Hier ist der komplette Text, von dem Du sagst, er erkläre die Freifall-Phase.
Wie Du siehst, habe ich mir sogar Mühe gegeben die textliche Übersichtlichkeit zu wahren. Du musst nur noch
fett markieren, wo erklärt wird,
wie es zur Freifall-Phase kam.
Ich sehe da nämlich nichts, außer der Feststellung das es sie gab.
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AUSZUG NIST REPORT
SUMMARY OF FINDINGS
A global finite element model of WTC 7 was developed using LS-DYNA to calculate the structural response of WTC 7 to a fire-induced initial failure event and to determine the sequence and timing of the events that led to the global collapse of the building. Four simulations were performed with this model.
The first was based on NIST's best estimate of both the debris impact damage from WTC 1 and the fire-induced damage, from the ANSYS analysis. This occurred at 4 h in the ANSYS computation.
The second simulation differed only in the input of a lesser degree of fire-induced damage at 3.5 h in the ANSYS computation. The purpose of this simulation was to determine whether a lesser degree of fire-induced damage could lead to the collapse of WTC 7.
The third simulation was the same as the first, except that no debris impact damage was included. The purpose of this analysis was to determine the contribution of debris impact to the WTC 7 global collapse sequence and whether WTC 7 would have collapsed solely due to the effects of the fires.
In the fourth simulation, the building experienced no debris or fire-induced damage. A section of Column 79 between Floors 11 and 13 was removed. The purpose of this analysiswas to determine the potential for a classic progressive collapse, i.e., disproportionate structural damage from a single failure, regardless of the cause of that failure.
The following are the principal findings from these simulations.
The best estimate simulation led to global collapse of WTC 7 and provided a reasonable sequence and timing of the events leading to the initiation of global collapse. As expected, there were some deviations from the observable evidence at computation times after the global collapse was underway.
The simulation with identical debris impact damage, but reduced fire damage, did not lead to a collapse-initiating event or to collapse itself. This result indicated that the debris impact damage, by itself, was not sufficient to initiate global collapse. It also indicated that the long duration of the multiple floor fires was the primary cause of the eventual collapse.
The simulation with the same fire-induced damage as the best estimate simulation, but with no debris impact damage, identified the same initiating event and timing of that event as did the best estimate simulation. This simulation also led to global collapse of the building, further indicating that the debris impact damage was not a principal contributor to the occurrence of collapse. However, the mechanics of the collapse differed from the best estimate simulation in the later stages of the horizontal progression of failure.
WTC 7 was prone to classic progressive collapse in the absence of debris impact and fire- induced damage when a section of Column 79 between Floors 11 and 13 was removed. The collapse sequence demonstrated a vertical and horizontal progression of failure upon the removal of the Column 79 section, followed by buckling of exterior columns, which led to the collapse of the entire building.
The simulations supported the leading collapse hypothesis and provided the following additional insights into the sequence of events leading to the global collapse of WTC 7. Except as noted, these findings were derived from the best estimate simulation.
Initial Local Failure for Collapse Initiation
- The collapse of Floor 13 onto the floors below, some of which were already weakened by fires, triggered a cascade of floor failures in the northeast region of the building.
- The floor failures progressed down to Floor 5, where debris accumulated, and spread across the east floor area, due to the effects of impact loads from falling debris on thermally weakened floor areas.
- Critical Column 79 became laterally unsupported between Floors 5 and 14 in the east-west and south directions as a result of the progression of floor system failures. There was still some lateral support in the north direction at Floors 8 to 12 and Floor 14, as the erection bolts in the seated connections had all failed at these girder ends, but the girders had not walked off the bearing seat.
- The increase in unsupported length led to the buckling failure of Column 79, which was the collapse initiation event for WTC 7.
Vertical Progression of Failure
Once Column 79 buckled between Floors 5 and 14, the column section above began to descend. Column 79 began moving downward at the roof level approximately 0.2 s after Column 79 buckled and 0.6 s before Column 80 buckled.
As Column 79 moved downward, the floor framing adjacent to Column 79 was pulled downward up to the roof and east penthouse, which led to the observed kink in the east penthouse roof framing on the north side.
As Column 79 moved downward, the floor framing connections to interior Columns 76 and 80 and exterior Column 44 failed under increasing tensile in-plane floor forces, resulting in a vertical progression of failure of the floor systems around Column 79.
Column 80 became laterally unsupported in both directions between Floors 5 and 15, as a result of the progressive floor system failures. Column 80 buckled about 0.7 s after Column 79 buckled.
Column 81 became laterally unsupported in both directions between Floors 7 and 15, as a result of the progressive floor system failures. Column 81 buckled about 1.3 s after Column 79 buckled.
As Columns 79, 80, and 81 each buckled, their column sections above Floor 14 began to descend downward and pulled the floor structures downward with them, thereby creating a vertical progression of floor collapses that spread across the entire east region of WTC 7.
The east penthouse, which was supported by Columns 79, 80, and 81, fell downward.
Horizontal Progression of Failure
Interior column buckling failures continued to propagate across the core in the lower floors from east to west, due to (1) loss of lateral support from floor system failures, (2) impact of falling debris from adjacent floors and columns, and (3) load redistributed from other buckled columns.
During the vertical progression, the falling floors pulled on adjacent columns until the floor connections failed on the east side of Columns 76, 77, and 78.
Truss 2 collapsed due to accumulated weight and impact loads from falling debris. The collapse of Truss 2 precipitated the buckling of Columns 77 and 78, which together with Column 76 had already lost lateral support.
The occurrence of this westward progression indicates that the failure of Truss 2 was not essential to the failure of Columns 77 and 78, as they would have buckled in a similar fashion as the other columns.
The horizontal progression of failure was sensitive to the extent of the estimated initial structural damage in WTC 7 due to debris impact from the collapse of WTC 1.
For the debris impact damage scenario, some of the interior columns on the west began buckling as the horizontal progression of interior column failures approached the middle of the core. For example, the middle line of interior columns directly behind the initially severed exterior columns buckled before the two north-south lines of columns immediately to their east. The middle line of interior columns lost lateral support in the north-south direction over 11 floors in response to the structural damage that was imposed.
For the no debris impact damage scenario, the horizontal progression of column failures occurred in order from east to west after collapse initiation (i.e., buckling of Column 79) and the vertical progression of failures on the east side. This scenario matched the observations where the screenwall on the roof fell downward before the west penthouse. This suggests that the damage scenario that was imposed in the best estimate analysis was slightly more severe than actually occurred.
Global Collapse
- The exterior column failures were sensitive to the extent of the estimated initial structural damage in WTC 7 due to debris impact from the collapse of WTC 1.
For the debris impact damage scenario, the exterior column buckling began at the southwest corner Column 14, adjacent to the WTC 1 debris impact zone, between Floors 10 and 12. The exterior columns adjacent to the (seven) columns severed in the southwest region due to the collapse of WTC 1 were the first to buckle because additional load was distributed to them as a result of the damage. The analysis with debris impact damage closely simulated the observed failure of the exterior façade, where the façade moved downward as a single unit.
For the no debris impact damage scenario, the exterior columns buckled near mid-height of the building, approximately between Floors 17 and 29. If the exterior columns had buckled at mid-height, it would have been visible in the videographic records. The analysis without debris impact damage did not closely simulate the observed failure of the exterior façade.
- The observed behavior of the exterior columns during the global collapse was more closely simulated by the analysis with debris impact damage.
- As the interior columns buckled at the lower floors and the corresponding upper column sections began to move downward, the exterior columns buckled inward at the lower floors as a result of floor pull-in forces caused by the downward movement of the building core. The floor connections to the columns had not yet failed in this region, as there were no fires observed on the west side of Floors 10 through 14 at any time during the day, so the floors were intact and able to pull the exterior columns inward.
- The south and west exterior columns buckled first, followed by the north and east face columns.
- All exterior columns buckled between approximately Floors 7 and 14.
- Once column support was lost in the lower floors, the remaining exterior structure above began to fall vertically as a single unit.
- WTC 7 was prone to classic progressive collapse in the absence of fire-induced damage and debris impact damage when a section of Column 79 between Floors 11 and 13 was removed. The collapse sequence demonstrated a vertical and horizontal progression of failure upon the removal of the Column 79 section, followed by downward movement at the roofline due to buckling of exterior columns, which led to the collapse of the entire building.
- The observed descent time of the upper 18 stories of the north face of WTC 7 (the floors clearly visible in the video evidence) was 40 percent greater than the computed free fall time. A more detailed analysis of the descent of the north face found three stages: (1) a slow descent with acceleration less than that of gravity that corresponded to the buckling of the exterior columns at the lower floors, (2) a freefall descent over approximately eight stories at gravitational acceleration for approximately 2.25 s, and (3) a decreasing acceleration as the north face encountered resistance from the structure below.
http://www.nist.gov/customcf/get_pdf.cfm?pub_id=861611
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