A steel girder 40 m long with a cross-section of 0.050 m² is warmed by 25 K…

Step 1. Free growth. $1.2 \times 10^{-5}\ \text{K}^{-1} \times 40\ \text{m} \times 25\ \text{K} = 0.012\ \text{m}$.

Step 2. Stress. $2.0 \times 10^{11}\ \text{Pa} \times 1.2 \times 10^{-5}\ \text{K}^{-1} \times 25\ \text{K} = 6.0 \times 10^{7}\ \text{Pa}$.

Step 3. Force. $6.0 \times 10^{7}\ \text{Pa} \times 0.050\ \text{m}^2 = 3.0 \times 10^{6}\ \text{N}$.