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Re: x brace question
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- Subject: Re: x brace question
- From: "Michael Valley" <mtv(--nospam--at)skilling.com>
- Date: Wed, 11 Mar 1998 19:06:13 -0800
- Comments: Authenticated sender is <mtv(--nospam--at)linux.skilling.com>
- Priority: normal
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 brace.) 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Michael Valley E-mail: mtv(--nospam--at)skilling.com Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200 1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
- References:
- Re: Structural I Study Guides
- From: Richard Lewis
- x brace question
- From: Tarek Mokhtar
- Re: Structural I Study Guides
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