15 Mar 2007, 6:03 pm
Jacob_Landshore wrote:
In this photo we can see the upper section of the South Tower leaning forward as the building starts to break up. What makes this interesting is that physical law demands this upper section of the building continue its forward momentum and fall away from the tower unless another force counteracts that motion.
Your physics theory (which I am not sure holds weight anyway) is meaningless as the upper and lower parts of the building were still meshed together.
The building was not primarily "breaking up" at this point, it was beginning to collapse. What was being ejected into the air was the powdery remnants of what was being crushed.
What caused the collapse:
Quote:
After the columns bowed, the weight was no longer going straight down. Like taking a straw and bowing it in the middle, it no longer can hold the same weight as it did when it was straight. The building tried to transfer the load to the core columns and massive hat truss on the roof. The weakened core, weakened by fire and impact, couldn't hold the massive weight from tilting. As with the perimeter column, the massive load on the deformed core columns gave way.
Diagram followes, after which followed a pancake-type collapse.
Quote:
As generally accepted by structural engineering and structural mechanics experts (though not by some laymen and fanatics seeking to detect a conspiracy), the failure scenario, broadly proposed by Bazant (2001), and Bazant and Zhou (2002), on the basis of simplified analysis, and supported by very realistic, meticulous and illuminating computer simulations and exhaustive investigations by S. Shyam Sunder's team at the National Institute of Standards and Technology (NIST, 2005), may be summarized as follows:
1. About 60% of the 60 columns of the impacted face of framed-tube (and about 13% of the total of 287 columns) were severed, and many more were significantly deflected. This caused stress redistribution, which significantly increased the load of some columns, near the load capacity for some of them.
2. Fire insulation was stripped during aircraft impact by flying debris (without that, the towers would likely have survived). In consequence, many structural steel members heated up to 600±C (NIST 2005) (the structural steel used loses about 20% of its yield strength already at 300±C, NIST 2005, and exhibits significant visco-plasticity, or creep, above 450±, especially at high stresses that developed; see e.g. Cottrell 1964, p. 299; the press reports right after 9/11, indicating temperature in excess of 800±C, turned out to be groundless, but Bazant and Zhou's analysis did not depend on that).
3. Differential thermal expansion, combined with heat-induced viscoplastic deformation, caused the floor trusses to sag. The sagging trusses pulled the perimeter columns inward (by about 1 m, NIST 2005). The bowing of columns served as a huge imperfection inducing multi-story buckling. The lateral deflections of some columns due to aircraft impact and differential thermal expansion also decreased buckling strength.
4. The combination of six effects
a) overload of some columns due to initial stress redistribution,
b ) lowering of yield limit and creep,
c) lateral deflections of many columns due to sagging floor trusses,
d) weakened lateral support due to reduced in-plane stiffess of sagging floors,
e) multi-story buckling of some columns (for which the critical load is an order of magnitude less than it is for one-story buckling), and
f) local plastic buckling of heated column webs finally led to buckling of columns (Fig. 1b). As a result, the upper part of tower fell, with little resistance, through at least one floor height, impacting the lower part of tower. This triggered progressive collapse because the kinetic energy of the falling upper part far exceeded the energy that could be absorbed by limited plastic deformations and fracturing in the lower part of tower. (Bazant, Verdure, 2006)
1. About 60% of the 60 columns of the impacted face of framed-tube (and about 13% of the total of 287 columns) were severed, and many more were significantly deflected. This caused stress redistribution, which significantly increased the load of some columns, near the load capacity for some of them.
2. Fire insulation was stripped during aircraft impact by flying debris (without that, the towers would likely have survived). In consequence, many structural steel members heated up to 600±C (NIST 2005) (the structural steel used loses about 20% of its yield strength already at 300±C, NIST 2005, and exhibits significant visco-plasticity, or creep, above 450±, especially at high stresses that developed; see e.g. Cottrell 1964, p. 299; the press reports right after 9/11, indicating temperature in excess of 800±C, turned out to be groundless, but Bazant and Zhou's analysis did not depend on that).
3. Differential thermal expansion, combined with heat-induced viscoplastic deformation, caused the floor trusses to sag. The sagging trusses pulled the perimeter columns inward (by about 1 m, NIST 2005). The bowing of columns served as a huge imperfection inducing multi-story buckling. The lateral deflections of some columns due to aircraft impact and differential thermal expansion also decreased buckling strength.
4. The combination of six effects
a) overload of some columns due to initial stress redistribution,
b ) lowering of yield limit and creep,
c) lateral deflections of many columns due to sagging floor trusses,
d) weakened lateral support due to reduced in-plane stiffess of sagging floors,
e) multi-story buckling of some columns (for which the critical load is an order of magnitude less than it is for one-story buckling), and
f) local plastic buckling of heated column webs finally led to buckling of columns (Fig. 1b). As a result, the upper part of tower fell, with little resistance, through at least one floor height, impacting the lower part of tower. This triggered progressive collapse because the kinetic energy of the falling upper part far exceeded the energy that could be absorbed by limited plastic deformations and fracturing in the lower part of tower. (Bazant, Verdure, 2006)
(italics in original)
(highly recommended source link where can find links to the NIST report)
I am finished with this topic.
