Much has been made of the fact that NIST only analyzed the events up to the point where the Towers were poised to collapse before runaway collapse began and failed to pursue the remaining collapse. This was largely because after collapse began the chaotic impacts of the floors, walls and columns colliding could not possibly be analyzed accurately with even the strongest computers. As it was, it was a severe strain on computer capabilities to analyze the mechanism of collapse up to the point of runaway disintegration.
It is clear from the computer studies that the heat from the fires caused differential expansion of the steel parts in the long span, floor trusses with the resulting thermal bowing in some floors directly exerting pull-in forces on the exterior columns or this thermal bowing could have detached a floor which would have impacted the floor below destroying composite action by separating the concrete slab from the trusses and inducing strong tensile (suspension) forces in the double weighted floor. In other floors thermal expansion of the floor against the columns compressed the trusses which along with shear forces within the trusses that buckled the diagonal struts collapsing the trusses which went into suspension (catenary action) and this also helped pull-in and eventually buckle the exterior column walls. Differential thermal expansion of the concrete and steel has also been shown by NIST to disconnect the knuckles (knuckles are the steel tops of the bent over bars in the trusses which are imbedded in the concrete slab) from the concrete slab causing loss of composite behavior in the floors.
All these adverse floor truss effects were caused by steel expansion which begins immediately as the steel is heated. Bowing and buckling can happen at low temperatures (400 C to 500 C) even before the steel would have weakened excessively from higher temperatures. Thermal contraction caused by cooling of sagging trusses or ‘I’ beams after the fire ‘burns out’ or dies down can cause strong pull-in forces on the exterior columns and core columns due to the contraction of the steel trusses or ‘I’ beams.
In order for a column to support the loads it has to be plumb and in line with the columns above and below. The fact is, columns have to be axially (in line and centered) aligned to support the weight of the building above. If they get out of alignment by 10 to 20 degrees they buckle and can no longer support the weight. The buildings collapsed because the floors first caved in from restrained thermal expansion and from thermal bowing or delamination of the slab and bar joists affecting floor truss stability. The sagging, 60 foot long, floor trusses gradually pulled in the 59 columns in one exterior wall in each tower and these column walls eventually buckled removing support on one entire side.
Once the exterior columns buckling spread, along an entire wall removing support on one face, the buckling spread around the towers exterior and into the core and the towers began to tilt. With all the columns buckled the leaning top sections of the tower began to fall straight down. Although the North tower antenna appeared from some northern angles to have began falling straight down it actually tilted to the south because the south wall buckled first and the cantilevered top building section pulled the core over to the south. This is especially telling since with all the damage from the plane impact on the north side, the tower should have leaned over to the north. The South Towers’ top tilted to the east because its east wall buckled first. Once the tower’s tops began tilting all the columns across the buildings would eventually be out of alignment enough to have easily buckled.
Once the core columns got out of plumb, there would have been little resistance to their buckling at the weak splices. After the upper part of the buildings began descending, with the incredible weight of the top of the buildings’ gaining momentum, like a heavy wedge or sledge all it had to do was break the welded, and single bolt connections holding the floors to the columns. This is coupled with the fact that the falling top section’s momentum increases as the square of the number of floors impacted as the floors were detached and added to the weight of the descending top. There would have been little resistance to slow the top section's increasing mass of impacted floors acceleration to the ground. Because this acceleration due to gravity increased the speed and momentum of the collapsing floors and building top, the impacts were increasingly violent as shown on the seismic graphs; increasing in amplitude until maximum when the mass of accumulated floors hit bedrock seven stories into the cellar.
There have been some engineering analyses about the impacting floors slowing down the collapse so that the time to collapse should have been much longer than 'free fall' times of an object dropped from the towers tops. Once the buildings started to tip over from loss of column support on one side, the tremendous excess eccentric weight began buckling all the columns across the building. Once the tilted building's tops began descending they hit the floors or columns at eccentric angles which easily detached the floors and buckled the columns. In order for the lower building section to offer any meaningful resistance to the falling building top, the columns would have had to hit each other exactly in line and plumb and this was impossible with the top of the building leaning causing eccentric angles of impact.
Once the top building section began tilting the columns on the side that originally buckled did not line up at all. These columns would have been hitting the floors and would have easily detached or buckled them. After the east wall buckled in Tower 2, the adjacent perimeter wall columns buckled from overloads and the columns on the opposite west side of the building, which were still attached and acted as a hinge would still be bearing on each other but at an eccentric angle which means they also would have also eventually buckled as the top tilted. These columns along with some of the core columns as they buckled are probably what kicked the bottom of the top building section to the west as reported by NIST. Because of the weight of the accumulating collapsing floors, there was a release of incredible potential energy changing to kinetic energy and building momentum as the accumulating chaotic mass of debris accelerated into the cellars.
Since the Tower's outer wall columns, especially in Tower 1, pealed out like a banana after the building top began to impact the floors, these wall columns may have been able to break the connections to the floors ahead of the floors being impacted? In other words, with the weight of the wall columns pealing outward from the vertical along with the added horizontal forces of impacting floors projecting debris outwards onto these columns, these columns, while leaning out, might have been able to break the wall-to-floor connections ahead of the level of impacting floors? If this is possible than I believe that the connection failures could have traveled down the sides of the buildings at a speed faster than free fall times. This might explain the rapidity of the collapses especially in Tower 1. The wall-to-floor connection failures could have traveled down the building sides faster than 'free fall' times and in effect started the floors falling before they were impacted by the accumulating mass of impacted floors above.
The heavy exterior wall columns in the 1500 foot high buildings while pealing off could project the column sections outwards a great distance. This distance (300 to 400 feet) was proposed as only being made possible by explosive forces. I disagree. If a wall is strong enough and doesn’t break up as it falls outward it can fall out flat to a distance equal to its height. The Tower walls, however, did break at the weak splices as they fell. The fact that the exterior wall columns can be seen in the videos and pictures falling outward and downward and reaching the ground before the tower finished collapsing proves that the tower did not collapse faster than free fall times. These outer wall sections were falling at free fall speed and apparently were the first to hit the ground.
The compression of the12 foot chunk of air on each floor down to a fraction of an inch in a fraction of a second as the floors came together would propel the air smoke and dust outward from the building at great velocity. The lightweight aluminum cladding as it broke free from the buckling columns also would have been propelled outward a great distance by this expanding cloud of air and dust. This air compression would account for huge dust clouds and pieces of aluminum seen projected outwards from the upper sections of the collapsing buildings. The light reflected off the aluminum pieces at the north wall of Tower 2 would be interpreted as flashes from explosive 'squibs'. The flashes below the buckling east wall may have been from the aluminum cladding breaking free from the lower columns as they expanded after being unloaded of axial weight by the buckling of the wall above and their expansion breaking the connections to the cladding. Also explosives leave characteristic tears and fractures in steel and especially in aluminum, and such indications were not found anywhere in the debris pile. The compression of air in the elevator and air-conditioning shafts by the collapsing upper building section and floors, would project air, smoke, and dust down these shafts and out any path of least resistance or any of the HVAC air intake or discharge openings on the lower mechanical equipment floors in the exterior walls. This accounts for the plumes of smoke seen projecting outwards sideways from the buildings well below the collapsing floors. There were quite extensive Heating, Ventilating, and Air Conditioning (HVAC) shafts built into the building. These vertical shafts are connected to air conditioning exhaust and intake ducts open to the exterior on the mechanical floors.