laterally bracing compression flange 9short question0

huangyhg

laterally bracing compression flange (short question)
how far below the compression flange can a lateral support be and still be considered a laeral support.  is there a rule of thumb?  
weron4u
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weron4u,
to resist lateral torsional buckling, look at where the neutral axis is located and determine the location of the compression and tension zones. theoretically, you would only need to extend bracing at a depth equal to the depth of the compression zone. the lateral support is only required to resist compression, not tension. however, keep in mind that the shapes of the compression zones are different for elastic and plastic analysis. the elastic is triangular while the plastic is rectangular since it assumes that each fiber is equally stressed.
unless the cost would become a significant factor, i usually try to provide lateral support members with the same depth as the main member. also for ease of bolting or welding, the deeper the   
sorry motorcity but that made a whole lot of no sense to me.  i have an s10x25.4 that spans 21.5ft and is lifting 5000lbs.  it needs a lateral support at midspan to be capable of this while having a safety factor of 1.5 based off of fb from aisc asd.  i will most likely use a double angle to provide this lateral support.  
my question is, how far below the top(compression) flange of the s10x25.4 can i weld those double angles and still have them provide lateral support for the compression flange.  is there a rule of thumb? or should i try to weld the lat support directly to the compression flange?
yura stated that a lateral support near the neutral axis of a section will not brace that section properly.  i would weld a plate to the underside of the top flange, and to the web, and then bolt the double angles to this plate.
daveatkins
let me re-state my response: weld the double angles
at least 5" below the compression flange.  
if yopu want a rule of thumb answer, stabilize the compression flange, not somewhere below it.
cheers, jwb46
weron4u,
is this a mono-rail? welding stiffener plates under the top flange (and to the web) is the way i'd use. the plate can be half height of the beam.
lateral support at neutral axis location is not enough. the buckled shape of top flange is still whole length sway. that's why in some analysis softwares (such as staad), unsupported length of top flange needs to be specified in addition to lateral unsupported length.
daveatkins - yura also says that if you brace at the neutral axis and resist torsional movement, you are almost as good as if you braced the compression flange translationally.  
so dropping your brace down to the center of the   
thank you all.
the most common practice i see for providing lateral support during erection is using a seated connection from the end of the lateral support to the web of the beam in question, with the depth of the connection being approximately 3/4 the web depth, which would provide resistance to lateral movement and torsional movement.  
i also occasionally see beams that were field welded into place after erection and use the lateral support on either the top flange or directly underneath it.  this is where my question came from.
dave i'm going to follow your lead on this one with the plate.  i like that idea.
thank you all for your insight, greatly appreciated,
weron4u
i have a thought, i think the effective distance for resisting ltb is the 1/6 of beam depth from the beam extreme compression side fibers
i got this thought from the value ot (rt) stated in the aisc maual 9th edition which use the interia of compression flange + (1/6 of the web) about there vertical axe.
this value is used when i reduce bending strenght due to given limits of unbrced lenght.
i would disagree that rt has any correspondence to the ltb brace height.