After rigid bar $\mathrm{ABCD}$ rotates, member $\left(1\right)$ is elongated. To determine the deformation, consider its final length (${\mathrm{L}}_{\mathrm{f}}$) and its initial length (${\mathrm{L}}_{\mathrm{i}}$).

The deformation in member $\left(1\right)$ can be expressed as:

$$\begin{gathered} {\mathrm{\delta }}_1={\mathrm{L}}_{\mathrm{f}}-{\mathrm{L}}_{\mathrm{i}}=({\mathrm{L}}_{\mathrm{i}}+{\mathrm{v}}_{\mathrm{B}})-{\mathrm{L}}_{\mathrm{i}} \\ ={\mathrm{v}}_{\mathrm{B}} \end{gathered}$$