stemwalls?  If not, what is the reason?  I was trained by the
> oldtimers to always specify and design with 2500 psi for the
> "foundation", but I see that as only permissible for "footings".
>
> Ed Tornberg
> Tornberg Consulting, LLC
> 503-551-4165
>
> -----Original Message-----
> From: Dennis S. Wish, PE [mailto:dennis.wish(--nospam--at)verizon.net]
> Sent: Wednesday, February 16, 2005 10:53 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: 3000 psi concrete and special inspection
>
> Ed Tornberg wrote:
>
> > For engineers in "high seismic risk" regions, including the West
> Coast:
> >
> > Do you specify minimum 3000 psi concrete on all your foundations,
and
> > thus trigger special inspection?
> >
> > You are required to do this per ACI 318 21.2.4.1: "Compressive
> > strength f'c of the concrete shall be not less than 3000 psi"
> >
> > - ACI 318 Table R21.2.1 states that Foundations are included in the
> > scope of section 21.2
> >
> > - 2003 IBC confirms and reinforces this, by stating that .Seismic
> > Design Category D, E, or F, .foundations complying with Sections
[ACI]
>
> > 21.2 through 21.10 shall be used to resist [seismic].
> >
> > So I understand that all foundations, including stemwalls, PEMB
> > pedestals, etc. require special inspection. The only exception would
> > be isolated and strip footings.
> >
> > If you state that "design basis is 2500 psi", you are violating ACI
> > 318 21.2.4.1, correct?
> >
> > Ed Tornberg
> >
> > Tornberg Consulting, LLC
> >
> > 503-551-4165
> >
> Ed,
> In a sense, this is a loaded question. Most of the West Coast still
uses
>
> the 97 UBC and not the 2003 IBC. There is a section in, I believe,
> chapter 19 of the UBC that covers the use of 3,000 psi concrete. This
is
>
> especially true in areas that have specific municipal requirements
based
>
> upon modifications to the state adopted code (which in California is
> based on the 97 UBC). The use of 3,000 psi concrete, beyond its
specific
>
> need for strength in the design, triggers the requirements for special
> deputy inspection and structural observation. This can be required as
> part of the municipal code if in a region where the 97 UBC is not
used,
> but the local municipality has deemed it to required.
> The issue you raised is what governs. While I would like to say that
> ACI-318 governs, it is only the method, not the law. What is written
in
> the code that you use is law - it was adopted and ratified by the
state
> and local municipalities. You can use your judgment to argue the
issues
> or to design to the more conservative, but you can not, without a
change
>
> in the code, design to a standard less than that required within in
the
> code.
> I think that some years ago when design methods were part of the UBC
> (prior to 1994) this was an easier issue to deal with as discrepancies
> were less common since the section of the code dealing with any
material
>
> contained the general verbiage of the material organizations such as
> ACI, AISC, MIA, AITC etc. I don't particularly think that the split to
> sell separate references was good from the position of a practitioner
> since it left the rhetoric open to confusion and possible error.
> Finally, I don't specify a "design basis" but rather state that the
> minimum strength of concrete to be used must be 2,500 psi or as noted
on
>
> the plans. The contractor is free to use 3,000 psi concrete without
> special inspection and while I have a note that states that all grade
> beams and foundations resisting lateral forces are to be designed per
> section xxxx.xx.x.x of the current code. Furthermore, I specifically
> note the requirements for special inspection and structural
observation
> on the plan and on the details. There are times when, for example, I
> need to show a retrofit of an epoxy anchor if omitted or changed
during
> construction. If the anchor is for shear only, then deputy inspection
> requirements for epoxy are generally not enforced within most
> municipalities - the local building inspection can check the
cleanliness
>
> of the holes. The same is true when I use rebar dowels to tie a new
> foundation to an existing so as to prevent potential differential
> settlement. Here again, the anchor acts in shear and not tension so
just
>
> sticking a dowel into concrete without epoxy or grouting will still
> achieve the same goal when lateral movement (tension) is not the case.
>
> Hope this helps - just remember that code is law and ACI-318 documents
> are not enforced by law but are assumed to be the basis for the law.
If
> the law is worded wrongly and not corrected then theoretically you can
> design to the mistake and be covered. I think this is risky as your
> education and professional intuition can easily be questioned in a
court
>
> of law.
>
> *Dennis S. Wish, PE*
> *California Professional Engineer
> Structural Engineering Consultant
>
> dennis.wish(--nospam--at)verizon.net*
> *
> 760.564.0884 (office - fax)
> *
>
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