3. The composite beam shown in Fig. 1 is simply supported and carries a total uniform load of 40 kN/m on a span length of 4.0 m. The beam is built of a southern pine wood member having cross-sectional dimensions 150 mm × 250 mm and two brass plates of cross-sectional dimensions 50 mm × 150 mm. Determine the maximum stresses σB and σw in the brass and wood, respectively, if the moduli of elasticity are E_B = 96 GPa and E_w = 14 GPa. (Disregard the weight of the beam.)(20分)
Fig. 1