To: SEAOC Newsletter <seaint(--nospam--at)seaint.org>
Subject: RE: ASD vs. LRFD
From: Scott E Maxwell <smaxwell(--nospam--at)engin.umich.edu>
Date: Thu, 27 Apr 2000 19:18:06 -0400 (EDT)
> This eludes me. It sounds like you're saying that researchers can't
> handle the initial elastic response so they just note where the thing
> falls down, which doesn't make a lot of sense. Surely the transition form
> elastic to plastic behavior is part of the work. Any examples you can put
> your finger on?
Yes, the full range of behavior is examined during research, including the
"yield" point. I am not really sure what my point is/was (sometimes I
open my mouth before I think <grin>). I guess I look at the ultimate load
as being more significant...especially in relation to the yield point.
You also need to keep in mind that a significant part of research taking
place today structurally is seismic related. Ultimate strength plays a
much more important role in this research (i.e. how much ductility/energy
absorbtion can certain elements/connections with stand).
I will admit that both methods are somewhat arbitrary. After all, we
don't design to yield point but something that is approximately 60 to 67%
of yield (assuming fully braced and all that other stuff). Realistically,
when you are doing ASD design, you still have to take into account
ultimate loads. After all, if you have a beam that is completely
unbraced, it is not going to even reach yield point before it fails...it
will buckle/sway out of plane.
I guess I am rambling again...
I do agree with Charles in that changes to codes/specifications do seem to
occur too rapidly. I am not one who has problems with change in general,
but change things too often and it is more likely that mistake will occur
(both in the development and application of the code).
Take the seismic codes as an example. Since I have graduated (not that
long ago), the BOCA code has under gone SIGNIFICANT changes in the seismic
portion. The 1987 BOCA code had virtually no seismic portions, if I
recall correctly. The 1990 BOCA had seismic but was VERY simplistic. The
1993 BOCA introduced the first "real" seismic code in the BOCA. It was
based off of the 1991 NEHRP. The 1996 BOCA code was essentially the same
as 1993 with minor changes. I have not seen the 1999 BOCA (I ain't
shelling out the $$$ until someone adopts it around here), but would
assume it is based upon the 1997 (?) NEHRP, which was a drastic change.
While I don't use the UBC, I believe it may be worse in recent years.
>From what I understand, there is significant changes when going from 1994
to 1997 and there will be significant changes when going from 1997 to IBC
We are all familir with problems that people have encountered while trying
to use the 1997 UBC. Just think that we might go thru this again when the
2000 IBC is adopted.
Now I am all for incorporating in new, state of the art knowledge that
should make our designs better. But do we lose some of that benefit of
better techniques when people have to keep relearning methods and some of
the small things slip through the cracks?
Just to close out my rambling, my personal feeling is that people should
be about to use what tools (i.e. ASD or LRFD) they are comfortable with
until it gets to the point that one of those tools is so out of date that
it presents safety issues or finicial issue. I use ASD 100% of the time,
yet was taught LRFD in school. I use ASD because I am used to it and
don't want to take the time to make a switch at this time (I am lazy after
all <grin>). But, I foresee a time where I will have to switch and that
will not bother me in the least. In fact, I just might become unlazy when
the 3rd edition is released and make the switch.