Even if they had identified the steel from the affected floors, the damage done by the collapse itself might have been difficult to distinguish from the damage done BEFORE the collapse (by the planes and fires).
I love how truthers always have some goalpost that they claim would have clinched it for them "if only they had done that."
The steel followed gravity.
You survey the site and mark the visible pieces of steel.
You create a database.
You remove the steel layer by layer, surveying and marking.
The steel gets thinner as you get higher up.
Some of the steel retains its positional markings.
These will be used to reference the unmarked ones.
Run the clock backwards.
This will identify the steel from the floors.
The plane impact will cause one particular kind of shearing damage. Shearing plus heat indications by the fire will also get you close.
All these clues to look for, and look at the 1-3B inventory list.
Look at where they positioned the steel they could get (and keep) relative to the impact.
All these clues that they had to identify the steel needed for a robust model of the events, so where is that steel?
I am not criticizing NIST, they got stuck with just enough evidence to say it WAS a building. It was the FBI's Pasquale D'Amuro fault, he controlled the site.
If you are wondering why I don't like the FBI, Pasquale D'Amuro is a perfect example of corruption. The FBI could answer a lot of the questions about 9-11, so why don't they?
That was then, this is now. What has changed?
Since we don't have any more information from the government, then the people who ran the corrupt FBI, are still running the corrupt FBI.