Are you now not making the same mistake as Yandros? Asuming a homogenous lower and upper structure? Next argument will be thermodynamics.
Please, NO...don't, No Yandrification of this problem.
If you are using this to represent one floor, OK
Agree?
Keeping
Is
Simple for yandeo
S
I did mention that F1 would become smaller once the lower structure was damaged.
Where in God's name does he get that the momentum is all transferred to the Earth? The upper section contains a certain amount of momentum. It loses that momentum as its kinetic energy is transferred to the lower structure. This transfer of energy takes place over time and distance travelled and as it occurs it is marked by a reduction in velocity which by definition means that the momentum of the moving mass gets smaller.(only if the forces acting downward are less than those acting upward. If downward forces are greater then the velocity of the falling mass increases and so does the momentum of that falling mass)
ONLY if the lower structure was of the same strength as the bedrock would yandross be able to claim that the momentum was all transferred directly to the bedrock. He is ignoring centuries of structural engineering. The structure has a failure point. If enough force is applied that is greater than the force the structure can resist then the structure begins succumbing to that force, IT MOVES, it bends, it fractures. No office building is a solid structure, they are made of floors held up by their attachments to load bearing columns. If anything buggers up those columns then they will not efficiently transfer that load to the ground. Only load on those columns will be transfered to the ground.
In the case of the towers first of all something did bugger up the columns at the impact/fire floors. The ability to resist the dead load was reduced throughout the time the buildings were on fire , at the fire floors. As a result the columns started to bend uder the load as they did so the load was taken up by other columns, other columns continued to weaken as the fire moved through the building. Some columns, having already foreshortened under heat and load would then cool BUT they would no longer be straight and if not straight they could not possibly regain their ability to carry more load. Eventually a point was reached when there was no more reserve capacity in the columns and any additional load tranfers resulted in rapid failure of the columns(they bent and/or fractured).
The upper portion of the building then fell through the first floor height BECAUSE the columns no longer lined up where they had bent and/or fractured.
The impact forces and the gravity forces due to the mass of the falling upper section then could only be transferred to the columns as it was imparted upon the floor space and transfered via the truss system to the columns. The truss seats were never designed to be able to carry the force of the entire building above them let alone that plus the impact forces. Thus the vast bulk of downward forces were not on the columns and thus very little of that downward force was taken by the columns.
As this happened the floors were ripped away, the columns of the lower section lost lateral support and at the same time were being buffeted by the falling debris and they also buckled, bent and/or fractured.
The acelleration of the collapse was about 10-15% less than gravity meaning that the resistive force offered by the lower structure was 10-15% of the total force of the falling debris The lower section only offered the resistance it did because the force was not being taken on by the strength members, the columns, of the structure. Those columns in turn relied upon the trusses of the floors for lateral support. The columns of the intact building held the mass of the entire building BUT once they lose lateral support they could not remain standing (look up Euler and critical load), would not have been able to even stand on their own let alone while being buffeted by large dense moving debris.
As columns bent during collapse they would be transfering some of that energy as heavy lateral vibration , at the speed of sound, down their length. This may well have been large enough to snap some truss connections well ahead of the collapse zone itself and certainly would result in the sound reaching the ground by two paths, that through the columns and that through the air. In this way you have witnesses saying they heard booms ahead of the collapse coming from the basement. The sound coming via the columns would be travelling much faster than that through the air. It also explains why things were falling from the ceilings of the ground floor while the towers collapsed. It was also happening on other floors but no one there lived to tell of it and only a couple on the ground floor did.
Better Sylvester?
No matter, I know what you are trying to say 
Wouldn't dream to be so pretentious as to assume I could correct or teach you. But thank you anyway 
