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RE: SCBF Connections

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I asked this same question a few months ago.  Below are some of the answers
that were posted.  I hope this helps.

I originally asked:
UBC 97 section 2213.8.3.1 states that bracing connections must be designed
for the least of three (3) different cases.  One of the cases is "The
maximum force that can be transferred to the brace by the system".  It has
been suggested that the only way to obtain this value is thru a static
push-over analysis or a non-linear time history analysis.
Could someone explain (to a person not experienced in high-seismic design
requirements) the definition of "maximum system force transferred to the
brace" and how this value is normally obtained?
The building is Seismic Zone 3, Soil Profile SD, Importance 1.0, Ca 0.36, Cv
0.54, concentrically braced steel frame.

Mark Deardorff replied:
Look at the capacities of the elements that transfer force to the brace.
Collector elements, while designed for a max force of Em, have an upper
bound based on the capacity of the diaphragm capacity. Thus, the diaphragm
capacity may be lower than the the collector capacity which will limit the
force transferred to the frame.

Mark

Charlie Carter replied:
This option (maximum system force) is a limit that can be applied whenever
there is some feature of the system that limits the force that can be
transferred to the element in question by the system. That sounds like
circular logic, so I'll explain with a specific example. The maximum force
in a brace in a braced frame may be limited by uplift or overturning if the
structural system is one that utilizes spread footing. This limit may not be
applicable however for the same superstructure combined with pile
foundations.

Hope this helps. 

Charlie 


> -----Original Message-----
> From:	Tom.Hunt(--nospam--at)d-fd.com [SMTP:Tom.Hunt(--nospam--at)d-fd.com]
> Sent:	Tuesday, September 19, 2000 11:12 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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