When and how to consider stability requirements (AISC Chapter C) for Steel beam-columns?

[MJC6125]

This is a rather broad question, but I'm trying to work on my understanding of the AISC specification's chapter C. Specifically, where do I need to be taking additional steps in the design process to accurately consider the stability requirements of chapter C? Below are some of my specific questions:

1. What are some common steel compression member situations where chapter C needs to or does not need to be applied? Does it always need to be applied?

2. One of the programs I use for design is Enercalc. I don't think the steel column module of this program considers 2nd order effects, but I could be wrong. Does that mean this program shouldn't be used for any sort of steel design problem where chapter C stability requirements are critical?

3. The other main program I use for steel design is Risa 3D. I understand this program accounts for P-Δ effects and can account for P-δ effects if you introduce intermediate nodes along the length of a member. When do you consider adding these supplementary nodes to take into account the P-δ effects? I have read the Risa white paper about chapter C which had good insight into this, but I'm curious when and how other practicing engineers on here are considering P-δ effects?

4. When do you consider notional loads in steel design? Is this something you always consider for steel column design or is it just in certain situations?

5. How do you consider notional loads in design? I see that Risa 3D has an option for applying these loads, so I can probably look into their help topics to get a better understanding. But I assume you only apply these loads when you are modeling a lateral force resisting system within your structural model? And are the notional loads only applied as a point load at elevated floor and roof levels of building structural models?

Any responses are appreciated. Thanks.

 

[Lomarandil]

You should always apply the provisions of Chapter C (when AISC applies, e.g. steel building structures)

Now, that's different than always applying DAM. Chapter C can be satisfied using DAM, amplified second order effects, or traditional "KL" methods.

I'd highly recommend the commentary on ChapternC if you haven't read that already.

----
just call me Lo.

 

[JoshPlumSE]

1. Chapter C is a requirement. So, it always "applies". Different aspects of it become important for different applications.
a) In general, the more slender the column, the more important the 2nd order effects are. The more flexible the lateral stiffness, the more important. Whereas for a shear wall or braced frame (i.e. the old no sway systems) the 2nd order effects would not be as important....
b) When does notional load becomes important is when you have a compression member not really subject to lateral load. Think interior mezzanines in non-seismic areas. If you don't have any wind load applied to this mezzanine, then your P-Delta effect doesn't have any initial deflection to amplify. Really important to have notional loads there.

2. If your program doesn't direction account for 2nd order effects (i.e. geometric non-linearity) then you can account for this in other ways. Using the old B1-B2 method for example.

3. When in doubt, just add the extra nodes. I think the paper I wrote (and the accompanying NASCC presentation) does a pretty good job of discussing when the little delta effect is minor. Since most moment frames are strong axis frames only, the weak axis buckling controls the capacity of the member and your slenderness effects (which cause p-little delta amplification in the strong axis) become pretty minor. Its really only when you have HSS (which have similar weak and strong axis buckling strengths) or weak axis moment frames when little delta becomes important.

4. Notional loads are rarely important for real structures under typical code loads. You have to have cases where the axial load during gravity loads is a lot higher than for the lateral cases. Or, when you have very little lateral load. A good example might be internal mezzanines in non-seismic areas. Or, tanks in petrochem facilities that have a "hydrotest" load that is much, much higher than the "operating" load that gets combined with wind or seismic.

Another good example when notional loads may be necessary is when you have an arched truss / bridge. You want to introduce either notional loads or initial imperfections that will mimic the controlling buckled shape. This is because KL isn't really valid for these compression members. The buckling just isn't as simple as it is for a straight column.

5. The automated Notional loads in RISA only works for structures with a rigid diaphragm at each floor level.