I think they said that a floor is able to carry the weight of 11 or 12 floors, which is an extreme amount (only the connections then could do that, then imagine how strong the core itself must be!!) but that it could carry only 6 floors that move dynamically. In the old FAQ they explicitly stated that they rejected the pancake for collapse initiation.
This is what Nist says (not very clearly or convincingly):
"1.
Was there enough gravitational energy present in the World Trade Center Towers to cause the collapse of the intact floors below the impact floors? Why was the collapse of WTC 1 and 2 not arrested by the intact structure below the floors where columns first began to buckle?
Yes, there was more than enough gravitational load to cause the collapse of the floors below the level of collapse initiation in both WTC Towers. The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors
if the load was applied suddenly (
as was the case). Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings. Details of this finding are provided below:
Consider a typical floor immediately below the level of collapse initiation and conservatively assume that the floor is still supported on all columns (i.e., the columns below the intact floor did not buckle or peel-off due to the failure of the columns above). Consider further the truss seat connections between the primary floor trusses and the exterior wall columns or core columns. The individual connection capacities ranged from 94,000 lb to 395,000 lb, with a total vertical load capacity for the connections on a typical floor of 29,000,000 lb (See Section 5.2.4 of NIST NCSTAR 1-6C). The total floor area outside the core was approximately 31,000 ft2, and the average load on a floor under service conditions on September 11, 2001 was 80 lb/ft2. Thus, the total vertical load on a floor outside the core can be estimated by multiplying the floor area (31,000 ft2) by the gravitational load (80 lb/ft2), which yields 2,500,000 lb (this is a conservative load estimate since it ignores the weight contribution of the heavier mechanical floors at the top of each WTC Tower). By dividing the total vertical connection capacity (29,000,000 lb) of a floor by the total vertical load applied to the connections (2,500,000 lb), the number of floors that can be supported by an intact floor is calculated to be a total of 12 floors or 11 additional floors.
This simplified and conservative analysis indicates that the floor connections could have carried only a maximum of about 11 additional floors if the load from these floors were applied statically. Even this number is (conservatively) high, since the load from above the collapsing floor is being applied
suddenly. Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated. In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation. Thus, more than the 12 to 29 floors reported above actually loaded the intact floor
suddenly. "
Crystal clear? Question remains how you load just one (lose) floor
suddenly on the uppermost intact floor. Had it, the first floor above, been ripped off all the columns. Maybe, it had. How about the second, third, etc floors above? Also ripped off the
upper block. At the same time? The
upper block is supposed to be rigid! Nist says it goes into pieces.
Anyway, loading of lose floors on top of one intact floor cannot be simultanesouly and suddenly.
Another question is, of course, what happens when you load a floor on a floor? Answer is, that only the weakest part breaks!
Where is the weakest part? At the bolted floor connection to a column or at mid-length between the columns?
In the first case the bolts shear off at one side and the floor hangs on to the other side columns. The potential energy will be directed sideways out of the way down. In the second case the floor collapses at mid-length and hangs on at the columns both sides. The potential energy drops through a hole in the middle of the floor.
Nist suggests that all bolts - on both the outer wall and the inner core side - shear off simultaneously. No evidence for that of course. Etc, etc.
I sometimes wonder what monkey wrote the Nist reply!
