The wood joists supporting a plank floor (see figure) are 40mm × 180mm in cross section (actual dimensions) and have a span length L = 4.0m. The floor load is 3.6kPa, which includes the weight of the joists and the floor.
Calculate the maximum permissible spacing of the joists if the allowable bending stress is 15Mpa. (Assume that each joist may be represented as a simple beam carrying a uniform load.)