Only the summary is available, not the actual calcs so this isn't known. A later study was done for an impact at the 80th floor, but it was to analyze building sway only.
Do you know many structural engineers? All the ones I know (and I work for an engineering company) would exactly overdesign a building that way. Architects don't design structural systems for buildings so they would not do so.
Sure, but architects typically have a significant input on the final design.
A decade or so ago, I served with a few Navy Civil Engineers, reservists, who worked in building construction in their civilian jobs. It's been a few years since I spoke with any of them.
My remark on the design is that designing for a (load * safety factor) based on an expected approach speed of a 707 (200-300 knots) requires roughly One Fourth of the strength (well, ability to absorb the impact load and recover) of a design for a bit over twice the speed. The (higher impact spec * safety factor) would tend to increase material and cost siginficantly.
I learned a bit about safety factors in my Mechanical Engineering days, but I am not a civil engineer so I am left groping a bit at skyscraper design. Bridge trusses, our favorite model, aren't that good of a comparison.
Applying safety factors is an art, and is an exercise in Risk Management. (Are you an engineer, or another sort of engineering firm employee?) Cost/risk decisions happen on any construction project. The WTC designers obviously applied a generous safety factor to the design, whatever the final spec, as witnessed by the fact that the buildings took an impact load coming in at 400+ to 500+ knots -- and stood. The fire (per previous comments and NIST investigation) provided the critical chink in the armor of the structure's strength.
DR