Collapse of the World Trade Center Towers, 7

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Considered Effect of Impact on South Tower

At 9.03 am the plane impacted the South side of the south tower towards the South East corner. It struck the building at an angle across that corner and on a slight downwards heading, as shown in Fig.2. The plane passed into the building and then exploded out of the adjacent East side, causing a large fireball to erupt from both the entry and exit sides adjacent to the South East corner. Fig. 8 shows the plane immediately before impact and Fig 11 shows the expanding fireball erupting from both sides of the building.

In contrast to the North Tower impact, in the case of the South Tower only one corner of the core would have been directly in front of the plane’s path through the building, along with the floor slabs over several levels in the South East corner. It is likely that the initial impact destroyed all the floor slabs in that corner over at least four levels and maybe over as many as six. It would have also severely damaged the South East corner core, removing an unknown number of columns there, buckling many more and destroying most of the core walls ( which are drywall construction). We do know that at least one stairwell in the core remained intact after the impact, as there have been five survivors from the floors above the impact floor who must have had access to an intact stairwell to escape.

Immediately after the impact the perimeter frame in the South East corner would have been severely weakened, being reduced to an unknown number of intact box columns in towards that corner on each of the two sides. However, these columns would have lost the lateral support from the floor slabs over many levels and would have had to function as isolated columns spanning multiple story heights. They would likely have suffered blast damage and loss of alignment, however immediately following the impact they still retained sufficient compression capacity to resist their share of the loads from the 30 or so floors above the impact region.

Fig.11 Fireball from impact on South Tower
(Associated Press)

 Fig.12 Top of South Tower collapsing (CNN)

The fires started by the impact would have then progressively weakened the vertical load carrying capacity of the remaining core, causing progressively more load to have to be carried by the perimeter frame system. In my opinion, based on the footage taken of the building over that time, the fire would have had little impact on the strength and stiffness of the perimeter frames, even in the damaged corner. The stiffness of this system above the impact region would have distributed this load approximately uniformly around the perimeter frames, increasing the loading on these frames through the impact region, including on the residual columns in the damaged corner.

Finally the combination of increasing compression load on these damaged columns, with second order effects from this load acting on the buckled shape of these columns over their unsupported length, would have caused their collapse. This collapse would have initiated in the damaged corner and spread rapidly over the impact region, causing the tower above to fail by toppling sideways with the floors above the impact region momentarily in an intact condition. This stage of the collapse is shown in Fig. 12.

However, even with the top floors toppling sideways, sufficient material would have impacted straight down on the floors below the impact region to have caused these to start to pancake downwards, leading to the tower below the impact region collapsing in much the same manner as the North tower.

With both towers, the forces created by the falling floors above on the floors below would have been orders of magnitude greater than the resistance of these floors, leading to the complete collapses then observed.

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