$$\begin{gathered} 20-\mathrm{mm} \mathrm{diameter} \mathrm{steel} \mathrm{rod} \\ \mathrm{A}=314.16 {\mathrm{mm}}^2 \\ \mathrm{E}=200 \mathrm{GPa} \\ \mathrm{\alpha }=12.0\times {10}^{-6}{/}^{\circ }\mathrm{C} \end{gathered}$$

First, we observe that the $15-\mathrm{mm}$ diameter bolt is used in a double shear configuration. This fact along with the $70 \mathrm{MPa}$ allowable shear stress limitation will determine the force that can be supported by the bolt.

Next, we can write the force-temperature-deformation relationship for an axial member. From this relationship, the temperature drop $∆\mathrm{T}$ that will produce the allowable bolt force can be determined.