Subject: Re: ACI 318 - request for comments for improvements
From: Rick Burch <rburch(--nospam--at)conterra.com>
Date: Sat, 09 Jun 2001 15:51:22 -0400
> I would like to see if you would provide us with a little assistance, which
> ultimately could end up helping yourself.
> ACI has heard and received comments about the complexity of the ACI 318
> concrete code.
Well, I'm pretty amazed at the replies that you are getting along the
lines of "I find concrete and steel equally complex to design." If this
were the really the case, I don't think you would have ever had to ask
Take shear in a steel beam for example. In steel you take
V/(d*tw)<0.4*Fy and you are done. Or, to be more accurate, you don't
even do anything in most cases since you know from experience that shear
doesn't come close to being a problem in steel beams.
For a concrete beam, you open up ACI 318 to Chapter 11 and first find
FOUR PAGES OF VARIABLES. Then this is followed by 44 pages of text and
commentary. Granted, this chapter also includes torsion, but can anyone
actually say this is equal complexity?
Take designing a simple span steel beam. S = M/(.66*Fy), pick the beam
out of the Sx table, check deflection, and you are done.
For a concrete beam, you have to determine the end negative moments as
well as the positive moment, since there are few simple span concrete
beams. Then design the top bars and the bottom bars. Determine where to
cut them off, where to lap them and how much. Remember that laps depend
on bar spacing and cover. Be sure the beam is wide enough. If you
actually have to compute deflections, you first have to figure the
cracked moment of inertia, the cracking moment, the gross moment of
inertia. Then, for long term deflections, you figure another factor. For
stirrups, figure one, maybe two different spacings.
Beams are actually one of the easiest concrete members to design. If I
had really wanted to show the difference in complexity, I would have
used a slender column as an example. Check out all the requirements for
figuring sway moments and non-sway moments at each end, then the factors
for magnifying these moments, etc., etc.
I have worked at the engineering department of a large industrial
company, at a very large consulting engineering firm, and at a very
small consulting engineering firm, and I can say that at all three
places I know of plenty of jobs where one of the main deciding factors
between steel and concrete was the fact that steel is much easier to
design. (This if off my topic here, but steel is also easier to repair
if there is a mistake in design or construction, and it is easier to
modify in the future as needs change).
I may be the only one here to say so, but I vote for anything that can
be done to simplify the code. In my book, "simplify" is not a bad word.
If anyone thinks that all the complexity in the code is producing safer
or more economical structures, they are mistaken. To prove it, just pick
one of the obscure requirements in the code, ask some older experienced
engineer about it, and watch the blank look that you get. A lot of
simplifying is being done out in the real world, where engineers are
mainly interested in getting a safe job out, not in analyzing or
designing something to extremes. It is a little counterintuitive, but I
think that a simpler code would lead to more economical and safe
structures, since engineers would actually know what was in the code and
what it means, and would therefore be more likely to use it.
I know that concrete will always be more difficult to design than steel
just due to its nature. I think this is all the more reason to not make
it any more difficult than is absolutely necessary.
Simplify, simplify, simplify, every area that you can. To answer your
specific question, anything that can be done to make column design
easier would be my first priority.
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