We've been over this issue thousands of times.
There was a simple analysis done to assess such an unlikely, high-speed impact.
It showed the structure would be likely to survive the impact. It, however, was done in the 60's on only a few pages of paper. It's what's called a "back of envelope" estimate, or a Rough Order of Magnitude, because you really can't carry out such a complex calculation with pen and slide rule. To do it right, you need to model details, manage thousands of model elements, vary hundreds of material parameters, treat lots of impact angles and locations, and so on. This was finally done in 2005 by Purdue University and cost millions of dollars, required supercomputers, and took lots of grad student labor.
You can perform a similar ROM calculation to show that a human being is likely to survive being shot by a .22. Which is true. But don't get shot by a .22. Your odds of survival are good, but they are NOT guaranteed.
The original simple calculation did not include the fire. Just the impact. As has been noted, the Towers survived both impacts. The fire is what did them in.
The original calculation is not evidence that the Towers were designed to survive such an impact. If I crash a car at 100 MPH and survive, it doesn't mean the car was designed to such specs. It just means that it performed better than expected. There was never any requirement to survive a high-speed impact, so says Leslie Robertson, the guy who's job it was to verify those requirements.
Finally, even if all the above was not the case, it does not prove the Towers were destroyed by make-believe. It would only mean the calculation was wrong. Engineers make mistakes and science gets better over time. When our calculations don't agree with reality, we fix the calculations, not stamp our feet and scream that reality is wrong.
Unless we're Truthers.
Let's table this one, please? It was stupid in 2005 and it's stupid now.
