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Re: Steel bracing b/t[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Steel bracing b/t
- From: Rick.Drake(--nospam--at)fluor.com
- Date: Mon, 20 Sep 1999 07:25:43 -0700
b/t ratios are usually based on conventional "Timoshenko plate theory" for statically applied axial compression loads. However, this theory does not accurately predict the local buckling performance of HSS sections which have not performed very well in cyclic load tests, often experiencing local buckling after only a few cycles. As a result, b/t ratios in all of the codes tend to be very restrictive for HSS sections in seismic applications. Not to pick nits, but most mathematicians consider a square to be a special case of a rectangle. I would consider code provisions that refer to rectangular sections to include square sections. Rick Drake, SE Fluor Daniel, Aliso Viejo ******************************************** "Uthoff, Steve" <uthoff(--nospam--at)hninc.com> on 09/17/99 03:10:07 PM Please respond to seaint(--nospam--at)seaint.org To: 'SEAOC list' <seaint(--nospam--at)seaint.org> cc: (bcc: Rick Drake/AV/FD/FluorCorp) Subject: Steel bracing b/t 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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