Why Trade Center Towers Stood, Then Fell — ‘Exactly which failure began the sequence — which occurred under extraordinary conditions never envisioned in the buildings’ design — remains a matter of intense debate. Some analysts hold that too much evidence was destroyed in the collapses to say for certain.

But a leading theory has emerged as teams have sifted through the wreckage, examined photographs and videos and run computer simulations on aspects of the disaster. Many engineers now believe that relatively lightweight steel trusses holding up the reinforced concrete floors sagged in the heat and failed first when the connections that held them to the tightly spaced palisade of steel columns on the outside of the buildings gave way.’ NY Times [“FMHreader”, “FMHreader”]