I have applied axially compressive load on a I-column and performed the buckling analysis keeping the material to be brass and found the buckling load factor 16. Now I found the Buckling load=16*applied load. Comes out to be 771742

But Theoretically Critical buckling load=pi^2 EI/L^2

If i calculate critical buckling load through this, i get 9817835. How is this possible?

Have you refined your mesh size?