AS3600 - Calculating PhiNu for Column Fire Design

[cam_b]

When designing a column to AS3600, I find it a bit unclear exactly how PhiNu should be calculated. Figure C5.6.3 in the commentary shows an interaction diagram where a straight line is drawn from 0,0 and through M*,N*. PhiNu is then taken at the point where this line intersects the curve. But it seems to me that this would only apply to a short column. In a slender column, the moment magnifiers increase as N* increases. Therefore, wouldn’t the path of M* and N* follow a curved line, so PhiNu should be lower for slender columns?

Does anybody have any thoughts on this? I am asking specifically related to fire design, as some of the tables in AS3600 rely on the ratio of Nf*/PhiNu. But for some slender columns, increasing Nf* by a relatively small amount can change the ratio quite dramatically due to the increasing moment magnifiers.

Thanks.

 

[SteynvW]

The clause that you can halve the effective length of the column for the FRL typically negates that issue.

 

[cam_b]

Thanks SteynvW. I believe the option to take the effective length as 0.5Lu is only for the restricted tabular method, while Nf*/phiNu also applies to columns designed as walls. Also, do you know why the option to take the effective length as 0.5Lu is in the code in the first place? I'm not sure why they included that, especially considering clause 10.5.3 gives an effective length of 0.7Lu for a braced column that is fully fixed at either end.