During the design, STAAD. Foundation Advanced is reporting a certain strength load as the critical load case but I can see there are other strength cases where the magnitude of loads are higher. So why is it not considering the one with the worst loading as the critical case ?
In STAAD.Foundation Advanced (abbreviated as SFA) the critical load case is determined based on the reinforcement requirement and not based on the magnitude of the load. During the design process, SFA carries out the design for the first strength load case and finds out the reinforcement requirement. Based on the rebar input details, it calculates the actual area of reinforcement that it needs to provide for that load case ( say x ). It then moves on to the next strength case and calculates the reinforcement required for that case ( say y ). If the later is lesser than or equal to the former ( or y < = x ) then the reinforcement provided for the first load case is good enough for the second load case too. So SFA retains the first load case as the critical load case even though it may so happen that the second load case may be higher in magnitude. In this manner it designs all cases and if the reinforcement provided for the first one satisfies the requirement for the later cases, the first load is retained as the critical load case. If one of the later cases has a higher reinforcement requirement ( > x ) then that case now replaces the first one as the critical load case. The software then calculates the reinforcement to be provided for that case ( say z ) and proceeds in the same manner as discussed till all strength cases are designed.