Dr. Frank Legge has a knack at explaining difficult concepts logically and succinctly, as exemplified in his latest paper: "WTC 1 Collapse – What if the Columns Miss?" Dr. Legge considers four possible ways in which the columns in WTC 1 could have interacted in a gravity-driven collapse, and concludes:

"None of these four outcomes was observed. Clearly all the evidence points to the use of explosives to sever the columns in a precise sequence designed to produce vertical collapse at near free fall speed. As felling of such a tall building had previously not been attempted, it is likely that a considerable excess of explosive was used to make absolutely certain that the collapse would be vertical and complete. As a probably undesired consequence of this excess, most of the concrete was pulverized, as we observe.

"We can therefore safely conclude that, regardless of how the columns of the upper section interact with the columns of the lower section, the official explanation for the collapse is false."

Please read this three-page letter, here: http://journalof911studies.com/letters/c/ColumnsMissLegge9.pdf