follow-up on app. 6

huangyhg

follow-up on app. 6
i figured i would start a new thread so that jae's wouldn't get hijacked.
while i agree that the tension force in the flange keeps it from twisting, i believe that is only true for a section that is braced against twist at its ends.  this is, after all, the basis of all the aisc equations.  if that condition isn't met then all the equations go out the window.
additionally, if it were true that bracing the compression flange against lateral tranlation prevents ltb, then 1.) how do you explain the sidesway web buckling phenomenom?
and 2.) why does app.6 explicitly state that the section must be braced against lateral translation of the compression flange and twist of the section?  these are two distinct requirements.
check out our whitepaper library.
i think it would depend on the magnitude of the tension force.  a tension force increases the flexural stiffness of a   
streit:
i think you knew the foundamental on structural deformation, but let's just re-examine and review the geometries of member subjects to simple concentric axial & gravity loads.
for a member under axial compression, it has to shorten. when the material reaches buckling stress, it tends to bend side way, which is free to translate (buckling up or down is prevented by web).
on the other hand, a member subject to axial tension, it lengthens and stiffens (in all directions but axial). we all know the fact that a stiffened/tensioned string would not deform side ways without external forces to force the issue. thus, the member remains straight in its horizontal plane until something else happens.
now project these two members on a common plane, the distances between the two lines (representing the members) form the twist planes in between the   
correction:
"i think you knew the foundamental on structural deformation, but let's just re-examine and review the geometries of   
kslee-
i'm not sure how your post relates to my question.  if bracing against lateral translation of the compression flange is the same as bracing against twist of the section, then why does aisc explicitly state that the equations are based on the sections being braced against twist at the ends?
streit:
sorry the above does not help. no comment on aisc.
according to article 9.2.4 and 9.2.5 of the csa commentary (condensed):
quote:
additionally, if it were true that bracing the compression flange against lateral tranlation prevents ltb, then 1.) how do you explain the sidesway web buckling phenomenom?
i thought sidesway buckling is more a local consideration from point load bearing, and a function of the web slenderness. the web is in compression and the tension flange may not have the stiffness needed to restrain web sidesway.
quote:
2.) why does app.6 explicitly state that the section must be braced against lateral translation of the compression flange and twist of the section?  these are two distinct requirements.
where does it say that?

quote:
if bracing against lateral translation of the compression flange is the same as bracing against twist of the section, then why does aisc explicitly state that the equations are based on the sections being braced against twist at the ends?
sounds like preventing lateral displacement of the compression flange at the ends would qualify as preventing twist.  
quote:
if bracing against lateral translation of the compression flange is the same as bracing against twist of the section, then why does aisc explicitly state that the equations are based on the sections being braced against twist at the ends?
it may be that they defined it that way since the moment is often zero at the ends, so it wouldn't be correct to provide a brace only at one flange at the ends of the beam to prevent twist.