lateral bracing for long span beam

huangyhg

lateral bracing for long span beam
hi,
  i have a question about lateral bracing for a beam i'm designing.  it's an existing 2 story structure, built 2 years ago.  the setup is repetitive.  anyway, they want to remove the 2nd story mezzanine and and 2 interior columns from the interior of the building and hang the roof from a new beam stubbed up from the roof.  the span is 75', with 2 point loads (interior columns)acting symmetrically.  i sized it as continuously braced, but have questions about the lateral bracing.  i am going to attach another w section horizontally to the top flange of the beam.  this horizontal section will resist the lateral forces due to lateral torsional buckling.  i'm sizing it for lateral load of 2% * half the reaction of the beam.
the point loads are equal to 35 kips.  the reaction of the beam is 35k + .262k*75'/2 = 44.9k.  2% of this is .8985 kips.  this is applied per foot so the moment is .8985*75^2/8 = 632 k*ft.  i also designed it for l/360 -> ix required =8823 in^4.  i get a w24x250.  does this sound reasonable for this kind of span?  or is it 2% of the reaction from self-weight only?  i don't think that's the case, buy it yields a much smaller section, w21x93.  i know it's a 75' span so the lateral loads are large but i just want to justify my calc.
thanks
i am going to attach another w section horizontally to the top flange of the beam.  this horizontal section will resist the lateral forces due to lateral torsional buckling.  i'm sizing it for lateral load of 2% * half the reaction of the beam.
i am unsure about what you mean in the first sentence. are you setting a second w section on its side, atop and parallel to the main section?
if so:
using the simplified method you are suggesting, i would size this for 2% of the compressive force in the compression flange, not 2% of half the beam reaction. with the second section spanning the entire length of the main section, this will result in a varying force with the same shape as the main beam's moment diagram.
however, an analysis of the actual moment resistance of the new section may result in a smaller section being required, as the combined section will have greater lateral and vertical stiffness than the two added together. i would go this route, as you will need to design the connection between the two   
i think gwynn has answered your question correctly.  the capping beam only has to take 2%, or whatever percent is applicable, of the compression flange force.  as the capping beam will in itself be most of the compressive flange, i would first do the permutations necessary to select the sections to resist the vertical load, then check horizontally.
i think you'd be better off in your circumstance to use torsional bracing to laterally stabilize the beam.
it will make your profile smaller as the strut beams will frame into the side of the girder (versus sitting on top)
this allows for a cleaner moment connection to the girder too. also, you can design the other end as a pin connection and don't have to worry so much about adding any additional loads into the original building components(aside from the vertical reaction)
appendix 6.3 of aisc 13 does a fine job of walking you through the process.
i wouldn't use the 2% rule of thumb in this case.
what you are doing is creating a new section comprised of the two wide flanges.  simple calculate new section properties and design the beam accordingly as a unit.
with lrfd, you'd use appendix f, calculating all the required λ factors and using the beams new cross sectional properties (you will still have a singularly symmetric shape).
with asd, you now have a different rt factor (a very large compression flange) and design according to the beam formulae given.
by compression flange force do you mean decoupling the moment and just taking that load.  when i want to calculate the horizontal shear at the edge, it's 0 obviously because it's at the end of the   
actually, the horizontal shear is maximum at the ends.
pretending that the new capping beam is somehow an independent thing that laterally braces another beam is pie-in-the-sky simplification.
the two will work together as a single flexural element in reality.
by adding the wf on top, you are greatly altering the cross sectional properties, and the neutral axis location.  
design it as a combined section.  calculate the horizontal shear using q = vq/i and design the longitudinal connecting welds accordingly and in accordance with chapter b in the aisc spec.
correction the decoupled moment is 345 k 2% of this 6.9 kips.    even if i do design as a combined section, how do i know determine what the lateral force to resist is?  that's what i'm struggling with.  as of right now i'm using 2% of the beam reaction. which is .02*44.925 = .8985 k applied per linear foot.  i'll look into the combined section.  am i supposed to take 2% of the horiz shear?
i was reading in section 2.3 of blodgett that the horizontal shear is 0 at the ends, and it sounded suspicious to me.
it should be designed as a combined section with no lateral restraint (assuming that there are no transverse   
thanks you guys have been a great help.  i sized the beam for l/360 deflection not based on strength.  this is 75'*12/360=2.5".  i calculated the ix based on this deflection (which governed and easily satisfies strength requirements) and sized it for this.