he defination of laterally support fot w-shapes

huangyhg

the defination of laterally support fot w-shapes
hello everyone,
i am working on an analysis of a w-shape beam, it is laterally supported by the floor wood joists at the bottom, but the top flange is not supported, can the wood joists be the w-shape beam's lateral support?
thank you very much!
best regards,
tom
how are the wood joists framing into the bottom flange?
can you post a sketch?
after sketch can be sure as s...eit said. looks like bottom flange is but if wood joist are within 2" of top flange, than top flg is also braced laterally.  
     i would recommend you refer to appendix 6 of the latest aisc specification (aisc 360-05). appendix 6 deals with stability bracing for columns and beams. with regards to lateral bracing for beams it states "bracing shall be attached near the compression flange...". additionally the commentary to appendix 6 with reference to bracing for beam stability states "beam bracing must prevent twist of the section, not lateral displacement..." and also goes on to say "lateral bracing systems that are attached near the beam centroid are ineffective."
     it is the beam flange in compression that is the cause of instability, as such this is where the lateral bracing needs to be placed to be effective. if the lateral bracing does not provide restraint near the compression flange it will be ineffective at preventing beam instability. in terms of how near to the compression flange the bracing needs to be, i would say that would be a function of the height of the w-section. the 2" given by dgkhan would seem reasonable, however for something like a w6 section this would be near the beam centroid, which would be ineffective as already mentioned.
     as the specific w section, joist size, connection details, and joist spacing was not mentioned i would also recommend quickly checking into the strength and stiffness requirements also presented in appendix 6 of the aisc code as a quick sanity check. the strength and stiffness of the bracing must be commensurate with the beam to be braced. a large built up plate girder would require far stronger bracing than a small standard rolled section.

thepenguin-
while it is true that the brace must be located near the compression flange to be considered to brace said flange you can have a detail that braces both flanges.  i am thinking specifically of a beam over column connection where the bottom flange is attached to the column.  the stiffeners provided over the column and the positive attachment of the bottom flange to the column provides torsional restraint and qualifies as a brace point.  while i'm not thinking of a similar detail for wood joists, i have to admit that i'm not thinking of why they would be framing into the bottom of the w shape either.
either way, it is possible for the physical brace to be at the bottom flange and still brace the top flange.
structuraleit -
     i agree that bracing in general need not be located near the compression flange to prevent beam instability. the aisc code refers to beam stability bracing as "lateral bracing" and "torsional bracing" with different stiffness and strength equations for each type. the bracing to which you refer is as you said torsional bracing. however both types of bracing referred to in the aisc code have the same goal, to eliminate or reduce the possiblity of beam instability (lateral-torsional buckling). as you had mentioned a detail of a purely torsional brace involving wood joists framing to a steel beam does not come to mind. unless the joists in question provide some form of direct torsional restraint, they will be ineffective at providing beam stability if connected to the bottom flange (bottom flange assumed in tension). i would refer those interested to "fundamentals of beam bracing" by yura, which can be read here
structuraleit:
sounds like they are framing into the bottom so the ceiling will read flush, still allowing the deflection to be limited by the depth of the steel beam.
i would consider a shallower steel beam that would fall into the joist depth, allowing the top and bottom flanges to be braced with no concerns.  moreover, by not extending the beam line above the level of the plywood, a flange width wider than the wall above can be utilized, affording a greater i to resist the deflection.  if need be, the plywood can be nailed off to blocking, which can be nailed off to a wood filler bolted between the flanges of the steel beam.   
mike mccann
mmc engineering
mike could you elaborate on this statement...
"moreover, by not extending the beam line above the level of the plywood, a flange width wider than the wall above can be utilized, affording a greater i to resist the deflection.  if need be, the plywood can be nailed off to blocking, which can be nailed off to a wood filler bolted between the flanges of the steel beam.  "
it sounds interesting but i am a bit confused by how u worded it.
the steel beam, as orginally described, extended above the line of the plywood floor.  in order for the steel beam flange not to protrude outside the width of the wall, the flange width would have to be limited to the width of the studs, as 5.5" for a 6" wood stud wall.  
if the beam is kept within the depth of the floor joists, then the flange width has no visual impact on the wood stud wall above, and the flange can be wider, affording a greater i value.  
my only concern here would be providing a nailing surface for the edge of the plywood at the steel beam.  normally, this is handled by using a 2x or 3x top plate attached to the top of the steel beam with nelson sill plate anchors.  hoewever, in this case, to increase the depth of the beam the and gain greater i value, i chose to use a wood infill between the top and bottom flanges and bolted to the web, with 2x blocking nailed to the filler to which the plywood can be nailed.  
hope this is a little clearer.
mike mccann
mmc engineering
clear, but i don't know if that is what the op was describing.  i interpreted the original question to be that the steel beam and joist were about the same depth, but he was wondering if because the joist load was applied to the bottom flange if it would brace the top.