To: "SEAOC Newsletter" <seaint(--nospam--at)seaint.org>
Subject: Re: ASD vs. LRFD
From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Mon, 14 Jun 99 22:59:35 -0500
>I'll give you a few examples of situations when "LRFD is better."
Appreciate your taking the time to provide a couple of 'f'rinstances. I
think that's a first. I do have a couple of comments though. (I realize
I'm sticking my neck way out. Before the LRFD thread started I'd always
figured keeping well clear of building design was only a good career
move. Now I'm wondering if it might be a matter for prayerful thanks. ;->
I'll agree that seismic loading is a gray area, and that plastic response
often needs to be assessed. I daresay playing around at the margins of
real honest to god non-linear dynamic analysis isn't going to help much.
Not unless someone chooses to risk his life and publically propose time
history seismic analysis. So we're left with response spectra methods and
a sort of quasi-linear approach. Like we've always used.
The wind loading is puzzling because it seems like you're just playing
around with the ratio of lateral load to dead load, neither of which
should be affected by the method of analysis. Hurricane Andrew blew just
as hard for structures designed by limit methods and for elastic
analysis. Or have I missed something. Seems to me that I should be
designing against the same thing with linear elastic methods, simply by
using the same proportion of lateral to dead load.
And the comment about service loads really doesn't address 'better' as
much as 'different.' seems like you could accomplich the same thing by
doing the design based on service loading and then checking for collapse
mechanisms afterward, rather than figuring collapse mechanisms first and
then doing the deflections. Besides the very real cumulative effects of
service loading, my guts are telling me that we're still using all the
linear asumptions (notably superposition) and pretending that they hold
for non-linear response.
I'm a little reluctant to get more and more involved in this since
service loads govern the stuff I do, and it'll be ever thus. Moreover the
problems I hear about on this list seem service based--very few
discussions involving ultimate collapse. I wonder if my uncle felt the
same way when he came back from France in 1919 to find a popular majority
had somehow passed a law so wildly unpopular (and culturally irrelevant)
Christopher Wright P.E. |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)