고체역학 (Beer) — Shearing Stresses in Beams: Problem 6.30

6.30 Two $20 \times 100\text{-mm}$ and two $20 \times 180\text{-mm}$ boards are glued together as shown to form a $120 \times 200\text{-mm}$ box beam. Knowing that the beam is subjected to a vertical shear of $3.5\text{ kN}$, determine the average shearing stress in the glued joint (a) at $A$, (b) at $B$.