AISC 15th Edition Section F11 Question

[zrck99]

I'm confused by the purpose of Eqn F11-1.

Mn = Mp = F[sub]y[/sub] Z <= 1.6 F[sub]y[/sub] S[sub]x[/sub]

if Z = (1/4)(width)(thickness[sup]2[/sup]) and S[sub]x[/sub] = (1/6)(width)(thickness[sup]2[/sup])

and 1.6*(1/6) = 0.267 on the right side of the equation and 1/4 = 0.25 on the left side of the equation then won't the left side of the equation always control?

Is this a situation where the S is always calculated about the x axis but Z could be calculated about a different axis?

Thanks.

 

[JoshPlumSE]

Round bars.... But, it barely, barely controls.