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Actually, some research has been done on this, but I cannot remember the names of the authors except Nethercote and Galambos.  If you apply a load to the top flange, i.e. above the neutral axis, the load will magnify any top flange lateral buckling due to any eccentricities (P x e_sub_x). On the other hand, a load applied to the bottom flange( or below the neutral axis) tends to pull the beam back into toward its vertical axis.  If you wish I can send you a copy of a page from Galambos that gives K factors for various beam support situations depending on where the load is applied.
Regards, Gary


David Topete wrote:
I don't believe the point of load application matters as much as the stresses in the beam.  if you sit a post on a beam or hang a load from the bottom flange, the top flange will still be in compression while the bottom flange is in tension.  therefore lateral bracing points are still to be added to avoid the buckling.  unless I am missing something in the discussion...

On Sat, Nov 21, 2009 at 5:57 AM, Gary L. Hodgson and Assoc. <design(--nospam--at)hodgsoneng.ca> wrote:
If the load is applied to the bottom flange only, the beam is unlikely to buckle sideways.  However as the building code (here in Canada) does not differentiate between points of load application, I would be obliged to brace the beam laterally, based on Limit States Design.  One brace in the centre will work if using 50ksi steel.  However, using A36 steel, I would need 2 brace points.  This last one was done rather quickly using old fashioned ASD, so don't hold me to it.
Gary


G Vishwanath wrote:
Thanks Mark and Harold for your quick response.
 
Yes,I inadvertently omitted to mention the span.
You both correctly guessed it as 25 feet from the BM value.
 
Threre is no horizontal brace to restrict the unsupported length of the compression flange.I  I later got the opportunity to do the calcs.
 
W8x18 is confirmed as being hopelessly inadequate.
However if the beam is somehow laterally held at mid third points , it will pass muster.
I checked this out later.
 
Without lateral support the smallest beam that I could suggest was W12x30.
 
This is in response to a quick query from the field. They have a W8x18 ready which they can support on walls and they need to lift 4000 pounds just once and they don't want to spend too much money.
 
I have noted Mark Johnson's point also alerting me to the need to prevent rotation at the ends of beam resting on the wall.
I am advising the field staff accordingly.
 
Thanks once again.
Regards
Vish
 
 

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David Topete, SE
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