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RE: RC column Axial Load Equation - ACI

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Scott,
I regret that I offended you with my comments.  I have to admit that
from the perspective of the code writers, they may seem like cheap
shots.  The point I was trying to make is that engineers in general, and
reinforced concrete engineers in particular, like to live under the
impression that they know how their structures behave with a certain
amount of precision; and that we need to be alert to the fact that that
precision usually isn't real.

When I was writing the concrete column design software that I mentioned,
I first did hand calcs (as I always do when I write programs) solving
one interaction point for a round column with eight bars.  That
calculation took a full page, and a lot of very careful effort just to
ferret out the simple arithmetic mistakes.  I was able to match CRSI's
values to the accuracy with which they are given, for which I was proud.
Then it dawned on me that the results probably carried at least two more
significant figures than they should have.  Let me rephrase that:  the
real accuracy of that calculation is probably one hundredth of what I,
and CRSI, state as fact in our calculated interaction points.

Now, with the apology-and-explanation portion of my message behind us,
let me quote from the Commentary to Appendix C of ACI 318-99:

"...The load factors in Section 2.4.3 of ASCE 7 are said to be based on
a survey of "reliabilities inherent in existing design practice."  For
reinforced concrete buildings in countries where the ACI Building Code
and similar codes have been used, the best and most compact survey of
"reliabilities inherent in existing design practice" are the load and
strength reduction factors used in the ACI Building Code.  Currently,
the strongest support for the strength reduction factors in Appendix C
is the fact that, used with the load-factor combinations from ASCE 7,
the results are generally compatible with those obtained using Chapter
9."

Seems a little cocky to me.

And, earlier on, "..It is unsafe to use the load factors from Reference
C.1 (i.e. ASCE 7)with the strength reduction factors from Chapter 9."

Three years later, the Code told us to do exactly that, for the bending
case that I mentioned.

Again, we don't know as much as we think we do.  It gets under my skin
when Code writers need to pump themselves up like this, only to reverse
themselves in the next cycle.

And that's all I am going to say today (applause in the background).

Mike Hemstad, P.E., S.E.
TKDA
St. Paul, Minnesota



-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Monday, August 09, 2004 6:38 PM
To: Michael L. Hemstad
Subject: Re: RC column Axial Load Equation - ACI


Michael:

I would offer that if you excercise some care when offering such
attacking comments.  You don't help matter when you play it a little
"fast and loose" with your comments.

I can tell that ACI 318 did not "blithely" change anything.  There is a
LOT of heart burn that still exists on the committe over the change in
the load factors.  The change was ONLY approved because there was an
extensive realiability study done that showed that it was OK to switch
to the 1.2/1.6 factors but remain at the same phi factors for bending
(to a degree...after all the phi factor only remained the same for PURE
bending...but the main body of the code now uses the "unified method" in
which it is more likely that the phi factor will be some where between
true bending and true axial...and the phi factor DID change for the
axial "side").  The idea that you seem to offer that ACI just
"willie-nillie" changed to the new load factors gives the false
impression that 318 was neglegent in this.  I certainly hope that is not
your intent.

And, FWIW, I am sure that there are those that still believe that
1.4/1.7 is still the true path.

And there are those that wonder why members of code committees don't
participate on the list...they will get blasted from all sides.  ACI 318
members get blasted by you for changing to 1.2/1.6 that tons of other
people have been blasting them for years to do.  Why would anyone want
to show their face on this list only to get blasted no matter what they
do? For that matter, I don't really no why anyone (myself included)
would want to do the thankless job of being on a code committee, because
undoubtly someone will always be unhappy with what they come up with and
will comment (from the "cheap seats") that they could have done it
better, even though they won't take the time and/or effort to get
involved with a code committee (and I am not not necessarily refering to
you).

Regards,

Scott
Adrian, MI


On Mon, 9 Aug 2004, Michael L. Hemstad wrote:

>
> rooms after dinner (according to Ted Galambos, who would know).  ACI, 
> after all, is the group who cavalierly said that the 1.4/1.7 load 
> factors were the only true path in the commentary of one issue of 318,

> then blithely changed to 1.2/1.6 in the next issue without changing 
> anything else (like the phi factor) for flexural design.
>

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