To: "SEAOC Newsletter" <seaint(--nospam--at)seaint.org>
Subject: RE: Who's using ASD or LRFD?
From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Mon, 31 Jan 00 10:26:25 -0600
>You're both right and wrong on this, Chris.
I figured as much (and no offense taken. I daresay we'd've had a lot less
bickering over relative merits with a little more understanding of
origins and differences). Just the same, how much difference can there
be? No question that incorporating plastic bending response is a jump,
but column response already includes the effects of yielding. Moreover
tensile and shearing limit loads (excluding stability considerations) are
pretty much defined by first yield. Not too much left beyond expanding
the details, unless you start getting into real non-linear response like
actual collapse loading. Specifically it doesn't sound like there's any
big change in real safety margins, such as might require less explosive
to be used in demolition.
>The 1999 LRFD Specification will have, among other things,
>explicit treatment of srength and stiffness requirements for stability
>bracing for beams, columns, frames, etc., a section on evaluation and load
>testing of existing structures, and a major improvement to fatigue
Is this going beyond the current practice of specifying limiting
width/thickness ratios to stave off elastic buckling or are you getting
into post-buckling strength or improved correlations of buckling response
of thin elements? Maybe a little more detail on bracing stiffness
Tell me a little more about the fatigue improvements. I presume this
isn't going to incorporate anything too revolutionary like crack growth
mechanisms and fracture mechanics, so it'll be an upgrade to the old way
of specifying cyclic response based on construction details. Harking back
to the SMRF problems with Northridge my guts tell me that a little more
attention to basic fracture prevention in the area of weld quality and
plastic response might also be on the way.
This isn't to argue about which approach is better, because it's still
not obvious to me that they're all that much different, at least in the
results produced. The sorts of changes (especially fatigue and bracing
provisions) you mentioned look like general improvements which aren't
peculiar to LRFD and would be advisable for ASD design like the machinery
and the associated structures I do. Which will likely always be based on
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)