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 average normal stress in the bolts is 35 mpa and that the bearing stress between the washers and the planks must not exceed 8.5 mpa.