An upper floor of a school class-room extension is to be constructed of solid timber floor-joists, spanning 5m c/c and supported on 100mm wide concrete block walls. The joists are overlaid with 25mm softwood floor-boards and this floor surface is to be left uncovered. Below the joists is a 13mm thick plasterboard ceiling. Select and check a suitable layout and size of joists for the floor.(For a design UDL of c. 5 kN/m2: c. 75x275 joists in, say, C16 timber @ 400 c/c)