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RE: UBC Section 2213.8.2.5

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Chapter 10 of Ductile Design of Steel Structures, by Bruneau, Uang,
and Whittaker (1998) describes some of the analytical (elastic and
post-elastic buckling) and experimental work that went into
establishing element slenderness criteria for elastic, plastic, and
seismic design.

The slenderness limits are related to the desired ductility before
local post-elastic buckling takes place.  That's why the values for
plastic design are different from those for seismic design (for
instance, 65/Fy^1/2 versus 52/Fy^1/2 for wide flange bf/2tf; to get
more ductility in the seismic design).  Based on work by University of
Michigan Professor Subhash Goel (and his co-workers), the AISC Seismic
Committee decided that post-buckling flexure controls the response of
concentric braces.  Because it's a flexural ductility mode, the same
slenderness criterion as for flanges of I-shaped members (52/Fy^1/2)
was adopted.

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Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: Acie Chance [mailto:achance(--nospam--at)lacsd.org]
Sent: Tuesday, May 23, 2000 4:36 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: UBC Section 2213.8.2.5



     I have jest started my first braced frame structure under the 97
UBC.
Section 2213.8.2.5 limits the width to thickness ratio of angles to
52/Fy^1/2.
This puts the minimum thickness of a 6 in. angle at 3/4 inch. Under
the 94 UBC
Table B5.1 the width to thickness ratio was 76/Fy^1/2. The 94 UBC
would allow a
6 in. angle to be 1/2 in. thick. I would like to know of any reports
of
failures and/or testing which brought about this change. Most of my
structures
are not buildings and I could use section 2213.8.3. However there size
and cost
makes it desirable for these structures to  go through a major
earthquake with
out following down or needing major rework.

Thanks
Acie Chance