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Re: UBC97-- Redundancy Factor

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In a message dated 2/25/99 1:30:10 PM EST, 1548(--nospam--at) writes:

<< Re: UBC97, Reliability/Redundancy Factor
 1. What is your interpretation of the UBC97, section1630.1.1, eq. 30-3, if
 have a chevron brace?
 2. Do you still consider chevron brace as two brace elements when calculating
 r(max.) or one?
 3. Is there any difference between X-brace & chevron brace when calculating
 4. Maybe X-brace counts for two brace elements since each system (diagonal
 column) could be an "independent",
 statically stable system?
 5. However with  chevron is a different story -- buckling, let's say,  of one
 diagonal make the whole system unstable ( assuming that there are no special
 provisions for beam).
 6. What is your opinion?
 Lee Konczak SE
 Industrial Design Corp.
 Portland, Oregon

I numbered the responses relative to above and will try to address them

1.  I personally think the redundancy issue speaks for itself, the more frames
you have the lower the forces which you have to design for, especially the
connections on a SCBF.  You can rearrange the redundancy equation to determine
the preliminary minimum number of braces required so you won't be penalized by
the redundancy factor.  The analysis should be done on a floor by floor basis.
The  UBC allows you to start at the two thirds level of the building height
and check from that floor level down. 

2.  A chevron braced bay would be considered as being two brace elements (the
steel section 2213.8.2.3 talks about limiting the horizontal force component
of all members acting in compression to more than 70% of the total force
acting at that level).

3.  I see no real difference if you are using an "X" brace (two story x brace)
or a chevron.  I would consider the x brace as two seperate chevrons (one
inverted and the other normal).  The bottom part of a two story x will have
higher axial loads in the brace.  For a single story x brace (I might try a
single diagonal instead), I would consider this as two seperate brace

4.  The  x brace is two seperate elements, therefore you can use it as two
elements.  I would not consider the column as a bracing element.   

5.  The two story x brace is a unique animal.  You do not need to do the
special beam provisions since if one brace buckles, you have bracing above or
below the beam to resist the UBC unbalanced load combination acting on the
beam.  This can save about 180#/foot in beam weight (example: SCBF two story x
brace could use a W18x50 where a multistory chevron would require a beam in
the range of  W24x220 for TS8x8x5/8 tubes).

6.  To be cost effect for steel weight,  the two story x brace should be
encouraged.  Not necessarily cost effective for the designer though when using
the SCBF, getting the geometry correct is time consumming regarding out-of-
plane buckling of the brace gusset plates.  Performance wise, the SCBF is
better than the OCBF, and the final brace sizes may be very similar due to b/t
thickness ratio requirements. 

My two cents

Michael Cochran S.E.