The stepped bar in Figure 1 is fixed to the wall at A. Segment AB is steel (d = 30 mm) and segment BC is aluminium (d = 20 mm). A total load of 30 kN acts on the collar at B towards the wall, and 20 kN acts at the free end C away from the wall. Determine the total change in length of the bar AC (answer in mm; + for elongation).