18. A member is made from steel and cast iron, as shown below, and fits in between immovable walls. The member is rigidly attached to the walls. If the temperature decreases 120°F, calculate the stress in each bar. $E_s = 30 \times 10^6$ psi, $E_i = 10 \times 10^6$ psi, $\alpha_s = 6.5 \times 10^{-6}$ in./in./°F, $\alpha_i = 6 \times 10^{-6}$ in./in./°F