Collapse of the World Trade Center Towers, 6continued The strength, stiffness and redundancy of the perimeter frames would also have been more than adequate to redistribute vertical load around the severed members on the impact side, thus preserving the integrity of these frames above the impact region. The likely influence of the fire in the time from impact to collapse would have been to progressively weaken the residual vertical load carrying capacity from the remaining core columns, increasing the need for slab panel action from the floor slabs above the impact region back to the perimeter frame. This would have been transmitted up through the floors above the impact region through the tension tie effect from the core columns, increasing the severity of shear action between the top floor or floors and the perimeter frame. The sagging of the core region on the upper floors could have been the reason for a phone call from the upper levels shortly after the impact saying that the building was breaking up. The sagging around the core and the impact damage would also have made the stairs impassible through the impact region, cutting off escape from the upper floors. The strength, stiffness and redundancy of the perimeter frames would also have been more than adequate to redistribute vertical load around the severed members on the impact side, thus preserving the integrity of these frames above the impact region. The likely influence of the fire in the time from impact to collapse would have been to progressively weaken the residual vertical load carrying capacity from the remaining core columns, increasing the need for slab panel action from the floor slabs above the impact region back to the perimeter frame. This would have been transmitted up through the floors above the impact region through the tension tie effect from the core columns, increasing the severity of shear action between the top floor or floors and the perimeter frame. Finally, it is likely that the interconnection between one or more floors and the perimeter frame failed at or near the top of the building. This would have resulted in the immediate collapse of these floors. From the video footage this collapse appeared to occur uniformly around the building and spread very rapidly down to the floor above the impact region. That region then pancaked causing a brief gout of flame to be expelled most noticeably from around the South and East sides as the areas within the impact region still on fire collapsed. The collapse then continued down the building, with the floors pancaking leaving the perimeter frames briefly standing unsupported until they too collapsed. The effect of the floors pancaking nearly straight down inside the perimeter frames lead to the North tower effectively imploding, with some sections of the perimeter frame remaining standing unsupported for a few seconds before collapsing. This is seen from a number of video footages and pictures, including the collapse sequence shown in Fig 10. 
Fig. 10 Sequence of collapse of North Tower
( Photo from CNN) Click to enlarge
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