I am not convinced that the material pouring from the corner is iron or steel. Color on the video is simply unreliable and uncontrolled as a metric for what the material is. There are other metals which could have been melted at that corner.
The collapse of the core did not require a single mechanism. There were 24 perimeter columns in the core which supported the OOS floors. Clearly you don't have to destroy ALL 24 for the floors to drop or for the top come down.
What most likely happened... since what happened seem to TAKE TIME for it to happen was that there was a PROGRESSION of columns failures from MULTIPLE causes:
mechanical destruction from the plane strikes
weakening from the elevated temps from fires
uploading as columns failed and loads were redistributed
possible explosive or incendiary devices attacking the joints/splices
possible incendiary devices cutting away parts of the columns sections.
As you can see from the 3 attached slides the perimeter core columns did not carry equal loads. The 4 corners carried 28% of all the loads, the columns on the sort span side carried 20% of the total load and the columns on the long span side carried about 52%
You can also see that as columns are destroyed the loads are redistributed to the remain columns. If we accept the NIST assessment of destroyed columns from the plane crash... (red backgrounds)... this comes to about 33% of the axial load was removed. But.. perhaps NIST was over estimating the extent of damage?
Working with the assumption that the perimeter of the core lost about 33% of its capacity to support the floors the remaining 16 columns were then supporting these loads. Of course the floors around the destroyed columns would collapse locally, but the floors above them remained *intact* and attached to the columns which now had no support beneath them. The floors above acting like membranes/plates had their loads supported by the 16 intact columns.
Since the tops did not collapse from this amount of core damage we know that the FOS was at exceeded. This means that a column can support more load than it was designed to support... In this case 1/3 of the loads were now being re distributed to the remaining columns. If it were an equal load redistribution... 16 columns shared an extra 33% of the load or about 2% each.
If we look at any particular column... such as one of the strong corners which supported 7% of the loads prior to the plane strike ... it now was carrying and extra 2% bringing it to 9% of the total floor load. Its load has increased 35%
I did an Factor of Safety study of the WTC steel and determined it was about 1.65. This is not a precise number because it is derived from a ratio of the entire set of column's yield strength divided by the mass of the tower.. the load those columns support. My 1.65 number was based on a mass of 500,000 tons... with the A36 steel with a total yield strength of around 800,000 tons. If the towers mass was lower, the FOS would increase. Urich came to a mass of about 400,000 tons... giving an FOS of 2. My guess is that GU's mass is too low and perhaps 500,000 tons is too high. But it's safe to say the FOS was between 1.65 and 2.0. The average FOS for steel skyscrapers is apparently 1.42... so it appears that the twins were stronger than average.
Returning to the NIST assessment... 33% of the yield strength has been removed... the core can now support .67 x 800,000 tons or 536,00 tons. If the mass had been 500,000 tons the remain core was approaching its limit and the FOS was perilously now close to 1 at about 1.07! If the mass was 400K tons, the FOS was down to 1.34
Now we need to consider the effect of heat on the strength of the remaining steel. If steel is heated by 300°C it loses 20% of its yield strength. We do know that there were fires up there. We don't know how hot they were or how extensive they were. We do know that for wtc 2 they burned for about 1 hr. And so we don't know how much they might have weakened the steel.
If we look at another column such as the ones on the short span side. They too picked up an additional 2% of the total load. Those columns were each originally supporting about 2.5% of the loads... and now picked up 2% bring them to 4.5%. The problem here is that if they had an FOS of 2 they were seeing about an 80% increase in loads and this exceeded their yield strength. It would seem that they would have buckled from the redistributed uploading.
My hunch is that there was load redistribution to the 23 columns in the center of the core AND there was not an equal load redistribution to all remaining columns. The hat truss now was pulled into service and part of the core was in tension now *hanging* from the hat truss which was loading up the columns it was bearing on which were continuous to the foundations. The hat truss had 8 trusses with 4 in each axis over the outside rows and the two center rows.
What we can't know is:
How much did heat from fires weaken the remain columns?
How extensive was the plane damage to the core
If there were any devices to attack the core (there didn't need to be too many)
We do know that the progressive failure of the core took about 1 hr for wtc 2... and once the core's FOS descended below 1 the remaining columns buckled and the tops section descended and tilted favoring the most damaged side of the core.
The core collapsed up there because the remaining columns could not support the loads upon them... the redistributed loads overwhelmed those columns.
Could devices have been dislodged from the core and made their way to the NE corner? Perhaps. But that seems highly unlikely.
I think more study about the weakening of heat is in order. How much heat would it take to reduce the strength sufficiently to buckle those columns???
