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RE: steel brace

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Greg,
There are three problems with this system that need to be
solved.

1- All forces are resisted by tension-only or
compression-only braces. Seismic provisions requires that
at least 30% and maximum 70% of the total force is resisted
by tension braces. This is simply to avoid getting flat
hysteresis loops in load reversal cycles due to large
plastic deformation of compressive braces. The solution to
this would be using stiffer braces with less chance of
buckling and less plastic deformations, i.e. designed for
load combinations 4-1 & 4-2:
1.2D + 0.5L + 0.2S + Omega x Qe
0.9D - Omega x Qe
 
2- Using one single brace transfer an unbalanced load to
the beam. So the beams needs to be design for resulted
shear and moment. In standard Chevron bracing system,
Zipper column distribute unbalanced load to all beams in
that frame. Assuming that not all compression braces yield
together, this approach would resulted to smaller beam
sizes. But in your system, the unbalanced load exists on
all floors at the same time. So the only advantage of the
Zipper would be redistributing these forces and get
approximately same size for all beams, instead of having a
heavy beam in first floor.

3- Due to geometry of the bracing system, drift would be an
issue. This system will behave like EBF with large link
length. The efficiency of this system in drift control can
be as low as 20% of the similar elements configured as CBF.
If this is an issue, then stronger braces are required.
Therefore unbalanced load may increase as well (up to
system capacity) which will result in larger beam size.

Frank

--- Greg Meyer <gmeyer(--nospam--at)scahouston.com> wrote:
> Charlie & Scott, I wish I could change the bracing system
> but the architect
> has left me with limited options.
> Scott is correct in explaining the system that I am
> proposing here. (a 1
> brace chevron)
> Why is this a bad system?
> It seems to me like the brace system could be considered
> as both a
> concentric brace and a moment frame since the brace is
> putting the beam into
> bending?
> Maybe it would behave similarly to a knee brace type of
> system?
> What to you guys think?
> 
> Greg Meyer
> 
> 
> -----Original Message-----
> From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
> Sent: Wednesday, July 24, 2002 8:02 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: steel brace
> 
> 
> >Yes, for the descriptions provided it
> >appears to be what you descrbe (i.e.
> >half of a "chevron" type brace).
> 
> That kind of detail could get ugly quick in my opinion.
> 
> For SCBF, we have requirements that make V and inverted V
> braces robust to
> minimize the potential for losing a diagonal (which makes
> a nice concentric
> system eccentric). Then we have requirements for V and
> inverted V systems to
> ensure that, if a diagonal is lost, the beam can carry
> gravity load and the
> system does not destabilize. Seems like this
> configuration intentionally
> starts at the point we have a bunch of provisions in
> there to avoid.
> 
> Perhaps it could be treated as an EBF, but there are
> aspect ratios for links
> that must be satisfied. Also, a link adjacent to the
> column makes for a
> system that places a higher inelastic rotation demand on
> the beam to column
> moment connection than for the same interstory drift on a
> moment frame.
> Again there are specific provisions to keep out of
> trouble. So you wind up
> putting the link between the braces or really beefing up
> the moment
> connection.
> 
> Can you configure the system another way? That may be the
> best solution (and
> the simplest).
> 
> Charlie
> 
> 
> 
> 
> 
> 
> 
> 
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=====
Fariborz Tehrani,                      Phone: (310)801-4237Professional Civil Engineer              Fax: (877)743-6206BSCE, MSCE, PE                    Voice Mail: (877)743-6206email: general@fmtehrani.comweb: www.fmtehrani.com

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