I think that the strain should not exceed the elastic limit. So limit your stain to (Maximum Stress in the Column / Young's Modulus for steel). Doing that then you can calculate the shortenening in the column (strain * Length).
Compute the elastic strain shortening based on the cumulative loads of the structure.
DrAmmar has given one premise of shortening, limited by the steel. You will also need to check the concrete compressive strain to make sure you have strain compatibility. The concrete strain will occur first in order to mobilize the strain of the steel.