Residual stresses

[Lion06]

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.

 

[271828]

Do you mean "general softening" as in "reduction in the overall system stiffness"?

If so, then I still don't get it because overall system stiffness is a function of variables that are already in the analysis (member elastic stiffnesses, reductions using taub, member geometric stiffnesses, etc.).

What exact behavior, that's not already in the analysis somewhere, affects overall system stiffness?

 

[haynewp]

Before I continue, I will advise that I have had a couple of glasses of wine... As the load effects go past the linear elastic range (using ultimate, factored loads) there is partial plastification in some areas of the structure since the entire cross section of the entire frame is not remaining below the elastic limit. So how would you say that local yielding be accounted for in the model? We are not applying elastic range service loads in the DAM.

Softenting to me is a reduction in stiffness from a linear elastic model. If by using ultimate strength design, parts of cross sections along a frame are expected to pass yield points then I think this should be accounted for in an linear elastic model!!

Three cheers for AISC for having a free searchable datatbase of papers and presentations made available to AISC members, where ACI gives nothing.

 

[271828]

That is really funny.

I do think what you typed makes sense. Full yielding here or there is not taken into account in the model, so maybe that's it.

Three cheers? I agree completely, although perhaps the statement's placement was due to the wine? I'll go even farther: A fourth cheer because they include pdf EJ papers from way back. Whenever I need an ASCE paper before about 1990, I know I'm in trouble because I'll need an inter-library loan or hike down to the library and get a hard copy.

 

[Lion06]

I found something in a text I have. It is titled, "Structural Stability of Steel" and is written by Galambos and Surovek. It is a 2008 text and does have a discussion on the DAM.
It says, "The reduction in flexural stiffness accounts for both the general yielding that may occur at design load levels, as well as the effects of residual stresses on column inelasticity. In particular, the reduction due to the residual stress is handled on a member-by-member basis through the use of the inelastic stiffness reduction factor, or tau factor."
This suggests that haynewp is correct about the 0.8 factor. It also shows that 271828 is correct about the taub factor account for residual stress.