How does STAAD.Pro determine the Critical Load Case/Combination for Steel Design ?

STAAD.Pro does the design on a load case by load case basis. For each case, by default it finds out the forces at each of the 13 sections and finds out the highest utilization ratio amongst all sections for that case. It then moves on to the next case and follows the same procedure. If the highest utilization ratio for the second case is lesser than that obtained for the prior one, the former case is retained as critical case and the software moves on to the next one. If the utilization ratio for the second case being considered is greater than that for the first one, the second case becomes the new critical load case and it moves on to the next one. This way it scans through the entire list of cases/combinations and the case/combination with the highest utilization ratio is reported as the critical load case.