Excellent Analysis of the Collapses of the Towers
Go check out BasqueArch's post at JREF; this is solid analysis backed up by video and photographic evidence.
4. Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors.
5.Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated.