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RE: Seismic bracing system- Knee-braced Frames

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Gentlemen,

I have developed a procedure for ductile design of knee-brace used as
retrofit for existing moment connections and tested it in 1993. In this
procedure tension yielding as well as in-plane buckling of bracing are both
used as a mechanism to dissipate energy and transfer desired moment to a
column. The slenderness ratio of bracing within the range of 40-70 could be
selected for bracing which are to buckle but have the energy dissipation
capability and maintain good post-buckling capacity. The connections must be
designed for overstrength of tension members and overstrength moment
capacity of the bracing.

I designed and tested knee-brace consisting of double-angles and used them
as retrofit for existing riveted connections. Ref. ASCE, Structural Journal
1996, Vol. 122, No.7, July 1996. Pp. 762-775

Dr. M. Sarraf, PE
Senior Bridge Engineer/ Seismic Specialist
Imbsen & Asscoiates Inc
Sacramento, CA
916-366-0632


-----Original Message-----
From: Keith Fix [mailto:kefix(--nospam--at)yahoo.com]
Sent: Friday, December 06, 2002 1:57 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Seismic bracing system- Knee-braced Frames


I think you're right, except maybe the answer doesn't lie with AISC, so to
speak, but with the research community.  Somebody needs to put these braces
to
the test and see what happens.

The best argement for the knee-brace is the reliability of the connections
themselves.  The questionability seems to lie in the system.

Either way, without testing, we're stuck.

All that said, where should the ductility be developed?  By ductility in the
brace, or by ductile hinge formation in the beam at the brace connection?

-Keith Fix, PE
-Little Rock, AR

--- Igor Marinovic <IMarinovic(--nospam--at)vp.com> wrote:
> I think that previous discussion about KBF systems omitted several
important
> facts:
>
> 1)	Typical KBF system used by PEMB industry is intended for small loads.
It
> doesn't work, i.e., it becomes uneconomical, when the eave height exceeds
say
> 20 ft. (even less than that)
> 2)	Typical KBF system used by PEMB industry does not carry gravity loads,
> other than selfweight.
> 3)	Typical KBF system has a brace located not far from the beam-to-column
> joint. Consequently, the KBF system behaves similar to moment frames,
where
> the "triangle" at the top of the column transfers the moment across the
> joint. The shape of the deflected members and the actual deflections are
> almost identical to moment frames. However, the integrity of this system
> relies heavily on three details: beam-to-column connection, brace
connection
> to the column/beam, and brace strength for axial compression (and tension)
> loads. So, these would be the details subject to additional seismic
criteria.
>
> 4)	If the brace is lowered so it connects in the middle of the column
height
> the behavior of the system under lateral loads resembles the K-braced CBF,
> which, as mentioned by others, may have detrimental effects on the column
> (frame) integrity. This is not the Knee-braced system!
>
> So, lets recognize that in addition to apples and oranges, there are
pears...
> The KBF system is a practical system that can be correctly designed and
> detailed within certain height/load limits.
> Maybe we need Charlie to suggest another category for the next version of
> Seismic Provisions. Maybe to add a subset to OMF category, with special
> detailing requirements for KBF details mentioned above?
>
> Thanks,
>
> Igor Marinovic, PE, Memphis, TN
>
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