maximum span of a steel tube 150mm*50mm*3mm thickness
We want to construct a second floor using steel tubes 150mm*50mm*3mm at 40cm intervals (centres), either end supported by concrete block wall, on top of the steel tubes will be a floor of 3.2cm. Tongue and groove pine. The span is 3.5 metres. The total floor area will be 9m2 used as an occasional bedroom.
This is a job for a registered engineer. it exceeds the functions of this forum. A consultant would want to have a closer look at all the structure supporting this addition. You should contact and engineer for help with project. Many universities have engineering departments that might help. You should contact them directly.