RE: retrieve steel design result for each load combination after resetting the stiffness matrix using "CHANGE"

You do not need to add the CHECK CODE command after every REPEAT LOAD case and follow that up with a PDELTA 1 ANALYSIS and CHANGE command.

Please refer to the following wiki which should explain how to set the design up

communities.bentley.com/.../18198.how-to-get-design-utilization-ratio-for-each-load-case