Design of cold formed steel sections (CFS) as per IS 801:1975. Comparison of Manual and STAAD results.

Hi All,

I was trying to compare the results for the design of a channel section with lips using STAAD and manual calculation. Following are few observations which are hard for me to comprehend. I would be grateful if anybody could enlighten me.

1. Flexural Capacity:
   1. STAAD Analysis: The allowable bending moment in the beam is independent of the span of the member. Please see the attached STAAD file results, where the flexural capacity of both the members (Member #1 & 2) is the same irrespective of the member span.
   2. Manual Calculation: The flexural capacity is dependent on the length of the member as per section 6.3 of IS 801.
2. Compression Capacity:
   1. STAAD Analysis: I am not sure if STAAD calculates the effective area considering the flat width ratios, however the allowable compression capacity is lower than the value calculated by STAAD. Can someone please explain how STAAD ends up with a compression capacity of  29.67kN as per the attached STAAD?
3. Torsional-Flexural Bucking:
   1. STAAD Analysis: Does STAAD design members considering the possibility of torsional-flexural buckling? There is a parameter "FX" in the STAAD input. However, when this option is selected the interaction ratio of the member is displayed as infinity.

Regards,

Kaladhar Dheramcommunities.bentley.com/.../4774.Cantilever.std