Problem $2.$ A short column is made by filling a cast$-$iron tube with concrete. The internal and external diameters of the tube are $200$ mm and $300$ mm respectively and its length is $500$mm$.$Find the stress in each material and the shortening of the column under a compressive load of $400$kN$.$ Take E for cast$-$iron as $110$ GPa and E for concrete as $20$ GPa.