Write the force-deformation relationship for each member.

For member $\left(1\right)$, the force-deformation relationship is:

${\mathrm{\delta }}_1=\frac{{\mathrm{F}}_1{\mathrm{L}}_1}{{\mathrm{A}}_1{\mathrm{E}}_1}+{\mathrm{\alpha }}_1∆{\mathrm{TL}}_1$

and for member $\left(2\right)$:

${\mathrm{\delta }}_2=\frac{{\mathrm{F}}_2{\mathrm{L}}_2}{{\mathrm{A}}_2{\mathrm{E}}_2}+{\mathrm{\alpha }}_2∆{\mathrm{TL}}_2$