It is today quite simple to conclude how WTC7 was pulled down on 9/11!
According NIST Final report of the WTC7 collapse
http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR 1A.pdf
(20 November 2008) pp 47-48 one explanation is:
"The probable collapse sequence that caused the global collapse of WTC 7 was initiated by the buckling of Column 79 … The buckling of Column 79 led to a vertical progression of floor failures up to the east penthouse and to the buckling of Columns 80 and 81. An east to west horizontal progression of interior column buckling followed due to loss of lateral support to adjacent columns, forces exerted by falling debris, and load redistribution from other buckled columns. … Global collapse occurred as the entire building above the buckled region moved downward as a single unit."
However, NIST has agreed/confirmed the finding that the upper part of WTC7 free falls (acceleration 9.82 m/s²) for 2.25 seconds during the collapse, i.e. there is no support/resistance of the upper part above floor 16, when it displaces downward abt 32 meters. See e.g. figure 3.15 in the report. What a free fall drop is, is explained at
http://uk.youtube.com/watch?v=gC44L0-2zL8 .
NIST suggests that this free fall displacement was initiated by failure of Column no. 79 between floors 11/13 followed by a vertical and horizontal progression of failures (no details of course except some strange figures) below floor 16 that apparently removed all support/resistance of the upper part for 32 meters vertically down at one side only and the complete structure horizontally.
But now the problem! A free falling upper part of WTC7 (above floor 16) does not apply any load at all on the structure below floor 16! A free falling mass does not apply any force on anything! So how can the upper part/mass above floor 16 damage the lower part below floor 16 as suggested by NIST during these 2.25 seconds? What kind of structural analysis is done ... when no loads are applied?
Furthermore - NIST suggests that the upper part/mass deforms itself during these 2.25 seconds ... when no forces at all are applied to it (all masses of the upper part are in free fall!). Same question ... how can a free fall upper part/mass deform?
NIST has been asked these questions ... and could not reply! Thus the NIST WTC 7 report is of no value.
The NIST statement on page 57 - "Computer simulations of … the structural collapse can be used to predict a complex degradation and collapse of a building", has no foundation in the case of WTC7. NIST has not done a correct job! The WTC 7 structure does not collapse as shown in figures 3.10-14 due to removing a part of Column no. 79. Quite easy for anybody to verify! No loads are applied on anything by a free falling mass!
An explanation how the 24 core columns could suddenly 'disappear' to produce free fall of the structure above is given at
http://www.journalof911studies.com/volume/2008/Ryan_NIST_and_Nano-1.pdf .
Thus, the 24 core columns below floor 16 were simply simultaneously burnt off using, e.g. sol-gel nanothermites sprayed on the columns that then evaporated (!) instantaneously. That explains why no lower (below floor 16) core columns are found in the debris of WTC7 (and that the pull down of WTC 7 was an inside job, I am sorry to conclude)!
According NIST Final report of the WTC7 collapse
http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR 1A.pdf
(20 November 2008) pp 47-48 one explanation is:
"The probable collapse sequence that caused the global collapse of WTC 7 was initiated by the buckling of Column 79 … The buckling of Column 79 led to a vertical progression of floor failures up to the east penthouse and to the buckling of Columns 80 and 81. An east to west horizontal progression of interior column buckling followed due to loss of lateral support to adjacent columns, forces exerted by falling debris, and load redistribution from other buckled columns. … Global collapse occurred as the entire building above the buckled region moved downward as a single unit."
However, NIST has agreed/confirmed the finding that the upper part of WTC7 free falls (acceleration 9.82 m/s²) for 2.25 seconds during the collapse, i.e. there is no support/resistance of the upper part above floor 16, when it displaces downward abt 32 meters. See e.g. figure 3.15 in the report. What a free fall drop is, is explained at
http://uk.youtube.com/watch?v=gC44L0-2zL8 .
NIST suggests that this free fall displacement was initiated by failure of Column no. 79 between floors 11/13 followed by a vertical and horizontal progression of failures (no details of course except some strange figures) below floor 16 that apparently removed all support/resistance of the upper part for 32 meters vertically down at one side only and the complete structure horizontally.
But now the problem! A free falling upper part of WTC7 (above floor 16) does not apply any load at all on the structure below floor 16! A free falling mass does not apply any force on anything! So how can the upper part/mass above floor 16 damage the lower part below floor 16 as suggested by NIST during these 2.25 seconds? What kind of structural analysis is done ... when no loads are applied?
Furthermore - NIST suggests that the upper part/mass deforms itself during these 2.25 seconds ... when no forces at all are applied to it (all masses of the upper part are in free fall!). Same question ... how can a free fall upper part/mass deform?
NIST has been asked these questions ... and could not reply! Thus the NIST WTC 7 report is of no value.
The NIST statement on page 57 - "Computer simulations of … the structural collapse can be used to predict a complex degradation and collapse of a building", has no foundation in the case of WTC7. NIST has not done a correct job! The WTC 7 structure does not collapse as shown in figures 3.10-14 due to removing a part of Column no. 79. Quite easy for anybody to verify! No loads are applied on anything by a free falling mass!
An explanation how the 24 core columns could suddenly 'disappear' to produce free fall of the structure above is given at
http://www.journalof911studies.com/volume/2008/Ryan_NIST_and_Nano-1.pdf .
Thus, the 24 core columns below floor 16 were simply simultaneously burnt off using, e.g. sol-gel nanothermites sprayed on the columns that then evaporated (!) instantaneously. That explains why no lower (below floor 16) core columns are found in the debris of WTC7 (and that the pull down of WTC 7 was an inside job, I am sorry to conclude)!
