bracing for lateral torsional buckling

huangyhg

bracing for lateral torsional buckling
i need to brace the compression flange of a beam temporarily against lateral torsional buckling while a temporary load is added. i was wondering what the required force is to reist the buckling? i have heard that the rule of thumb is 2% of the flange force. has anyone else heard this or know the exact answer?
thanks
it depends if it's a relative or nodal brace.  this is covered in great detail in aisc 360-05 app. 6 of the spec.
for a relative brace, the required brace strength is 0.008mrcd/ho , and the required brace stiffness is 4mrcd/(lbho) either divided by phi or multiplied by omega depending on asd/lrfd.
the canadian steel code mentions the 2% rule. cant re  
a relative brace just braces the beam with respect to other brace points.  a nodal brace braces the beam with direct help from other brace points.  the way i think about it is that a nodal brace somewhat fixes the brace point in space (in the axis of the brace), relatively speaking of course.  a relative brace fixes the brace point with respect to other brace points, but not in space.  the relative braces can all move together.
i figured out the maximum spacing of my baces to be able to get the full allowable stress (lb < lc) using l/rt. the spacing of my braces can be at a maximum of 14 ft to get the full allowable stress of 0.60fy. now i need to find out what force do i need to design these discrete braces for? 2% of the flange has been a common rule of mine but i only heard that in a seminar once and never understood where it came from.  
my 4th edition s&j text has a discussion on this for the old asd.  why not just use app 6 in aisc 360-05?