$[25$p$.]$ Consider the end$-$loaded hollow shaf AB with a horizontal rigid bar, CD $,$ attached to the free end, $B,$ of the shaft. The shaf is made from aluminum $(E=70$GPa,$G=26$GPa $),$ The end of the bar CD is loaded by a vertical force of magnitude $P=8kN,$ see Fig. below. Calculate the downward deflection of point D using the strain energy approach.