A two-story building has steel columns AB in the first floor and BC in the second floor, as shown in the figure. The roof load P1 equals 400 kN and the second-floor load P2 equals 720 kN. Each column has length L = 3.75 m. The cross-sectional areas of the first- and second-floor columns are 11,000 mm2 and 3,900 mm2, respectively.
(a) Assuming that E = 206 GPa, determine the total shortening δAC of the two columns due to the combined action of the loads P1 and P2.
(b) How much additional load P0 can be placed at the top of the column (point C) if the total shortening δAC is not to exceed 4.0 mm?

P1 400 KN L 3.75 m P 720 kN L 3.75 m