To: "INTERNET:seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: RE: effective length factor for sway moment
From: Peter Higgins <76573.2107(--nospam--at)compuserve.com>
Date: Fri, 6 Apr 2001 16:36:54 -0500
Message text written by INTERNET:seaint(--nospam--at)seaint.org
>What I have seen is some people really just want K=1, so there can be
smaller columns, without any other limitations. The bottom line is that
IBC does not amend the AISC Seismic Provisions to allow specifically allow
K=1 in sway frames carte-blanch. Also nowhere in the code does it say K=1
carte blanch if a P-Delta check is employed.
I feel I must finally weigh in on this one. No one (that I know of at
least) is trying to make K=1 simply to get smaller columns, regardless of
the consequences. This is simply nonsensical.
There are two places where a K=1 is quite appropriate:
1) Where an "equivalent imperfection" design technique is employed. It
should be noted that virtually the entire world outside the US and Japan
has switched to this design method. To quote a european colleague: "nobody
uses effective lengths anymore. They went out with bell bottom pants."
[that was said before they had a resurgence]
2) Where you use an effective length method but certain criteria regarding
the story stiffness are satisfied. This is well documented in the
literature, but essentially most frames satisfy this criterion. Yura's
papers on tangent modulus stiffness techniques have been considerably
expanded in the 30 years since he published them, and we really should make
some effort to keep up, rather than waste vast amounts of steel simply
because we're too lazy to pick up a journal after we leave school.
Peter S. Higgins, SE