I Beam Sizes and Spans

12x5" beams allow 24' spans holding about 10 tons of weight. These beams are about 35 lb per foot.
Compare Rebar Truss-based Rebar Tubular Space Frame at 9.5 lb per foot. Reinforced 4x gets us 10 ton loads with 24 foot spans. At what deflection? Probably L/250 or 0.4%.
We had 1/4" deflection at Universal Prototyping Structure

Failure Modes

Fire protective coating on floor beams, and flexible vertical columns were found in the WTC collapse, according to the official reports. [1]. Heat was from molten aluminum (20 tons of aluminum per plane) reacting with water. Molten aluminum in water are explosive. Lesson: do not build aluminum buildings [2].