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RE: x brace question

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Thanks for your informative response.  I guess what caused the confusion between k=0.5 and k=0.65 was the definition of L.  If you consider L as the length of the brace from the bottom connection to
 the column to the upper connection to the adjacent column then you absolutely right, K=0.65 is overly conservative assuming that C/T ratio is met.


From: 	Michael Valley[:mtv(--nospam--at)]
Sent: 	Wednesday, March 11, 1998 11:06 AM
To: 	seaoc
Subject: 	Re: x brace question

Tarek Mokhtar wrote:

> For an ordinary x braced steel frame with tube steel braces
> can one assume the brace buckling length for in and out of plane
> to be 1/2 the actual brace length ?

I have read several other responses that indicate that out-of-plane
buckling of the compression diagonal is not braced by the tension
diagonal.  That is generally NOT the case.  You might refer to the
following articles (and their references) that discuss theoretical
and experimental work on this topic:

Picard and Beaulieu in AISC Engineering Journal, 3rd Quarter 1987,
pp. 122-126.

Picard and Beaulieu in AISC Engineering Journal, 4th Quarter 1988,
pp. 156-160.

Stoman's discussion of Picard and Beaulieu papers.  Discussion was
in AISC Engineering Journal, 4th Quarter 1989, pp. 155-159.

To summarize for common cases (braces pinned at ends and connected to
each other at the crossing, same section and material properties for
both braces):

When C/T < 1.6, K=0.5 for full length of brace.  (Where C is the
force in the compression brace and T is the force in the tension

When T = 0 in one brace, use K = 0.72 for full length of compression
brace. (Note that this is NOT both braces in compression, but is one
brace in compression and the other unstressed.)

Others mentioned using K=0.65.  The AISC codes (ASD and LRFD) use
K=0.65 instead of K=0.5 for a fixed-fixed column condition because
infinite fixity is impractical.  The conditions leading to K=0.5 (of
full length) for X-braces is different because it is based on bracing
due to axial stiffness and not flexural rigidity.  Using K=0.5 for
the full length is conservative as long as C is not greater than
1.6T.  Using K=0.65 for balanced conditions (C=|T|) would be
analogous to using 1.3 times the distance between braces for typical
compression members--that is, unnecessarily conservative.

Many designers (and authors of papers) use K<1 where they can
identify partial end restraint (for instance, in trusses with all
welded connections).  Similarly, some use K<0.5 for full length of X
braces where the design provides end restraint in the connection.

As a practical note, ductile detailing of X-braced systems assures
that the tension brace really can brace the compression brace because
the tension connections are strong enough to guarantee that
compression brace buckling controls (and the braces are usually
balanced).  Unless your braces pick up a considerable amount of
gravity load compression, one brace is much larger than the other, or
your braces do not cross at midlength, you should not end up with
conditions that would require using K>0.5 of the full length.

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Michael Valley                                   E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201