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Re: Steel bracing b/t

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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)> on 09/17/99 03:10:07 PM

Please respond to seaint(--nospam--at)

To:   'SEAOC list' <seaint(--nospam--at)>
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?

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.