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# Steel bracing b/t

• To: "'SEAOC list'" <seaint(--nospam--at)seaint.org>
• Subject: Steel bracing b/t
• From: "Uthoff, Steve" <uthoff(--nospam--at)hninc.com>
• Date: Fri, 17 Sep 1999 15:10:07 -0700

```I have TS8x8x3/8 braces in a base-isolated building.  These braces remain
elastic even under Max Credible Earthquake loads.

Title 24 section 2211A.8.2.5 states that the width-thickness ratio b/t shall
be per AISC Table B5.1 for compact sections = 190/sqrt Fy = 28 for square
and rectangular tubes.  The AISC width b is defined as the clear distance
between webs less the inside corner radius, which is the total section width
minus three times the thickness.  My b/t = (8 -1.125)/0.375 = 18.33 by this
definition, so TS8x8x3/8 is OK.

Title 24 has a provision stating that "rectangular tubes shall have outside
width-thickness ratio not exceeding 110/sqrt Fy=16.2. There is an exception
to this if the "compression element is stiffened to resist local buckling."
My b/t=8/0.375=21.33 by this defintion, so TS8x8x3/8 is no good, unless you
stiffen the brace.

My opinion is that the base isolation "stiffens" the brace to resist local
buckling, in fact the brace will never yield even due to MCE.  I also
believe that by a strict reading of Title 24, square tubes may have
b/t=190/sqrt Fy, and only rectangular tubes require b/t= 110/sqrt Fy.

Question:  For a base-isolated building, is it sound engineering judgement
to not allow TS8x8x3/8 braces, and require TS8x8x1/2 braces?

Thanks,
Steve Uthoff

P.S. Compact sections develop plastic capacity prior to local buckling.
Non-compact sections locally buckle between yield and plastic capacity.
Slender sections locally buckle prior to yield.

```