고체역학 (Beer) — Pure Bending: Problem 4.123

4.123 The C-shaped steel bar is used as a dynamometer to determine the magnitude \( P \) of the forces shown. Knowing that the cross section of the bar is a square of side 40 mm and that the strain on the inner edge was measured and found to be \( 450 \mu \), determine the magnitude \( P \) of the forces. Use \( E = 200 \text{ GPa} \).