고체역학 (Beer) — Pure Bending: Problem 4.114

4.114 To provide access to the interior of a hollow square tube of 0.25-in. wall thickness, the portion $CD$ of one side of the tube has been removed. Knowing that the loading of the tube is equivalent to two equal and opposite 15-kip forces acting at the geometric centers $A$ and $E$ of the ends of the tube, determine (a) the maximum stress in section $a-a$, (b) the stress at point $F$. Given: The centroid of the cross section shown is at $C$ and $I_z = 4.81 \text{ in}^4$.