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RE: SCBF Connections

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I believe the maximum force that can be transfered to the system
involves finding the "weak link" in the system such as the diaphragm's
strength the transfer the shear into the frame line.  Ron H. suggested
that rocking in the foundation could also be the possible "weak link"
thereby reducing the maximum force needed to be considered.

-----Original Message-----
From: Tom.Hunt(--nospam--at)d-fd.com [mailto:Tom.Hunt(--nospam--at)d-fd.com]
Sent: Tuesday, September 19, 2000 8:12 AM
To: SEAINT(--nospam--at)seaint.org
Subject: SCBF Connections


I am working on a steel structure which utilizes Special Concentric Braced
Frames (SCBF) per the 1997 AISC Seismic Provisions.  For the bracing
connection (gusset plate, bolts, etc.)  Section 13.3.a. requires the lessor
of:

a.  The nominal axial tensile strength of the bracing member, determined as
RyFyAg.

b.  The maximum force, indicated by analysis, that can be transferred to
the brace by the system.

Can anyone give me a good interpretation of item b. above??  Could you
assume that the upper limit of this would be a force based on R = 1.

The reason I ask is that in many cases the size of the brace is governed by
Kl/r < 1000/sq.rt Fy.  In my case I have a W8 x 28 brace, 16.4 feet long,
with only 31.4 kip factored seismic load, however, item a. above requires
the connection to be designed for 454 kips.  With an R = 1 the maximum
seismic load the system could see is (R = 6.4 for SCBF) 6.4 x 31.4 = 201
kip which I would like to use instead of 454 kip.

Does anyone see a flaw in this logic or any reason a building official
would not accept this reasoning?

Thanks,

Thomas Hunt, S.E.
Duke/Fluor Daniel


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