Nordenson's failure mechanism for the northeast corner floors below could not have occurred. He errs in not including the much lower stiffness of the falling girder, which will limit the impact load to well below the shear capacity of the girder seat at column 79.I spent a little more time going through these today and I have to say that the Nordenson report is quite interesting. He also independently modeled the collapse; however, unlikely Bailey, he modeled WTC globally. His actual model is described in the portions of his report in the second Nordenson report PDF in the OP. Section 4 (starting on pg 75 of that PDF) outlines the vulnerabilities he sees with the WTC structure, while Section 5 (Starting on page 115 of the pdf) describes the global collapse, and even includes very detailed diagrams of the likely collapse progression. These diagrams are the sort of thing I wish NIST had produced more of in relating their theory of the collapse. He has a pretty clear explanation as to how he believes the inner supports failed prior to the collapse of the outer frame.