Ignition of propellant at each floor level pushes the perimeter columns outward and creates a reactionary force into the core columns and out through the opposite side.
(Image by MesserWoland, Author: MesserWoland) Details Source DMCA
These outward forces caused the separation of the floor trusses at the channel at the bolted connections and accelerated the concrete floor outward and away from the core. After the initial perimeter columns were pushed outward, the leaning-out-of-alignment columns would then add to the outward tensile forces at the bolted connections. The propelling process then progressed at a constant rate down the structure. This process could be described "like an unzipping of the building" all the way down, similar to peeling a banana.
The sequence of the "banana-peeling" mechanism is illustrated in the series of images from Figure 4 through Figure 9. Figure 4 illustrates a six-floor segment before the demolition begins. The floor trusses traverse the area from channel seats (yellow columns) to the outside seats at the perimeter column (green). An interior core column and supporting cross members are also shown in Figure 4 (orange). The floors are identified as A through F for convenience.
Structure showing floor trusses on six floors — from the perimeter columns to an interior core column.
(Image by Wayne Coste, PE) Details DMCA
In Figure 5, the propellant has been activated above floor "A" and is exerting outward pressure on perimeter columns. This puts tension on the bolted connections at both ends of the floor truss. Then the propellant between floors "A" and "B" is activated. Some amount of propellant and associated debris would be ejected through the windows creating a debris cloud, such as was observed around each floor.
With channel connection at 'A' separating due to tension from events on the floor above, high temperature propellant accelerates toward perimeter wall through the office contents,scouring the floor truss structure.
(Image by Wayne Coste, PE) Details DMCA
Figure 6 shows the continued activation of the propellant between floors "B" and "C". At this stage the perimeter columns are leaning outward.
With connections at 'A' and 'B' separating due to the momentum impact of the propellant and accelerated office contents against the perimeter wall, the separation at connection 'C' begins.
(Image by Wayne Coste, PE) Details DMCA
Figure 7 shows the progression down the building with the activation of the propellant between floors "C" and "D". The outer-perimeter columns continue to lean outward and will eventually separate and fall away with some outward horizontal velocity. The perimeter columns are three-story-high sections that are staggered to provide multi-floor stability. Depending on where the perimeter column sections were joined, they could exert an enhanced outward force based on the principles of a cantilever. With the bolted connection broken between the channel and the floor truss, Floor "A" near the core is unsupported and begins to fall towards Floor "B".
With connections at 'A,' 'B' and 'C' separating due to the momentum impact of the propellant and accelerated office contents against the perimeter wall, the separation at connection 'D' begins.
(Image by Wayne Coste, PE) Details DMCA
(Note: You can view every article as one long page if you sign up as an Advocate Member, or higher).
4
2
1
