Three wooden planks are fastened together by a series of bolts to form a column. the diameter of each bolt is 12 mm and the inner diameter of each washer is 16 mm, which is slightly larger than the diameter of the holes in the planks. determine the smallest allowable outer diameter d of the washers, knowing that the aver- age normal stress in the bolts is 36 mpa and that the bearing stress between the washers and the planks must not exceed 8.5 mpa.