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).
I recommend that you check the reference ASCE Report "Design of Blast Resistant Buildings in Petrochemical Facilities". In this Report you will find excellent information on designing of reinfoirced concrete structures against blast loads.
Hi
You can refer to any book on steel design to see exactly where you had done a mistake! I recommend the book by Jack C. McCormac titled "Structural Steel Design". However note that you should be careful about the concrete section that you should include in the transformed section...