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AISC Seismic 15.6a, EBF brace design, brace connection to link be am design

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Hello:


I am looking at my first EBF up close and personal and am going to get
cross-eyed looks at church next week because of it.  All the braced frames
that I have seen before normally assume the braces have hinged ends.  97
AISC Seismic 15.6a says the required combined axial and flexural strength of
the diagonal brace shall be the axial forces and moments generated by the
expected nominal shear strength of the link multiplied by .....  I have a
couple of design examples that show the braces designed for moments factored
for (1.25*Ry*Vn)/Vu per AISC Seismic 15.6a but some of the typical
connection details don't show brace flanges welded to link beams  and have
gusset plates in the plane of the link beam web welded to brace flanges.
Also see "Fig. C-15.6. EBF with HSS bracing" on page 100 of the 97 AISC
Seismic.

One notebook from an AISC Seminar appears to show the gusset to link beam /
beam welds designed for only the moment resulting from the eccentricity from
the beam centerline to the weld, multiplied by the axial beam force
transmitted to the brace [Seismic Design and Detailing of Braced Frame
Structures, Lundeen July 1999 page 93].  It seems that if the brace flanges
are not welded directly to the beam then you would have to design the
gusset-to-brace connection for the axial force in the brace combined with a
couple between the flange welds. Also the gusset to beam weld appears to
need to be designed for the internal forces the brace is designed for.  Am I
looking at this right?  

* Is it standard to model to the brace to beam connection for EBFs as a
rigid connection?  Do some people model the brace to beam/link beam as a
hinge and design the brace for axial forces only and ignore moments other
than the gusset to beam weld?

*Should the brace-to-gusset and gusset-to-link beam connections be designed
considering the factored up moment and axial force that the brace is
designed for by AISC Seismic 15.6a?

*Is it  typical to weld the brace flanges to the link beam or what?


Thanks.

Scott M Haan P.E.
Plan Review Engineer
Building Safety Division 
Development Services Department
Municipality of Anchorage
http://www.muni.org/building
phone:907-343-8183  
fax:907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us


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