This is a point I have often raised in other debates... thanks for bringing it up here Lee, as I think it very important.
I also think it very important to note that acceleration due to gravity is only relevant when there is only the normal impedance due to air resistance and cannot be applied where further resistance is encountered.
People keep using this to demonstrate 'how it is mathematically possible for the towers to fall in such a time frame', when this really does not apply at all as the resistance of the supporting structure would significantly impede acceleration from taking place. By what factor seems impossible to conclude as there is no mathematical or evidential data on the resistance.
Yes, that factor is the key here. There's a downwards acceleration of g (gravity), for the buildings to fall as observed, then the net upwards force would have to be around g/10. (Then giving you a downward acceleration of 0.9*g)
There is actually a lot of evidence for the resistance the floors offered. It's been quite well studied. The problem is that the Truthers tend to use either faulty reasoning, or wildly inaccurate estimates (like assuming the fall has to "dustify" everything on the way down, rather than just break the connections). Since the math is beyond most people, you tend to get entrenched positions. There's the basic math here, but even this is still a simplified model (although vastly more accurate than the YouTube whiteboard explanation).
https://www.metabunk.org/files/Mechanics of Progressive Collapse.pdf
https://www.metabunk.org/files/00 WTC Collapse - What Did & Did Not Cause It.pdf
It think it would be productive to try to get a simpler explanation though.
Long Span flooring sections are in common use and I think it important that people should be assured that the same design faults as allegedly caused wtc1, 2, & 7 to fall, are not inherent in all these other buildings.
I agree, and so does NIST, which is why that was a primary point in their recommendations resulting from the investigations.
Citing its one new recommendation (the other 12 are reiterated from the previously completed investigation of the World Trade Center towers, WTC 1 and 2), the NIST investigation team said that "while the partial or total collapse of a tall building due to fires is a rare event, we strongly urge building owners, operators and designers to evaluate buildings to ensure the adequate fire performance of the structural system. Of particular concern are the effects of thermal expansion in buildings with one or more of the following features: long-span floor systems, connections not designed for thermal effects, asymmetric floor framing and/or composite floor systems." Engineers, the team said, should be able to design cost-effective fixes to address any areas of concern identified by such evaluations.
Long span flooring is not inevitably dangerous, it just raises additional issues. The recommendations on NIST are aimed at preventing progressive collapse, not eliminating long span floors.I cannot seem to find if the new wtc 7 has long span flooring?