Lets say, instead of the WTC, the terrorists flew a 767 into Sears Tower. Was it constructed in a similar way to the WTC, and do you think it would likely have collapsed? Do you think the aftermath of the attacks and response would be different somehow? I’d say Sears Tower was arguably less iconic than the WTC but…it still was the tallest building in the US.
No buildings on earth are built the way the WTC towers were built - AFAIK. Bin Laden targeted them twice specifically because of their potential fragility. Had the 1993 bombing been successful (and it was technically sound, just not big enough or in quite the right position), the bombed tower would have collapsed into the other one, which likely would have gone down or been totally destroyed.
An airliner into Sears Tower or most other giant buildings would be a horror and a firestorm, but would not necessarily bring the building down. It’s likely the damage would be too great to repair, though, and fire-weakened structure could never be trusted again. The building would likely have to be dismantled, a daunting prospect.
In this scenario, what floor of the Willis Tower is being hit by the 767?
This is partially correct.
You are correct that the WTC towers were a unique design. However, they certainly were not “fragile” nor do I think Bin Laden targeted them for their design (he targeted them because of what the symbolized and were nearly the tallest buildings in the US).
The WTC design moved the supports for the building from the inside to the outside in order to maximize interior floor space. This is why they had those closely spaced pillars that gave the outside it’s particular look. Most skyscrapers use an interior framework to hold the building up and the outside walls are mainly decorative (curtain wall construction).
The WTC towers actually performed very well considering the massive damage they suffered. They kept standing long enough for most people to escape.
The Sears Tower (Willis Tower) is built differently. Not only is it a more typical interior frame design it is also more like several skyscrapers bundled together. Think of it like a bunch of tubes bundled together.
So, while a plane hitting it would certainly cause substantial damage it is unlikely that it would collapse in the manner that the WTC did.