$($for this problem, determine the horizontal deflection of point D$)($Use Castigliano$$s second theorem$)$: A five$-$member plane truss in which all member have the same axial rigidity AE is loaded as shown in Fig P$10\mathrm{.}16.$ Apply Castigliano$$s theorem to obtain the vertical displacement of point D$.$ The answer for the vertical displacement is $8\mathrm{.}657$ WL$/$AE