Bracing for Lateral Torsional Buckling

[STEVE97]

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

 

[Lion06]

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.

 

[engn555]

The Canadian steel code mentions the 2% rule. Cant remember the exact clause but I know it is in Sect 9.
As far as I am aware, CISC makes no distinction between a "relative or nodal brace"
What is the difference between them?

 

[Lion06]

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.

 

[STEVE97]

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.

 

[Lion06]

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?