aisc appendix 6 - beam bracing

huangyhg

aisc appendix 6 - beam bracing
through past work by yura the aisc specification has added appendix 6 which provides requirements for bracing.
section 6.3 is for beams and includes both relative and nodal bracing requirements.  the general concept i understand but we've been trying to understand the explicit application of this section to brace designs.
the main issue is that the brace strength equations ((a-6-5 and a-6-7) provide values for required brace strength, pbr.  this is given as a force in pounds as a function of bending moment and beam depth (ho).  
but if we provide braces at 4 feet on center, we get a pbr value.  if we put braces at 8 feet on center we still get a similar pbr value since the moment is the same.  the beam might be a bit deeper since lb would be larger, but that means that the brace force actually gets smaller since ho is in the denominator....and that seems counter-intuitive...fewer braces means less brace strength required.
anyone have any views on this?

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nothing useful.  
one thought: i hate app. 6 because it doesn't make sense about 1/2 the time i try to use it!
you only get a smaller brace force if you have a deeper beam, right?  if you keep the beam constant, then the brace force stays the same regardless of the number of braces.  that is the part that didn't make sense to me.  
i can see the brace force decreasing as ho increases (for the same moment), because the force in the flanges will be smaller.
one other thing about app. 6.  the very first paragraph says, "beam bracing shall prevent the relative displacement of the top and bottom flanges, in other words, twist of the section.", but the spec specifically says that beam bracing needs to prevent lateral translation of the compression flange or twist of the section.  which is correct?
twist needs to be prevented. the driving force behind ltb is "buckling" of the compression flange (similar to column buckling). by preventing lateral translation of the compression flange you are, in essence, preventing twist.
to be quite honest, i don't really know how you could prevent twist and not prevent lateral translation of the compression flange.
i don't think you can prevent twist without preventing translation of the compression flange, but i think you can definitely prevent lateral translation of the compression flange without preventing twist.  
if you have a beam subject to pure moment such that you have a brace at the top flange only, no where else on the section, i can see the bottom flange curling up toward the laterally restrained compression flange.  the only thing preventing it is the weak axis bending strength of the thin web.  
isn't this similar to the sidesway web buckling phenomenom when a beam is subject to a high concentrated force, and the reason that a stiffener is required even if braced laterally?
the tensile force in the bottom of the flange prevents it from twisting, much like an axially loaded   
ok - but getting back to my question....  
the aisc forumla gives the req'd brace strength in lbs.  but it doesn't give guidance on the spacing of those braces and that doesn't make sense.
any ideas?

hi,
could it be interpretated that the brace force (lateral load) calculated is for a particular point along the beam.
for example if the beam is subjected to a udl then the maximum moment is at midspan and the lateral force is also applied to the beam at midspan hence a brace at midspan would have to take the entire load.
if you needed additional braces because the unbraced span of the beam was still too long, the brace force for each would be dependent on the moment in the beam at each brace location.

tclat has a valid point.
tclat, that makes sense and goes along with what i was thinking - the brace force equation is implied to be for a singular point on the beam - with the moment that occurs at that point.
suppose, though, that you design a beam spanning 30 feet taking a total distributed load w, with a single brace at midspan.  the moment is wl^2/8 and you get a beam size based upon lb = 15 feet.
you use the aisc forumlae to get a pbr capacity for your brace at that point.  so far so good.
but now if you look at using 3 brace points, the lb now equals 10 feet, and you get a new beam that is perhaps a bit smaller in weight but the same depth.  
with the second design, aisc implies that you need almost 3 times the brace strength of the first design since the pbr's for each brace will be approximately the same as for the single brace.
that, to me, just doesn't make sense.  i can see the three braces, together, providing a similar pbr capacity, but all three needing that much capacity compared to the single brace?
this is what has me questioning this issue.

jae,
i don't have access to the formula you are talking about, so it is difficult to comment on that.  i am attaching a page from csa s16-01 for your comparison.
you said:
quote:
but now if you look at using 3 brace points, the lb now equals 10 feet, and you get a new beam that is perhaps a bit smaller in weight but the same depth.
that would be for two brace points which divide the beam into three sections.  in the csa formula, delta(0), the initial misalignment, is based on the length of beam between brace points...if l is reduced, so is delta(0).  the term beta increases from 2 to 3 for one and two braced points respectively.  also, note that for two or more braced points, the forces pb alternate in direction.
  
ba