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Steel bracing b/t[Subject Prev][Subject Next][Thread Prev][Thread Next]
- 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.
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