14. A tunnel has a length of 150m. The material above the tunnel has a depth of 60m, and a width of 5.8m. The tunnel roof is to be supported by square steel columns. Each column has a cross sectional area of 960cm2 . The density of the material above the tunnel is 2800kg/m3. The ultimate strength of the material in the columns is 400 x 10'5 N/mz. For safety reasons you only want to keep the compressive stress on each column half of that. How many columns will you need to be able to support the material above the tunnel