You can use the traditional approach to determine the effective length factor as specified in the AISC-unified code.There is a standard alignment chart illustrated in the AISC -360-10 code (Fig-c-c2.3 ). This chart is based on some relationships explained in the the page-16.1-240. If you in use the AISC ASD code (9th edition) in Staad, then you can directly get the KY and KZ value calculated by similarly AISC nomogram (Page-5-137). All you need to do is click on the CALCULATE button.
The determination of K value is a cumbersome process for a big structure involving intricate framing system. In such case, it is suggested to perform the second order Direct analysis as specified in the AISC design code where K is assumed as 1 to determine Pn.