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RE: x brace question

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Alright, I guess I'll comment, too.

In an X-brace, generally only one brace is actually a continuous 
member, the other one is split by the gusset.  So for one direction, 
the section which is used to brace the other has essentially no 
bending stiffness - it's stiffness is going to result from catenary 
tensioning of the section as it is displaced laterally.  And a 
catenary must displace laterally quite a bit before is starts to 
provide very much lateral resistance.  So the use of an effective 
length factor on the order of 0.65 doesn't seem unreasonable.  Even 
if moment continuity was provided across the center intersection 
point, I might question using the theoretical 0.5L value, since the 
natural out-of-straightness permitted in AISC specs means that the 
tension section will have to bend a bit in order to straighten out.

Also, for seismic loads don't forget the beta factor defined in the 
UBC (or the equivalent reduction factor defined in the AISC seismic 
criteria).  As the members stretch and buckle a bit, they will become 
even less straight!!
--------------

Martin Johnson
EQE International, Inc.
mwj(--nospam--at)eqe.com