I am designing concrete column as per ACI 318-11 code. Does STAAD.Pro automatically account for the K value ?
No STAAD.Pro does not automatically account for the K. If you are using a first order analysis and using moment magnification to account for the slenderness, then you have to manually input the K ( option only available for design as per ACI 318-11 ) using design parameters like SKY , SKZ. Details on this can be found in the STAAD.Pro Technical reference manual section 3.5 titled Slenderness Effects and Analysis Consideration and also in section 3.4 titled Design Parameters.
Alternately you can do a PDELTA nn ANALYSIS ( nn stands for the number of iterations, generally 15 to 25 is fine ) with reduced stiffness as outlined in ACI code section 10.10.4 which should not require computation of K.