I am designing a concrete column in STAAD.Pro but STAAD is only designing for the first load case in spite of the fact that the other cases have higher forces.
STAAD.pro designs the columns for all load cases. The reporting of critical case is not based on the highest member forces but is rather based on the highest reinforcement requirement. The load case that requires the Maximum Reinforcement is reported as the critical case.
For example, let us say we have 10 load cases 301 to 310 for which we want to carry out the column design. The software would start with load case 301 and compute the reinforcement requirement for that case ( say required area A1). It would then check for load case 302 and compute the reinforcement requirement ( say A2 ). If A2 > A1, load case 302 would be considered as critical load case and the software would move to load case 303. If the reinforcement requirement for load 303 ( say A3 ) is lesser than A2, load case 302 would be retained as the critical load case. On the other hand if A3 > A2 then 303 would now replace 302 as the critical load case. In many situations you may find that magnitudes for certain loads are higher for certain cases but these cases may not require a higher reinforcement than a previously designed load case. In our example above let's say 304 has the highest magnitude of forces/moments. However once the software gets to 304, it would compute the reinforcement requirement for 304 ( say A4) . If A4 <=max(A1,A2,A3), the load case which is already listed as the critical load case so far, would still be retained as the critical load case in spite of the fact that load 304 may have higher forces/moments. The software would go through all cases up to 310 following the same procedure.
Following the same principle, if it so happens that the reinforcement requirement is minimum for all the load cases, then only the first load case in the load list would get reported as the worst case. The reason being, no other load case that follows the first load case, requires higher reinforcement than what is required by the first load case in the list. The output should also contain a statement that "Only Minimum Steel Is required".
If you want to verify this, use separate load list & column design commands for a few load cases which you feel may be critical & check the reinforcement. You will find that the reinforcement requirements for all of these subsequent load cases are either the same as that required for the first load case (301 shown in the above figure) or are lesser than that required for the first load case.