Tonic: Explanation only on the condition that when you have your completed lightweight concrete airframe design that you post it here first.
Flexure design means design of the beam for bending (at a maximum over the middle support and near mid-span. There is also shear (near supports) and bearing (on supports). Also deflection, minimum steel to prevent cracking etc. to consider.
The following is roughly the method:
Firstly, there is insufficient information, as you need the maximum bending moments to size the section and the reinforcement and you cant get those without the length of the spans.
If you had the length, a continuous span beam is statically indeterminate, so you need a method such as moment distribution or other to determine from the distributed load, the maximum bending moment in the section. I should know the magic formula for 2 spans but its been too long.
Reinforced concrete flexure design requires a "balanced beam" approach, the idea being that the resistive couple (compression at the compressive face resisted by concrete and tension at the tensile face in the embedded steel) should be such that the yield strength of steel is reached before the concrete fails compressively. Why? Because if it does fail, it does so progressively rather than suddenly.
So there is a formula which includes the material strengths provided and the section shape (rect) which says: the ultimate moment = {strength factors} * b * d ^2. So from the breadth, you can get the depth.
Once you have the overall dimensions, you then use a more exact estimate of the cross-section layout (position of steel) to determine the resistive couple locations, and from the yield strength of steel and the force, the area of steel needed. Then the number of bars is based on what will fit, standard sizes etc.
Then on to the next beam...
There is elegance in concrete design (prestressed especially), although much more flexibility in steel (and Al of course). I wonder what would have happened to the WTC if it had been a concrete structure?
Regards.