Single Angle Bending

[MER3]

I am trying to wrap my head around AISC 9th edition angle bending. I know I should be using newer codes, but we aren't allowed to where I work.

Anyway, going through the specification for single angle bending, for an equal leg angle I am allowed to use the geometric axis provided I calculate Fob using equation 5-4. I also have to increase compressive stresses by 25%. Being that an angle has two section moduli, a top and a bottom, since it is not symmetrical, is it wrong to calculate stresses for each section moduli and use as needed?

For example, if I have a simply supported angle, vertical leg down with a concentrated load in the middle. The moment at the middle is PL/4. The tensile stress in the tip is obviously M/S. S in this case is the S value from the book as it is the governing section modulus. However, for compressive stress in the horizontal leg of the angle could I use the distance from the N.A. to the top of the horizontal angle as my 'c' value, thereby increasing my S value and resulting in lower stresses? This would partially negate the 25% increase.

 

[Leftwow]

I just get a slenderness ratio kl/r. R values for an angle should be in the steel book. Of course I use the 14th edition, not sure if you have that. Then I figure out my elastic critical buckling stress in (ksi). Then my flexural buckling stress in (ksi). Then you should be able to multiply the flexural buckling stress by the cross-sectional area of the beam to find your Allowable Force.

 

[Triangled]

https://uedata.amazon.com/Single-Design-Manual-Whitney-McNulty/dp/B004KUH7D4