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RE: "X" Bracing

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I am pretty sure they are talking about the braces and not other members.
The rationale for this, as far as I know, was that if all your braces, along
a line of resistance, go into compression at once, when the load is
reversed, due to probable buckling that may have occurred in the previous
direction, the capacity may be substantially reduced. As for tension only
rods, if some of them fail in tension, then in the next cycle there would be
loss of capacity also. The crux of the matter is that, there is a lack of
redundancy in a system that places all braces in tension or compression at
once. Hence the 70% rule. 

And as Michael Cochran correctly indicated, for compression members there is
an exception that allows their use in 100% compression if they meet the
specified loading combination. The exception was a much needed addition in
the 97 UBC. I remember that in 1990 I was reviewing a 40 story dual frame
system that had all the braces in 2 lines of resistance in the same
directions all the way through the height of the building. Since 1988 UBC
had just been adopted the designer was not aware of this requirement and the
steel had already been ordered in advance. So we compromised and suggested
that if all the braces could resist 3/8 Rw (Omega in the new code) times the
design loads we would accept the design. They ran the numbers and
fortunately ended up needing to upgrade only a few of the braces.

Ben Yousefi, SE
San Jose, CA

	-----Original Message-----
	From:	David B Merrick [SMTP:mrkgp(--nospam--at)]
	Sent:	Friday, May 25, 2001 9:52 AM
	To:	seaint(--nospam--at)
	Subject:	Re: "X" Bracing

	Mr. Yousefi:

	Is not 2213.8.2.3 using the term "members" (not "bracing members")
for those
	horizontal members collecting the load and delivering it to the
bracing members?
	This would insure bracing members were not all at one end of a line
of force.

	2213.8.2.1 controls slenderness stability, ruling out cable

	2213.8.2.2 uses the term "bracing members", that would be different

	David Merrick


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