The important thing to understand about indeterminate structures is that the distribution of forces is dependent upon the relative stuffnesses of the members in the structure. So if your model incorporates SELECT commands that are likely to change the sizes of some members in your model, it is important to reanalyze the model after the SELECT command. Reanalyzing does two important things. First, it incorporates the revised selfweights, if you are using the SELFWEIGHT command. Second, it captures the effects of the changed stiffnesses, and allows the loads to redistribute as necessary.
Finally, since there is likely to be some shifts in load during the second analysis, it is necessary to issue a final CHECK CODE command, to be sure that the selected members are still within allowable stress limits after the loads have shifted in the second analysis.
STAAD.Pro does a very efficient job in its optimization process, and selects members that have utilization ratios very close to 1.0 whenever possible. As a result, you may find that certain types of structures are particularly sensitive to the redistribution of stresses during that second analysis. As a result, some structures can be more difficult to optimize without many iterations. To address this effect, STAAD.Pro offers a parameter named RATIO, which can be used to set an upper limit on the utilization ratio that the program will allow when going through its optimization process. In this way, it allows us to build in a certain ammount of "tolerance" to these inevitable shifts in stresses, and to arrive at an acceptable design in fewer iterations.
Cheers,
Chris