Forum Sceptique

alobee a écrit :As a result of the aircraft impact damage, the north and south wall each carried about 7% less gravity load after impact, and the east and west walls each carried about 7% more load. The Core carried about 1 percent more gravity load after impact. NIST p. 150

si le NIST avait dit 100% more gravity load on aurait que les poutres supportaits 2x le poids d'origine... 1%

Only three of the recovered samples of exterior panels reached temperature in excess of 250 degre oC during the fires or after the collapse. This was based on a method developed by NIST to characterize maximum temperatures expireinced by steel members through observations of paint craking. NIST P. 181

Les tests 4 tests effectués afin de reproduire les conditions du 11 septembre par le NIST démontrent;

The result established that this type of assembly was capable of sustaining a large gravity load witout collapsing for a susbtancial period of time relative to the duration of the fires in any given location on September 11.” NIST P.143

Qui veut contredire ces faits ?
Non mais ce n'est pas a moi de les expliquer c'est a vous...

NIST NCSTAR 1, WTC Investigation pp. 179-186

8.3 FINDINGS ON THE MECHANISMS OF BUILDING COLLAPSE

8.3.1 Summary of Probable Collapse Sequences

WTC 1 was struck by a hijacked aircraft at 8:46:30 a.m. and began to collapse at 10:28:22 a.m. WTC 2 was struck by a hijacked aircraft at 9:02:59 a.m. and began to collapse at 9:58:59 a.m. The specific factors in the collapse sequences relevant to both towers (the sequences vary in detail for WTC 1 and WTC 2) are:

Each aircraft severed exterior columns, damaged interior core columns and knocked off insulation from steel as the planes penetrated the buildings. The weight carried by the severed columns was distributed to other columns.

Subsequently, fires began to grow and spread. They were initiated by the aircraft’s jet fuel, but were fed for the most part by the building contents and the air supply resulting from breached walls and fire-induced window breakage.

These fires, in combination with the dislodged insulation, were responsible for a chain of events in which the building core weakened and began losing its ability to carry loads.

The floors weakened and sagged from the fires, pulling inward on the exterior columns.

Floor sagging and exposure to high temperatures caused the exterior columns to bow inward and buckle—a process that spread across the faces of the buildings.

Collapse then ensued.



Seven major factors led to the collapse of WTC 1 and WTC 2:


Structural damage from the aircraft impact;
Large amount of jet fuel sprayed into the building interior, that ignited widespread fires over several floors;

Dislodging of SFRM from structural members due to the aircraft impact, that enabled rapid heating of the unprotected structural steel;

Open plan of the impact floors and the breaking of the partition walls by the impact debris that resulted in increased ventilation;

Weakened core columns that increased the load on the perimeter walls;

Sagging of the floors, that led to pull-in forces on the perimeter columns; and

Bowed perimeter columns that had a reduced capacity to carry loads.

8.3.2 Structural Steels


Fourteen different strengths of steel were specified in the structural engineering plans, but only 12 steels of different strength were actually used in construction due to an upgrade of two steels. Ten different steel companies fabricated structural elements for the towers, using steel supplied from at least eight different suppliers. Four fabricators supplied the major structural elements of the 9th to the 107th floors. Material substitutions of higher strength steels were not uncommon in the perimeter columns and floor trusses.

About 87 percent of the tested steel specimens (columns, trusses and bolts) met or exceeded the required yield strengths specified in design documents. About 13 percent had NIST-measured strengths that were slightly lower than the design values, but this may have arisen from mechanical damage during the collapse, the natural variability of structural steel, and slight differences between the NIST and original mill test report testing protocols.

The safety of the WTC towers on September 11, 2001, was most likely not affected by the fraction of steel that, according to NIST testing, was modestly below the required minimum yield strength. The typical factors of safety in allowable stress design were capable of accommodating the measured property variations below the minimum.

The pre-collapse photographic analysis showed that 16 recovered exterior panels were exposed to fire prior to collapse of WTC 1. None of the nine recovered panels from within the fire floors of WTC 2 were observed to have been directly exposed to fire.

None of the recovered steel samples showed evidence of exposure to temperatures above 600 ºC for as long as 15 min. This was based on NIST annealing studies that established the set of time and temperature conditions necessary to alter the steel microstructure. These results provide some confirmation of the thermal modeling of the structures, since none of the samples were from zones where such heating was predicted.

Only three of the recovered samples of exterior panels reached temperatures in excess of 250 °C during the fires or after the collapse. This was based on a method developed by NIST to characterize maximum temperatures experienced by steel members through observations of paint cracking.

Perimeter columns exposed to fire had a great tendency for local buckling of the inner web; a similar correlation did not exist for weld failure.

Observations of the recovered steel provided significant guidance for modeling the damage from the aircraft impact with the towers.

For the perimeter columns struck by the aircraft, fractures of the plates in areas away from a welded joint exhibited ductile behavior (necking and thinning away from the fracture) under very high strain rates. Conversely, fractures occurring next to a welded joint exhibited little or no ductile characteristics.

There was no evidence to indicate that the type of joining method, materials, or welding procedures were improper. The welds appeared to perform as intended.

The failure mode of spandrel connections on perimeter panels differed above and below the impact zone. Spandrel connections on exterior panels at or above the impact zone were more likely to fail by bolt tear out. For those exterior panels below the impact zone, there was a higher propensity for the spandrels to be ripped off from the panels. This may be due to shear failures as the weight of the building came down on these lower panels. There was no difference in failure mode for the spandrel connections whether the exterior panels wereexposed to fire or not.

With the exception of the mechanical floors, the perimeter panel column splices failed by fracture of the bolts. At mechanical floors, where splices were welded in addition to being bolted, the majority of the splices did not fail.

Core columns failed at both splice connection and by fracture of the columns themselves.

The damage to truss seats on perimeter panels differed above and below the impact zone in both towers. The majority of recovered perimeter panel floor truss connectors (perimeter seats) below the impact floors were either missing or bent downward. Above this level, the failure modes were more randomly distributed.

In the floor trusses, a large majority of the electric resistance welds at the web-to-chord connections failed. The floor truss and the perimeter panel floor truss connectors typically failed at welds and bolts.

The NIST-measured properties of the steels (strain rate, impact toughness, high-temperature yield and tensile strengths) were similar to literature values for other construction steels of the WTC era.

The creep behavior of the steels could be modeled by scaling WTC-era literature data using room temperature tensile strength ratios.



8.3.3 Aircraft Impact Damage Analysis

Both towers withstood the significant structural damage to the exterior walls, core columns, and floor systems due to the aircraft impact. WTC 2 was the more severely damaged building and the first to collapse. WTC 2 displayed significant reserve capacity, as evidenced by a post-impact rooftop sway that was more than one-third of that under the hurricane force winds for which the building was designed. The oscillation period of this swaying was nearly equal to that calculated for the undamaged structure. (Such an analysis was not possible for the less severely damaged WTC 1 due to the absence of equivalent video footage for the analysis.)

American Airlines Flight 11 impacted the north wall of WTC 1 at a speed of 443 mph ± 30 mph, banked 25 degrees ± 2 degrees to the left (left wing downward) and with the nose tilted slightly downward. United Airlines Flight 175 impacted the south wall of WTC 2 at a speed of 542 mph ± 24 mph, banked 38 degrees ± 2 degrees to the left (left wing downward) and with the nose pointed slightly downward and to the right.

The aircraft impact on WTC 1 caused extensive damage to the north wall of the tower, principally in the regions impacted by the fuselage, engine, and fuel-filled wing sections. Photographic evidence showed that 34 perimeter columns were completely severed, while four columns were heavily damaged, and two columns were moderately damaged.

The impact simulations of WTC 1 indicated that three to six core columns were severed, and three to four columns were heavily damaged. The floor trusses, core beams, and floor slabs experienced significant impact-induced damage on floors 94 through 96, particularly in the path of the fuselage. The wing structures were fragmented at the exterior wall, and aircraft fuel was dispersed on multiple floors. Aircraft debris substantially damaged the nonstructural interior partitions and the workstations and dislodged insulation in its path. The bulk of the fuel and aircraft debris was deposited in floors 93 through 97 with the largest concentration on floor 94.

The aircraft impact on WTC 2 caused extensive damage to the south wall of the tower and to the regions impacted by the fuselage, engine, and fuel-filled wing sections. Photographic evidence showed that 29 perimeter columns were completely severed, one was heavily damaged, and three were moderately damaged. Four perimeter columns on the north wall also were severed.

The impact simulations of WTC 2 indicated that five to ten core columns were severed and up to four columns were heavily damaged. The rupture of some column splices on floors 77, 80, and 83 contributed significantly to the failure of the core columns. The floor trusses, core beams, and floor slabs experienced significant impact-induced damage on floors 79 to 81, particularly in the path of the fuselage. The analyses indicated that the wing structures were fragmented due to the interaction with the exterior wall and, as a result, aircraft fuel was dispersed on multiple floors. The aircraft debris substantially damaged the building’s contents and also dislodged insulation in its path. The bulk of the fuel was concentrated on floors 79, 81, and 82, while the bulk of the aircraft debris was deposited in floors 78 through 80, with the largest concentration on floor 80.

Other effects of the aircraft impacts included (a) severing of the sprinkler and fire hose water supply systems, negating any possible fire suppression efforts; (b)dispersing of jet fuel and ignition of building contents over large areas; (c) increasing the air supply into the damaged buildings that permitted very large fires; and (d) damaging ceilings, enabling unabated heat transport to the floor structure above and over the floor-to-ceiling partition walls to the next compartment. These effects were consistent with photographic evidence and with the accounts of building occupants and emergency responders.

The simulations fairly closely matched the exterior wall damage patterns from each of the aircraft impacts and correctly predicted the collapse of five of the six stairwell walls and the lesser damage to the sixth, the trajectories of the engine and wheels that penetrated the buildings, and the accumulation of furnishings and debris in the northeast corner of the 80th and 81st floors of WTC 2.

8.3.4 Reconstruction of the Fires


In each tower, the fires were initiated simultaneously on multiple floors by ignition of some of the jet fuel from the aircraft. The initial jet fuel fires themselves lasted at most a few minutes.

The principal combustibles on the fire floors were workstations. The total combustible fuel load on the WTC floors was about 4 lb/ft2. Higher combusted fuel loadings resulted in slower fire spread rates that did not match the patterns observed in the photographic evidence.

Under these higher combusted fuel loadings, the fires likely would not have reached the south side of WTC 1 in the time needed to cause inward bowing and collapse initiation.

The aircrafts added significant combustible material to their paths (and the paths of their breakup fragments) through the buildings.

It is possible to reconstruct a complex fire in a large building, even if the building is no longer standing. However, this requires extraordinary information to replace what might have been gleaned from an inspection of the post-fire premises. In the case of the WTC tower, this information included floor plans of the fire zones, burning behavior of the combustibles, simulations of damage to the building interior, and frequent photographic observations of the fire progress from the building exterior.

The fires in WTC 1 were generally ventilation limited, i.e., they burned and spread only as fast as windows broke. Where the combustibles were not significantly relocated by the aircraft debris, they tended to burn out in about 20 min. This was consistent with the results of workstation fire tests conducted by NIST, in which the fuel load was 4 lb/ft2. Although there were multiple fires on some of the impact floors, the general trend was for the fires to move toward the south side of the tenant spaces.

The fires in WTC 2 had sufficient air to burn at a rate determined by the properties of the combustibles. This was in large part due to the extensive breakage of windows in the fire zone by the aircraft impact. In contrast with WTC 1, there was little spread in WTC 2. The early fires persisted on the east side of the tower and particularly in the northeast corner of the 80th and 81st floors, where the aircraft debris had pushed a lot of fractured combustibles

The Fire Dynamics Simulator can predict the room temperatures and heat release rate values for complex fires to within 20 percent, when the building geometry, fire ventilation, and combustibles are properly described.

The Fire Structure Interface, developed for this Investigation, mapped the fire-generated temperature and thermal radiation fields onto and through layered structural materials to within the accuracy of the fire-generated fields and the thermophysical data for the structural components.

Conventional office workstations reached a peak burning rate in about 10 min and continued burning for a total of about a half hour. Partial covering of surfaces with inert material reduced the peak burning rate proportional to the fraction covered, but did not affect the total amount of heat release during the entire burning.

Jet fuel sprayed onto the surfaces of typical office workstations burned away within a few minutes. The jet fuel accelerated the burning of the workstation, but did not significantly affect the overall heat released.

In the simulations, none of the columns with intact insulation reached temperatures over 300 °C. Only a few isolated truss members with intact insulation were heated to temperatures over 400 °C in the WTC 1 simulations and to temperatures over 500 °C in the WTC 2 simulations. In WTC 1, if the fires had been allowed to continue past the time of building collapse, complete burnout would likely have occurred within a short time since the fires had already traversed around the entire floor, and most of the combustibles would already have been consumed. In WTC 2, if the fire simulation were extended for 2 hours past the time of building collapse with all windows broken, the temperatures in the truss steel on the west side of the building (where the insulation was undamaged) would likely have increased for about 40 min before falling off rapidly as the combustibles were consumed. Temperatures of 700 °C to 760 °C were reached over approximately 15 percent of the west floor area for less than 10 min. Approximately 60 percent of the floor steel had temperatures between 600 °C and 700 °C for about 15 min. Approximately 70 percent of the floor steel had temperatures that exceeded 500 °C for about 45 min. At these temperatures, the floors would be expected to sag and then recover a portion of the sag as the steel began to cool. The temperatures of the insulated exterior and core columns would not have increased to the point where they would have experienced significant loss of strength or stiffness.


8.3.5 Structural Response and Collapse Analysis


The core columns were weakened significantly by the aircraft impact damage and thermal effects. Thermal effects dominated the weakening of WTC 1. As the fires moved from the north to the south side of the core, the core was weakened over time by significant creep strains on the south side of the core. Aircraft impact damage dominated the weakening of WTC 2. With the impact damage, the core subsystem leaned to the southeast and was supported by the south and east perimeter walls via the hat truss and floors. As the core weakened, it redistributed loads to the perimeter walls through the hat truss and floors. Additional axial loads redistributed to the exterior columns from the core were not significant (only about 20 percent to 25 percent on average) as the exterior columns were loaded to approximately 20 percent of their capacity before the aircraft impact.

The primary role of the floors in the collapse of the towers was to provide inward pull forces that induced inward bowing of perimeter columns (south face of WTC 1; east face of WTC 2). Sagging floors continued to support floor loads as they pulled inward on the perimeter columns. There would have been no inward pull forces if the floors connections had failed and disconnected.

Column buckling over an extended region of the perimeter face ultimately triggered the global system collapse as the loads could not be redistributed through the hat truss to the already weakened building core. As the exterior wall buckled (south face for WTC 1 and east face for WTC 2), the column instability propagated to adjacent faces and caused the initiation of the building collapse. Perimeter wall buckling was induced by a combination of thermal weakening of the columns, inward pull forces from sagging floors, and to a much lesser degree, additional axial loads redistributed from the core.

The WTC towers would likely not have collapsed under the combined effects of aircraft impact damage and the extensive, multi-floor fires that were encountered on September 11, 2001, if the thermal insulation had not been widely dislodged or had been only minimally dislodged by aircraft impact.

In the absence of structural and insulation damage, a conventional fire substantially similar to or less intense than the fires encountered on September 11, 2001, likely would not have led to the collapse of a WTC tower.

The insulation damage estimates were conservative as they ignored possibly damaged and dislodged insulation in a much larger region that was not in the direct path of the debris but was subject to strong vibrations during and after the aircraft impact. A robust criterion to generate a coherent pattern of vibration-induced dislodging could not be established to estimate the larger region of damaged insulation.

For WTC 1, partitions were damaged and insulation was dislodged by direct debris impact over five floors (floors 94, 95, 96, 97, and 98) and included most of the north floor areas in front of the core, the core, and central regions of the south floor areas, and on some floors, extended to the south wall.

For WTC 2, partitions were damaged and insulation was dislodged by direct debris impact over six floors (floors 78, 79, 80, 81, 82, and 83) and included the south floor area in front of the core, the central and east regions of the core, and most of the east floor area, and extended to the north wall.

The adhesive strength of BLAZE-SHIELD D to steel coated with primer paint was found to be one-third to one-half of the adhesive strength to steel that had not been coated with primer paint. The SFRM products used in the WTC towers were applied to steel components with primer paint.

The average thickness of the original thermal insulation on the floor trusses was estimated to be 0.75 in. with a standard deviation of 0.3 in. The average thickness of the upgraded thermal insulation was estimated to be 2.5 in. with a standard deviation of 0.6 in. Based on finite-element simulations, the thermal analyses for determining temperature histories of structural components used a thermally equivalent thickness of 0.6 in. and 2.2 in. for the original and upgraded insulation, respectively. For thermal analyses of the perimeter columns, spandrel beams, core beams, and core columns, the insulation on these elements was set to the specified thickness, due to a lack of field measurements.

Based on four Standard Fire Tests conducted for various length scales, insulation thickness, and end restraints, the floor assemblies were shown to be capable of sagging without collapsing and supported their full design load under standard fire conditions for 2 hours or more without failure.

For assemblies with a ¾ in. SFRM thickness, the 17 ft assembly’s fire rating was 2 hours; the 35 ft assembly’s rating was 1½ hours. This result raised the question of whether or not a fire rating of a 17 ft floor assembly is scalable to the longer spans in the WTC towers.

The specimen with ½ in. SFRM thickness and a 17 ft span would not have met the 2 hour requirement of the NYC Building Code.

There is far greater knowledge of how fires influence structures in 2005 than there was in the 1960s. The analysis tools available to calculate the response of structures to fires are also far better now than they were when the WTC towers were designed and built.