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Problem 7.22. A flitched beam consists of a wooden joist 10 cm wide and 20 cm deep strengthened by two steel plates 10 mm thick and 20 cm deep as shown in Fig. 7.28. If the maximum stress in the wooden joist is 7 N/mm", find the corresponding maximum stress attained in steel. Find also the moment of resistance of the composite section. Take Young's modulus for steel = 2 x 105 N/mm and for wood = 1 x 10' N/mm? 2 Sol Given : Let suffix 1 represent steel and suffix 2 repre- - 10 cm sent wooden joist. Width of wooden joist, b, = 10 cm Depth of wooden joist, d, = 20 cm Width of one steel plate, b, = 1 cm Depth of one steel plate, d, = 20 cm 20 cm Number of steel plates Max. stress in wood, E for steel, E, = 2 x 105 N/mm? E for wood, E, = 1 x 10' N/mm Now M.O.I. of wooden joist about N.A., byd, 10 20 12 12 Activate Wind 6666.66 cm Go to Sewings to a 6666.66 x 10 mm plate = 2 0 = 7 N/mm2 N 12= = Steel plate Wooden Steel joist