STAAD analysis; compact or noncompact

[rulljs]

EIT here so if I'm missing a simple step here please forgive me.

I have a C8x11.5 (36ksi) beam modeled in STAAD. My results show that the allowable weak axis bending stress in the member will be Fb=0.6*Fy=0.6*36=21.6ksi which is the allowable stress for a noncompact section (F2-2 in ASD), from looking at table B5.1 it seems to me that a C8x11.5 should be compact.

b/t < 65/sqrt(Fy)

2.26/0.39 < 65/sqrt(36)

5.8 < 10.8

Am I missing something here since 5.8 is less than 10.8 shouldn't this member be compact, which in turn would make the Fb=.75*Fy=27ksi?

 

[rulljs]

Just re-read and I may have found my mistake, I just noticed to use F2-1 the member needs to be doubly symmetrical, which a channel is not, so I assume this is what takes me to using F2-2?

 

[JLNJ]

I believe that you are correct.

.75Fy is reserved for shapes with a large ratio of plastic to elastic section modulus such as solid bars and WF shapes on the weak axis. Since your channel will have different values of S for bending in tension and compression there might be an argument to be made for using .75Fy for the flange tips and and .6 for web, but I'd stay with .6 for both.