Seismic Slenderness Limits

[JoshPlumSE]

I'm looking at Table I-8-1 of the 2005 AISC seismic detailing specicifations (AISC 341-05). This table lists the limiting element slenderness ratios (h/t, b/t, et cetera) at which an column, beam or brace member can be considered "seismically compact" or "highly ductile".

Now, some of the member's limiting slenderness will depend on a Ca factor which considers the level of axial load in the member.

Everyone with me so far? Now, I'll have some members that have two different levels of axial force in them. My columns may require that I design them to the regular ASCE load combinations 1.2DL + 1.0 EL as well as the overstrength combinations. 1.2DL + 1.0*Omega*EL.

When I'm classifying my column as seismically compact, what load combinations am I using, the regular ones or the Overstrength ones. The code doesn't make this particularly clear.

 

[JAE]

JoshPlum,
I'm away from my office right now but usually the [Ω] factor (i.e. the overstrength combination) is only applied where the code specifically requires it. This is usually brace connections and collectors. Columns usually have their own design requirements as well.

Just off the top of my head - don't think it necessarily answers your question. If I have time this week I'll take a look at 341.

 

[JoshPlumSE]

Obviously, if the column, beam or brace member is NOT required to be designed to the Omega load combinations, then I wouldn't be using them to check the slenderness ratios.

My question is when the code specifically requires that the member be designed to the Omega overstrength load combinations, then should we also use those overstrength load combinations to check the slenderness ratios.

 

[JAE]

Oh - I got you - slenderness is based on Ca which is based on axial - then which axial do you use?

I would again revert back to the code - if the brace in question requires the overstrength combo, then you use the overstrength axial in that combo to check for slenderness.

In using the codes - each load combination is sort of "a world of its own" - a separate and distinct check independent of all the other load combinations.