residual stresses

huangyhg

residual stresses
aisc says to use 0.8ei when using the dam, in part, to account for residual stresses and the section seeing plastic stresses/deformations before analysis tells us that it wants to.
my understanding is that the 0.8 is geared toward wf sections (since that is what is used in typical building construction).  aisc doesn't give any guidance on a stiffness reduction for hss sections.  i know it would be conservative to use 0.8, but i'm up against a wall and i believe that 0.8 is unnecessarily conservative as an hss is uniform thickness (except for the corners) and will cool much more evenly.  additionally, if you do consider the residual stresses, the tension stresses occur where it helps the section (at the corners), unlike a wf section.
does anyone have any literature that might address this?  i don't expect to find anything related specifically to the dam, but if i could at least find something generically relating the residual stresses in hss's to those in wf's it would be helpful.
you have a very good point.  however, i thought taub was for dealing with residual stresses and the 0.8 has a bit more mysterious origin.  for long, skinny columns, the 0.8 plays a similar role as the 0.877 factor for column buckling.  i honestly don't know exactly why it's there for stockier columns, but i've assumed it was a fudge factor to account for out-of-straightness and whatever else hasn't been explicitly accounted for.
i have never seen a residual stress plot for an hss.  i would've thought that the situation would be entirely different from that of a w-shape, though.  for a w-shape, residual stresses exist because the flange tips and web mid-depth cool faster than the web-flange junction.  threfore, there is compression at the tips and web mid-depth and tension at the junction.  for an hss, it would seem that the residual stresses would be due to welding at the seam.  i don't see why there would be residual stresses at teh corners other than those from the process of making the corner.  these plastic stresses are transverse, though.  i don't know enough about plasticity to know what longitudinal stresses should result from these.  i admit to never digging into this subject or running across a paper or reference, though.  if anybody has one, i'd be interested in seeing it.
for now, i think i would stick with the dam as is even for hss.  my reasoning is that we have to deal with ch. e inelastic buckling and elastic buckling with exactly the same equations for w-shapes and hss columns.  if there was some more favorable condition known for hss, then it seems like there would be different ch. e equations for those.
if anybody would know about this, it would be someone like fred palmer at the steel tube institute.  you might try to contact him.
the 0.8 is to deal with residual stresses for general softening, the tau just added extra softening for the increased stresses in highly loaded compression members.
the 0.877 factor in the fcr equation accounts for   
willisv, i do not read the commentary that way, but i admit that the info seems a bit sketchy to me at the moment.  i need to dig up the papers and read them.
check out the last sentence on page 436, continuing onto page 437.  it talks about how for elasic buckling systems, 0.8 is approx equal to phi*0.877 so we end up with a strength equal to 0.8 times the "elastic stability limit."  why 0.8 is our magic number, i do not know.  my guess is that it's simply to be consistent with ch. e, which is good enough for me.
below that, it talks about why we have 0.8*taub for stockier members.  
taub is described as "similar to the inelastic stiffness reduction factor implied in the column curve..." i interpret that as meaning that we have a tangent modulus situation because we can't count the portions of the flange tips that have yielded (residual stress + p/a exceeds fy for some part of the flanges).  we need a smaller ei for larger p/a because we've lost more of the flange tips.  finally, as p approaches fy*as, we have taub = 0.0.  if we have p/a < 0.5fy, we have no yielding at the flange tips, so taub = 1.0
0.8 "accounts for additional softening under combined axial compression and bending."  this doesn't say anything about residual stresses.  i think this is because ei is effectively smaller if the   
my comments weren't based on the commentary, they were based on seminar notes from don white and larry griffis (both on the committee that developed the provisions).   
i don't have any seminar notes, but the draft stability design guide was written by white and griffis.  at the bottom of page 29, they start a section on column inelasticity.
in the first paragraph, they state "the effect of these residual stresses is to induce premature yielding across the scross section, which in turn reduces the effective moment of inertia, i of hte cross section.  a simple but convenient  way to account for this effect is to modify the moment of inertia i from i to tau*i where tau is a stiffness reduction factor."
they then give a couple of different tau factors, one of which is taub at the bottom of page 30.  on page 31, they state "this equation is the original crc parabola equation for the column tangent modulus.  it may be considered approximately to include the effects of residual stresses but not include the effects of column geometric imperfections."
two paragraphs later, they state "it is appropriate to use taub when modifying   
271828-
any idea when that design guide might be out?
no clue.  last i heard, it was out of the authors' hands and in aisc's, but that's second-hand info.  third-hand now, lol.
aisc is planning to release it sometime this summer. atleast, this is what i heard from one of my friends who works there.
-strucguy
lol, ask him what 0.8 is for!
it's my understanding that the residual stresses are of a 10ksi to 15ksi range and the 0.8 is a rough measure of these.  residual stresses for very slender columns have little effect.
dik