The 9/11 Forum

Niclas wrote:If the dominoes are weakly glued together the whole structure would be destabilized, and yes it would COLLAPSE, the dominoes would not crush the lower structure from top to bottom, it is more likely that the structure would start to wobble and fall to the side.

Or, once the defect grew to sufficient size, it might drop straight down like this tower of wooden blocks:

http://www.youtube.com/watch?v=NBbz2eIoVDQ

This, despite being incompressible elements compared to the impulse the mass elements can exert on each other in a gravity-driven collapse. Contact points with adhesive which fails under bending and torsion would mimic welds and fasteners snapping.

When there are compressible elements (deformable steel lattice with an interior volume of mostly air), there would be an even greater propensity to drop downward since there is a crush path available, whereas incompressible elements are just that.

Niclas wrote:If we have a piledriver, CRUSHING the lower structure, that is not a COLLAPSE.

How is that so? It's a particular kind of collapse. A CD is an explosives and gravity-driven collapse.

I actually came here to show this:

http://commons.wikimedia.org/wiki/File:Mass-Spring-Damper.svg

from which I made this:



as a simple mass-spring-dashpot model. The mass-spring model was discussed upthread. A missing element type in that discussion is a damper. Once a spring breaks (failure of support denoted by red circle/slash), the displacement is downward through damped compression which resists the descent but does NOT provide the static capacity FOS*mg afforded by the intact spring, it wouldn't (in this case) even provide a resistive force of the static load mg. It will only be some fraction while buckling of a level occurs. The process need not be sequential as a more accurate model would account for the three story perimeter height and staggered placement. But this is the general idea.

Merely by having the springs break, displacement is permitted against the lower section with a retarding force less than the springs. Make the dashpots (plastic deformation) have a zero coefficient and the section drops at freefall until loading up the next spring.

There is no violation of Newton's third law. At every point in such a system, the law is satisfied. The error lies in assuming a rigid tower, that means non-deforming, which leads to an absurd result because the tower is not an incompressible medium. A physics teacher should know this. I invite David Chandler to put in an appearance and defend his claims.

It's time for the simple-minded and erroneous physics arguments to hit the dustbin.

After reading this thread, I am surprised by how late in the discussion the actual structure was discussed. The notion that columns hold up floors and interior walls and the skin of the building (which in this case were columns and not a skin as in curtain wall construction) has led to the discussion that the columns must be destroyed to cause the collapse of the twin towers.

And surely there was some failure which initiated the collapse, but what collapsed was the FLOORS and the columns or the core largely survived or were separated into neat 36' segments scattered like pick up sticks showing very little if any distortion.

The facade can be seen peeling off as it came off in large sheets. It was not crushed. The cross beams of the core seemed to have been severed from the core columns and these maintained it into a rigid grid. But even some of those lateral braces held two several pairs of core columns together to over 50 stories.

So the story has nothing to do with the energy of the upper block and the resistance of the lower block and the columns played little part in the destruction (collapse), because they didn't collapse onto the floors. The floors were likely overloaded and plunged, breaking apart upon collision with the floor below, because they were overloaded or the connections to the columns were destroyed.

This was, after the initiation, a collapse of the floors and considering that the floor section was 96% air the speed of collapse is perfectly reasonably considering the growing weight of the plunging mass of floors and contents. The only thing crushed in the collapse was the floors... and clearly most of that mass did not float away in air currents and land all over lower Manhattan. And what did became air born at the conclusion of the collapse.

Heavier than air debris spilled out, pushed out, knocked the facade over and created a plume of free falling heavier than air debris which ranged about 250' from each facade. Tower 1 standing but 114 feet away suffered no facade damaged in the collapse of tower 2, so whatever was coming off missed tower 1.

If people objectively looked at the many videos and understand the structure they can report more accurate observations.