The puzzling thing here for me is why Nordenson chose to exclusively use hand calculations for this particular part of the analysis. He says on page 251 of the 2nd pdf , “Through the use of simplified, conservative hand calculations and basic principles of physics and mechanics, this report demonstrates that the unseating of Girder 44-79 at Floors 13, or at a lower floor in the building, initiates a sequence of partial floor collapses all the way to the ground. The analysis approach undertaken is transparent and straightforward and does not rely on a complex computer analysis that may obscure assumptions inherent to the process.” .This is right at the heart of an analysis that is justifying an unprecedented event, given in evidence. It’s a big claim to make. It is entirely reasonable to expect that he would have used the combination of computer and hand calculation methods at his disposal to check his hand calculations with a computer analysis, even if only at this, the most crucial part of his evidence claim. Nordenson refers to the dual methods used throughout his analysis on page 52 of the same pdf saying, “In contrast to other “black box” analysis programs that could be used to study the collapse, the combination of hand calculations and straightforward computer models presented in this report provides a transparent and easily verifiable account of the collapse.” . .There are certainly justified questions to be asked of the decision making process that seems to have led to such an easily avoidable error going unchecked, and so be allowed to add impact to the claim made on the strength of an unchecked hand calculation.