AISC 360-10 Notional Column Loads

[StrEng007]

Per section C2 in AISC 360-10, the use of notional loads is required to represent initial imperfections in the stability of the structure.

Does this also apply to columns that support gravity-only loads (ie, not part of a braced or sway frame). I thought the imperfections on these columns were already handled through the reduction applied in the critical buckling stress equations (both elastic and inelastic). How would this apply to a column that is gravity only and pinned at the top and bottom?

My understanding is that these sorts of columns do not require moment amplification as B1 is not applied to members that lack applied moments. If a notional load is required that represents a moment due to a "modeled imperfection", are we saying that ALL columns, regardless if they are not intended to support an applied moment, need to have minimum moments applied and thus also need to consider moment magnification?

 

[Settingsun]

Yes, you still apply the notional load because those columns can (will) be built with a small lean, not perfectly vertical. The coulmns modelled as pinned-pinned aren't affected by this, but it does increase the load on the braced/moment frame which is stabilising the gravity-only columns.

 

[StrEng007]

Where is Chapter C does it state this? The way I see it, all columns need to be checked for the stability provisions of Chapter C, regardless if they are part of the gravity-only system, part of a braced frame, or part of a moment frame.

Gravity only columns can still be subjected to combined bending and axial loads, especially when single plate connections are used and don't transfer shear at the C.G. of the column.

Does Chapter C explicitly state that notional loads are only used to check stability of columns that serve as part of braced or sway frames?

 

[KootK]

I agree with steveh49 on this one.

Does Chapter C explicitly state that notional loads are only used to check stability of columns that serve as part of braced or sway frames?

I don't believe that is explicitly stated but, in practice:

1) For concentrically braced frames, the direct design method tends to be a bit of a waste of effort because there isn't enough P-Delta sway for it to be very meaningful.

2) You could use notional load techniques to evaluate the stability of gravity columns. However, conventional design approaches tend to be more expeditious.

So, yeah, in practice the direct design method (DDM) is mostly the domain of moment frame design (beams and columns really).

When I do DDM, the picture that I have in my head at all times is the one shown below. The entire structure contributes destabilizing perturbations but only designated parts of the structure resist them.

I recommend a copy of the design guide shown below if you don't already have one. And this: Link

 

[JoshPlumSE]

I 2nd KootK's comments. This is the "leaning column" effect on the lateral force resisting system, and (if I recall correctly) is described very well in that Design Guide 28.

 

[JoshPlumSE]

I should point out that is certainly possible to handle this manually for the gravity only columns that are not directly included in your structural model... by adding in the destabilizing effect of the gravity only columns' notional loads without having to model in the columns themselves.