Bar $\mathrm{AD}$ is assumed to be rigid, meaning that it does not bend when load $\mathrm{P}$ is applied. Since it is pinned, bar $\mathrm{AD}$ cannot translate at $\mathrm{A}$ when load $\mathrm{P}$ is applied. However, bar $\mathrm{AD}$ rotates about $\mathrm{A}$, and as it rotates, the connections at $\mathrm{B}$ and $\mathrm{C}$ move downward. This movement causes elongation in aluminum bar $\left(1\right)$ and brass bar $\left(2\right)$. Bars $\left(1\right)$ and $\left(2\right)$ resist this deformation, developing internal forces and internal normal stresses.