Problem $3.(20$ points$)$

Use the principle of complementary eneray to calculate the internal forces in the members of the singly redundant pin$-$jointed framework shown in Fig$\mathrm{.}3.$ The members $AC$ and $BD$ are $30m{m}^2$ in cross$-$section, and all other members are $20m{m}^2$ in cross$-$section. The members AD$,$ BC and DC are each $800mm$ long. $E=200000\frac{N}{m}{m}^2.$ Tips: suppose member $CD$ as the redundant member.

Fig. $3$ A pin$-$jointed framework of problem $3.$