I am using STAAD to do steel design per the AISC code. For 2 members with similar cross sections, one passes, the other fails. Fact is, the one which fails has almost no load on it. The other is significantly more stressed but still passes. Is something wrong in the steel design calculations that STAAD is doing?
You will notice that, for the member which failed, the cause of the failure is reported using the phrase "L/R-EXCEEDS". This means that the member has failed the slenderness check.
When STAAD performs steel design on a member per the AISC code, it adopts the following sequence :
It first sets the allowable KL/r in compression to 200 and the allowable KL/r in tension to 300.
For the member being designed, it goes through all the active load cases to see if the member is subjected to axial compression and/or axial tension.
Next, it compares the actual KL/r against the allowable KL/r. If this check results in a FAILure, the member is declared as FAILed, and design for that member is immediately terminated. The requirement to check this condition is in Section B of the AISC specifications.
If the member passes the KL/r check, only then does the program go on to do the remainder of the checks such as axial compression + bending, shear, etc.
It must be noted that failure to satisfy the KL/r check is a reflection of the slenderness of the member, not the capacity of the section to carry the loads which act on it. Even if the axial load or bending moment acting on the member is a negligible quantity, the fact is, failure to satisfy KL/r will result in the member being declared as unsafe as per the code requirement.
If you do not want the KL/r condition to be checked, you can switch off that check using a parameter called MAIN. Set MAIN to 1.0 for a specific member and it won't be checked for slenderness. See Table 1 in the STAAD.Pro Help Section Design > D. Design Codes > D1. American Codes > D1.A. American Codes - Steel Design per AISC 360 Unified Specification > D1.A.6 Design Parameters.