Ttiling the new section is an issue I cant say that I would agree that calling this information a criticism is accuratemaybe adding it to the collpase mechanisms and only by stating something like Cherepanov has examined the collapse and determined that Fracture waves may explain another mode of the collapse His calculations indicate that the fracture waves would have made the collapse happen quicker than progressive collapse and would explain why the buildings fell as quickly as they did once initial failure occurred Bear in mind that the issue of frefall speed is not based on fact as there is no proof of exactly ghow long the buildings took to fall We have some seismic readings and related material but they are not proof of free fall anyway The reality is that the lower floors especially of the north tower continued to collapse for several minutes The free fall was from floor  to about th levels