Collapse of the World Trade Center Towers, 5continued Considered Effect of Impact on North Tower The North Tower was hit first, with the plane slicing into the North face of the building, as shown in Fig.2. The video footage that has been shown of that impact shows the plane disappearing into the building followed by a fireball erupting from the West and East sides of the building and back out of the entry hole on the North side. Subsequent footage and photos of the South side – the side opposite to the impact – show a large amount of façade destruction and smoke being discharged. This shows that a considerable amount of material exited the building on the opposite side to the impact. 
Fig. 9 North Tower after impact, viewed from the impact side (Photo from CNN)
Fig 9 shows the view looking into the impact hole. Given that the floor slabs are at 3.66 m centers and the façade column centers are at 1.03 meter centers with façade window widths of only 0.48 m, the number of façade columns and perimeter beams severed by the airplane in its passage though the perimeter frame is considerable. From the major damage to the side opposite the impact it is also apparent that much of the airplane would have passed through the core. It is likely that the impact destroyed most of the floors, at least on the impact side, and the core on at least three levels, removing many of the core supporting columns, at least on the North side of the core, and leaving the remainder buckled and stripped of their passive fire protection. This would have caused the floors above the impact level to sag downwards in the center, with the gravity columns which had been severed or severed by the impact now acting as tension ties between each of the floors above the impact region, through to the top floor. Core columns that were undamaged or with only minor damage would have now had to carry an increased compression load. Also, each upper floor would now have had to be at least partially supported off the perimeter frame to a greater extent than was allowed for in the design of the floor to perimeter frame connections. HERA has developed a method ( based on UK fire research) allowing the design of unprotected secondary beams in composite floor systems by using the dependable inelastic reserve of strength from a region of floor slab supported around its perimeter. I have applied this in a very approximate manner to a typical WTC floor slab supported off only the perimeter frame and the result shows that this is just about possible with regard to the membrane capacity of the floor system, but not possible in terms of the vertical load carrying capacity of the connection between the floors and the perimeter frame. As the vertical load carrying capacity of the core diminished, requiring more load to be transferred from the floors to the perimeter frames, the mode of failure would have most likely been failure of this connection, leading to floor collapse. I contend that, immediately following the impact, the core region of all the floors above the impact region would have sagged downwards due to the partial loss of vertical support in the core region. This sag would have progressively grown as the fire and ongoing yielding of the remaining damaged core columns reduced the core vertical load carrying capacity. This would have placed severe overstress on the connection between floor and perimeter frame around each floor at every level, with the greatest effect at the top floor, due to the core columns interconnecting each floor above the impact region now acting as tension ties. This would also have put extra vertical load on the perimeter frames, however these are sized to resist the lateral loading and would have had more than sufficient capacity to resist this extra load, especially as it would have distributed itself symmetrically around the perimeter frames. 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. continued... Related Articles | 
