lateral torsional suppor

huangyhg

lateral torsional support
we have an existing catwalk that we are adding load to. the bar grating on top can be removed, so i don't consider that for lateral bracing of the beam.  the beams are 20' long, about 3' apart, shear connected at the ends.  there are beams connecting these two beams at third points.  do these beams provide lateral torsional buckling resistance for the two main beams, even if only shear connected?  without the beams  providing lat-tors support, the beams fail in the existing load condition.
tests have shown that full depth shear connections on beams that are of equal depth to the main beams do provide adequate rotational stiffness to be considered braced points for ltb.  if you have something considerably less than that, i'd be worried...
if you "heard" it on the internet, it's guilty until proven innocent. - dcs
(3) bolt double angle connections on 16" deep beams.  where might i find the test results of which you speak?
i would say no, unless proven otherwise, as directed by my design code as4100 (australian).
from the commentary;
"c5.4.2 restraints at a cross-section
the case of a cross-section that is restrained against twist rotation but free to deflect laterally is not included because of the difficulty of providing simple guidance.
some very stiff torsional restraints may induce buckling modes in which the cross-section acts as if fully restrained (clause 5.4.2.1), while in other cases torsional restraints may be comparatively ineffective, and the restrained cross-section may deflect laterally."
perhaps you can fillet weld a diagonal brace (typical 3 places) to the underside of the w16 flanges. the welding could be simple downhand, performed from the top of the existing beams. suitable angles, say 3x3, should be sufficient to stiffen the walkway and prevent ltb.

i don't have access to the tests - the information was from a stability conference given by yura several years back.
in your case, even though the braces don't technically provide support for lateral displacement, they provide enough torsional support to prevent ltb.
if you have any appreciable lateral load, however, i suggest slideruleera's solution.
if you "heard" it on the internet, it's guilty until proven innocent. - dcs
i have designed these kind of structures before, from my experience i have seen that as long as the bracing   
get your hands on yura's "fundamentals of beam bracing" paper.  the aisc seminar notes by yura and helwig are very helpful also and it seems like most offices have at least one copy of these floating around.  the subject is more complicated than most give it credit for.
for this problem, ask yourself the question: "does anything prevent the beam from laterally displacing *or* twisting at the proposed braced point?"  you might have a "lean-on" bracing system.
swearingen, i'd say your first statement is usually true, but surely has exceptions.  i've never heard of an actual test.
i like slideruleera's advice in this case.
when the grate is removed, the loads causing the buckling are also missing.  use the grate as top flange bracing.
how is the grating attached to the top flange?  it may or may not provide resistance depending on this.
we could specify welding for the grating, say at 12" on center.  but some beams would have the weak axis of the grating to provide lateral support.