Capacity reduction factor in new AS3600-2018

[NTCONLINE]

Hi,

Does anybody realise that in Table 2.2.2 (AS3600-2018) that the reduction factor for bending and axial compression is mathematically a function of ∅o - which is not a constant?

And for a concrete column ∅o = 0.65 for short columns or ∅o = 0.6 for slender ones. However, when I look at the determination of whether a column is short or slender, I realise that it is also dependent on the load cases (dependent on M1*, M2* and N*).
Thus for each load case, there would be 1 interaction curve. Therefore, we theoretically can't use 1 interaction curve to check the column for all load cases as we have normally approached. It's a bit weird or I am missing something?

NTC

 

[IDS]

I don't see the problem. You have to check if the column is short or slender anyway, and if it is slender (or Q/G < 0.25) you use phi0 = 0.6 and if it is short you can use phi0 = 0.65. How does this complicate the design procedure?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[NTCONLINE]

Hi Doug,

The problem is that a column can be classified as slender in one load case but short in some other load cases.
I used to plot an interaction diagram (i.e phiMoment vs phiNu)and then plot all load cases to see if they are well within the curve. It seems I can't do this anymore unless I use the most conservative curve.

But I agree for each load case I can easily calculate phi and then compare phi x Mu and M* and phiNu and N*.
(just the visual part of the interaction diagram is that I feel a bit weird)

NTC