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Re: AISC Seismic PRovisions for SCBFs

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I appreciate Mr.  Sabelli comments, and would like to add a few additional 
comments/questions.  Comments/questions  inserted within the original message.

<< I    The "R" factor for SCBFs corresponds to a brace yielding mechanism, 
 not to collector failure.  Analysis may indicate that not every connection 
 must resist brace yielding when the yield mechanism forms.>>>

I assume this would be more justified when doing a non-linear analysis.  When 
doing a linear three dimensional analysis I assume you would want to design 
all braces for yielding because of the uncertainty as to the exact brace 
yield sequence and redistribution of forces as each brace yields.

<<< II   The brace-to-brace connection must also be designed for these forces.
 III  To limit the connection demand there are a few things that can be done 
 with the overall frame design to permit using a smaller brace:
   A  Lower-strength steel should be considered for slender braces. >>

The use of lower strength steel seems like a good concept, but I imagine the 
availability would be very limited, especially  if you are specifying A36 
steel.  I imagine that most steels will probably have actual yields in the 50 
ksi range, if not higher, due to recycled steel being used in current 
domestic steel production. 

The use of pipes, I believe, requires that the gusset plate connection be 
designed for out-of-plane bending since the pipe will most likely always 
buckle out-of-plane unless the gusset plate is reinforced to force the pipe 
to buckle in-plane.
 <<  C  Flat-wall width not overall width, should be used when following the 
 AISC Specifications.>>

I would agree with you on that.  But, for those of us designing with the 1997 
UBC, I am afraid we are stuck using the out-to-out dimensions unless we can 
convince the building official otherwise.

<<   D  Full flexural and axial continuity through the brace-to-brace 
 connection can be provided; this will reduce the effective length to below 
 the half length.  Be aware that for out-of-plane buckling this may induce 
 significant torsion in the tension brace.>>

Excellent point about the torsion.

<<   E  If possible, the ends of the brace should be fixed.  If this can't be 
 done, measure the brace length from the line of restraint at the hinge 
 zone, not from the column centerline.>>

What types of connection would you recommend to obtain complete fixity at the 
end of the brace.  Are you suggesting to use a connection other than gusset 
plates.  This would be helpful in eliminating the need for the 2t offset 
requirements on the out-of-plane buckling of the gusset plate.  I assume 
there could be a possible problems with local wall buckling of tubes and 
pipes at the point of fixity.

For the design of the brace element, I would still use the face of column to 
face of column dimension for sizing of the brace.  I am not comfortable using 
the clear distance between gusset plates to size the unbraced length of the 
brace. To design the gusset plates, I would use the clear length between 
gusset plates.  This is only really applicable when (Omega x earthquake 
forces) govern (are used for) gusset plate design (UBC: SCBF, OCBF; AISC: 
OCBF).  In cases where the connection is designed based upon  tension 
yielding (AISC: SCBF), then the actual length of the brace is not necessary.
<<< John, if any of this is unclear or if you would like references for the 
 above, please feel free to contact me directly.  Good luck,
 Rafael Sabelli


Michael Cochran