STAAD is considering both primary load cases and load combinations for design. How do I instruct the program to only consider load combinations?
You can use the LOAD LIST command to achieve this. Say, for example, your model has load combinations numbered from 20 to 100. In order to ensure that the code check only uses these combinations, the following command sequence can be entered in the STAAD Editor:
LOAD LIST 20 to 100PARAMETERCODE AISC UNIFIEDCHECK CODE ALL
Once this is done re-run the analysis and you'll find the design results (both in the output file and Post Processing) show critical loads that consist of load combinations only, omitting all primary load cases.