Generic Description of the Collapse Progression of the Two WTC Towers:
Video evidence demonstrates aircraft impact destroyed or bent a number of perimeter columns. A lesser amount of core column severance or damage is expected. Some floor spans also directly damaged.
Aircraft fuel spread through several vertically adjacent floors of the structure and ignited. This created a condition of very large area office contents fire on those floors with various levels of "rubblization" of the contents. This is a fire condition well beyond design expectations for fire spread. Office fires are expected to usually begin small, in a waste basket, or an electrical fault in a device or circuit, for instance.
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The capability of the structure to resist any collapse was being eroded. One very obvious indication of this is the inward bowing of part of the perimeter. Each tower also twisted and tilted noticeably up to collapse. These demonstrate that the structure was responding to further weakening due to the heat. Loads were continually changing and redistributing during this time. At some point a redistribution caused one or more columns to exceed its capacity. Column capacity in this case does not necessarily mean it original engineered load bearing ability. Loads were not necessarily on axis (due to impact or heat induced creep, or expanding floor beams) and many columns were subjected to significant heating, both of which would tend to lower any individual column's strength.
At one point a rapid progression of column failures occurs. A column or columns fail and in redistributing the load, cause other columns to exceed capacity, causing another rapid load shift. Column failures rapidly progressed to all columns over one or more levels
With all columns now having failed over one or more adjacent levels, a large section of the upper building is now falling, accelerating to next contacts. A combination of factors result in few, if any, buckled columns impacting on lower parts of themselves. Upper falling section column ends are, in many locations, the first contacts with lower structure. That contact would be proximate to the lower section columns and thus the greatest impact occurs near or directly on lower floor truss to column connecting components immediately failing the lateral support provided by those trusses. This is occurring both to long span truss to column connecting components, at perimeter and belt truss, as well as with the core's beams running between core columns.
As upper mass continues its lowest floor begins contacting floorspace below the initial failure level. This first contact is between floor systems that are already severely compromised. They are among those that have been affected by impact and heat and one has suffered loss of connections to columns as per last paragraph.
With columns largely being bypassed by the vast majority of falling upper section, they contribute very little in opposing collapse.