Long story short, I have a very simple model that consists of a single angle with a compressive force on it. The member is a 6x6x9/16. I am unable to derive the same unity as STAAD when using AISC 360-05 (360-10 gives same answer). I am getting 2.97 and STAAD is getting 2.43. Axial load is 71.7 kips.
I applied the ASD for the METHOD and TRACK 2 to see how STAAD was arriving at the 2.43 unity. The output showed that the only code check with a unity ratio was for "Check for Compression," which was expected, and that unity was 2.43. After digging into the output more, it became apparent the difference between my calcs and model was the KL/r ratio. Looking at E5(a) and E5(b) in AISC, I was (and still am) convinced that my situation falls under E5(a) since this is an "individual member." Given that the L/r ratio is greater than 80, I used equation E5-2 to get a KL/r of 202, or 200 due it being the max. However, STAAD's KL/r is listed as 181.19 in the compression check. I was finally able to find that STAAD was actually using equation E5-4, which is in E5(b).
My question is why did STAAD use section E5(b)? Is it a default? It seems like E5(a) would be the default if there was one. Am I missing something? Let me know what you guys think.
