Originally posted by jtma508
There is simply no reasonable explanation for that other than the fact that the buildings were all pre-wired with explosives on all floors so that upper collapsing floors would meet no resistance as they fell. Otherwise, the collapse of the buildings could not possibly have occurred at a speed so close to free-fall. Anything else is a red herring.

Regardless of the precise failure mode, experience with buckling indicates that while many elastic buckles simultaneously coexist in an axially compressed tube, the plastic deformation localizes (because of plastic bifurcation) into a single buckle at a time and so the buckles must fold one after another. Thus, at least one plastic hinge, and no more than four plastic hinges, per column line are needed to operate simultaneously in order to allow the upper part to continue moving down. (This is also true if the columns of only one floor are buckling at a time.) At the end,
the sum of the rotation angles of the hinges on one column line, cannot exceed 2 pi. This upperbound value, which is independent of the number of floors spanned by the buckle, is used in the present calculations since, in regard to survival, it represents the most optimistic hypothesis, maximizing the plastic energy dissipation.

Calculating the dissipation per column line of the framed tube as the plastic bending moment of one column times the combined rotation angle and multiplying this by the number of columns, one concludes that the plastically dissipated energy is, optimistically, of the order of 0.5 GN (for lack of information, certain details such as the wall thickness of steel columns, were estimated by carrying out approximate design calculations for this building).

To attain the combined rotation angle of the plastic hinges on each column line, the upper part of the building must move down by the additional distance of at least one floor below the floor where the collapse started, and so the total release of gravitational potential energy is ~4.2 GN m. To arrest the fall, the kinetic energy of the upper part, which is equal to the potential energy release, would have to be absorbed by the plastic hinge rotations, i.e., Wp would have to be larger than Wg . Rather,

Wg /Wp ~= 8.4

So, even under the most optimistic assumptions by far, the plastic deformation can dissipate only a small part of the kinetic energy acquired by the upper part of building. When the next buckle with its group of plastic hinges forms, the upper part has already traveled many floors down and has acquired a much higher kinetic energy; the percentage of the kinetic energy dissipated plastically is then of the order of 1%. The percentage continues to decrease further as the upper part moves down. If fracturing in the plastic hinges were considered, a still smaller (in fact much smaller) energy dissipation would be obtained. So the collapse of the tower must be an almost free fall.

Your postings on energy betray your ignorance of the subject, irrespective of whether you think it was 1/10 or 1/100th of a nuclear weapon. You do not mention the strain energy capacity of the building, which, by necessity would be a multiple of all of the potential energy of the building. Without the addition of additional energies the energy balance would have remained in deficit and no collapse could have occurred.

Originally by esdad71:
You are expressing the same thing. My post was stating the amount of built up energy, that was suddenly released when the collpase occured. The balance of energy was shifted because 3/4 of the columns and support structure (elelvators) were destroyed, and there was uneven distribution. A scholar working on it expained that the amount of force released was equivelant to that of a partial nuclear explosion and that is what I posted. Also, the Last thing you want to do is call anyone ignorant, okay, just ask a question, or pose a theory and don't think that because you present the same old tired movies people will think different. I mean, your video shows the building falling forward, and then it collapsing?That is not a demo job....It was also seen that floors were actually bucklng so far that the floors seemed to merge . This was verified by Police helicopters who were radioing to make sure everyone was evacuated.

To a structural engineer, a skyscraper is modeled as a large cantilever vertical column. Each tower was 64 m square, standing 411 m above street level and 21 m below grade. This produces a height-to-width ratio of 6.8. The total weight of the structure was roughly 500,000 t, but wind load, rather than the gravity load, dominated the design. The building is a huge sail that must resist a 225 km/h hurricane. It was designed to resist a wind load of 2 kPa—a total of lateral load of 5,000 t.

In order to make each tower capable of withstanding this wind load, the architects selected a lightweight “perimeter tube” design consisting of 244 exterior columns of 36 cm square steel box section on 100 cm centers (see Figure 3). This permitted windows more than one-half meter wide. Inside this outer tube there was a 27 m × 40 m core, which was designed to support the weight of the tower. It also housed the elevators, the stairwells, and the mechanical risers and utilities. Web joists 80 cm tall connected the core to the perimeter at each story. Concrete slabs were poured over these joists to form the floors. In essence, the building is an egg-crate construction that is about 95 percent air, explaining why the rubble after the collapse was only a few stories high.