Reopen911 : Et cinq mois après les attentats, la perplexité était toujours de mise, comme le rapportait alors le New York Times : « les ingénieurs et autres experts peinent à expliquer pourquoi et comment les Tours se sont complètement effondrées, alors qu’elles étaient conçues pour résister à l’impact d’un avion » (New York Times, 2/02/02).
En effet, les Tours Jumelles avaient été conçues pour résister à l'impact d’un Boeing 707, le plus gros porteur de l’époque, lancé à 965km/h [4]. Et elles auraient dû également résister aux impacts des Boeing 767 du 11-Septembre, dont l’énergie cinétique était équivalente – voire inférieure – à celle du 707 [5]. Franck de Martini, directeur des travaux du WTC, avait même déclaré, 9 mois avant le 11-Septembre, que n’importe laquelle des Tours « pourrait probablement résister à de multiples impacts d’avions de ligne » (« 9/11 Mysteries », min 07:06 ; J. Dwyer et K. Flynn, 2005, p. 149).
Toute cette partie est soumise à controverse. En cherchant un peu, deux choses différentes apparaissent :
Rapport du NIST p. 55 5.3.2
A Port Authority document indicated that the impact of a Boeing 707 aircraft flying at 600 mph was analyzed during the design stage of the WTC towers. However, the investigators were unable to locate any documentation of the criteria and method used in the impact analysis and were thus unable to verify the assertion that “...such collision would result in only local damage which could not cause collapse or substantial damage to the building and would not endanger the lives and safety of occupants not in the immediate area of impact.”8 Since the ability for rigorous simulation of the aircraft impact and of the ensuing fires are recent developments and since the approach to structural modeling was developed for this Investigation, the technical capability available to The Port Authority and its consultants and contractors to perform such an analysis in the 1960s would have been quite limited.
On peut trouver une copie du rapport dans les annexes du NIST :
http://wtc.nist.gov/NCSTAR1/PDF/NCSTAR%201-2.pdf p. 306
Robertson, un des chief structural engineers (
http://www.nae.edu/File.aspx?id=7345)
The two towers were the first structures outside of the military and nuclear industries designed to resist the impact of a jet airliner, the Boeing 707. It was assumed that the jetliner would be lost in the fog, seeking to land at JFK or at Newark. To the best of our knowledge, little was known about the effects of a fire from such an air-craft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires.
Selon Robertson lors de la conception (et il devrait normalement bien placé pour le savoir), ils avaient pris en compte l'hypothèse d'un avion volant à 290 km/h. L'énergie cinétique entre un 707 à 290 km/h et un 767 à 950 km/h n'a rien à voir (figure 3 p. 8).
(1) La conception : qui a raison ?
Sources :
http://www.nytimes.com/2002/09/08/magaz ... =20&src=pm et
http://www.nytimes.com/2002/09/08/magaz ... =21&src=pm
he recalls [Robertson] that he addressed the question of an airplane collision, if only to satisfy his engineer's curiosity [c'était alors même pas prévu dans la conception]. For whatever reason, Robertson took the time to calculate how well his towers would handle the impact from a Boeing 707, the largest jetliner in service at the time. He says that his calculations assumed a plane lost in a fog while searching for an airport at relatively low speed, like the B-25 bomber. He concluded that the towers would remain standing despite the force of the impact and the hole it would punch out. The new technologies he had installed after the motion experiments and wind-tunnel work had created a structure more than strong enough to withstand such a blow.
Exactly how Robertson performed these calculations is apparently lost -- he says he cannot find a copy of the report. Several engineers who worked with him at the time, including the director of his computer department, say they have no recollection of ever seeing the study. But the Port Authority, eager to mount a counterattack against Wien, seized on the results -- and may in fact have exaggerated them. One architect working for the Port Authority issued a statement to the press, covered in a prominent article in The Times, explaining that Robertson's study proved that the towers could withstand the impact of a jetliner moving at 600 miles an hour. That was perhaps three times the speed that Robertson had considered. If Robertson saw the article in the paper, he never spoke up about the discrepancy. No one else issued a correction, and the question was answered in many people's minds: the towers were as safe as could be expected, even in the most cataclysmic of circumstances.
no study of the impact of a 600-mile-an-hour plane ever existed. ''That's got nothing to do with the reality of what we did,'' Robertson snapped when shown the Port Authority architect's statement more than three decades later.
En gros on a aucun rapport des deux versions. D'un côté on a la Port Authorithy qui nous dit que des études ont été faites pour une vitesse de 600 mph, mais personne pour confirmer ça. Et en plus c'était dans un contexte où y intérêt à exagérer pour rassurer tout le monde (voir p. 20 NYT) :
Lawrence Wien, who was continuing his fight against the towers, had begun to remind New Yorkers publicly of a Saturday morning in July 1945, when a B-25 bomber, lost in the fog, barreled into the 79th floor of the Empire State Building. Most of the 14 people who died were incinerated by a fireball created when the plane's fuel ignited, even though the fire was quickly contained. The following year, another plane crashed into the 72-story skyscraper at 40 Wall Street, and yet another one narrowly missed the Empire State Building, terrifying sightseers on the observation deck.
Wien and his committee charged that the twin towers, with their broader and higher tops, would represent an even greater risk of midair collision. They ran a nearly full-page ad in The Times with an artist's rendition of a commercial airliner about to ram one of the towers. ''Unfortunately, we rarely recognize how serious these problems are until it's too late to do anything,'' the caption said.
De l'autre côté il faut croire Robertson sur parole. Bref, pas grand chose à se mettre sous la dent.
(2) En fait peu importe...
Les tours ont effectivement résisté aux impacts selon la VO (de ce que j'en ai lu pour l'instant, je suis toujours pas arrivé au bout de ton article). Ce sont les incendies qui ont causé l'effondrement des tours endommagées. Or il semble que personne ne nie qu'au moment de la conception des tours, personne n'était capable de calculer et prendre en compte les dégâts et conséquences que pourraient avoir des incendies de ce type.
To the best of our knowledge, little was known about the effects of a fire from such an aircraft, and no designs were prepared for that circumstance. Indeed, at that time, no fireproofing systems were available to control the effects of such fires. (Robertson p. 7)
Potentially challenging other statements by Port Authority engineers, Dr. Sunder [du NIST] said it was now uncertain whether the authority fully considered the fuel and its effects when it studied the towers' safety during the design phase. 'Whether the fuel was taken into account or not is an open question,'' Dr. Sunder said. (
http://www.nytimes.com/2003/12/03/nyreg ... d=2&src=pm)
(3) Franck de Martini
Où est-ce qu'il est allé chercher ses "multiples impacts" ? Aucun document ne parle de ça.
Est-ce qu'il aurait une expertise spéciale par rapport à la question ? Non, il n'était pas en charge de la conception ni de la construction. Il n'est arrivé qu'en 1993.
Est-ce qu'il a parlé des conséquences d'un incendie après les impacts ? Non.
Ça ne me parait pas être une bonne idée de faire référence à lui sur ces questions précises. Tu le cites aussi à un autre moment, où on peut douter pour les mêmes raisons (peut-être qu'il a raison, mais on peut raisonnablement demander une autre source plus fiable que lui qui corrobore ça):
La structure des Tours « est comparable à [une] moustiquaire, et l’avion est juste un crayon perforant la moustiquaire » nous explique le directeur des travaux du WTC Franck de Martini. « Il ne lui fait absolument rien » ajoute-t-il (« 9/11 Mysteries », min 07:14 ; J. Dwyer et K. Flynn, 2005, p.149).
Voilà donc c'est assez controversé, ce qui est étonnant c'est qu'il y a au final aucun documents des deux côtés. Sachant qu'on peut pas vérifier les infos, il vaut mieux supprimer ces parties de l'article.
A