707 vs 767

Im Stadtgebiet von New York City liegen zwei der weltgrößten Flughäfen - John F. Kennedy International und LaGuardia. Im benachbarten New Jersey befindet sich zudem der Newark Liberty International Airport.
 

Quelle; 1=JFK, 2=LaGuardia; 3=Newark

Am 28. Juli 1945 irrte ein B-25 Bomber der US Army auf der Suche nach dem Flughafen Newark im dichten Nebel über Manhattan umher und stürzte ins Empire State Building - seinerzeit das höchste Gebäude New Yorks. (Quelle)

Vor diesem Hintergrund fanden Mitte der 60er-Jahre die Planungen für das World Trade Center statt. Verantwortlich für die Statik-Berechnungen war u.a. der Bau-Ingenieur Leslie Robertson, Er beschreibt das Szenario, das bei der Planung in Betracht gezogen wurde:
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 aircraft, and no designs were prepared for that circumstance. [...] The buildings survived the impact of the Boeing 767 aircraft, an impact very much greater than had been contemplated in our design (a slow- flying Boeing 707 lost in the fog and seeking a landing field). Therefore, the robustness of the towers was exemplary. At the same time, the fires raging in the inner reaches of the buildings undermined their strength. In time, the unimaginable happened .... wounded by the impact of the aircraft and bleeding from the fires, both of the towers of the World Trade Center collapsed. (Quelle, Videoclip)

Eine entsprechende Schilderung findet sich daher auch im Untersuchungsbericht der FEMA:
It was assumed in the 1960s design analysis for the WTC towers that an aircraft, lost in fog and seeking to land at a nearby airport, like the B-25 Mitchell bomber that struck the Empire State Building on July 28, 1945, might strike a WTC tower while low on fuel and at landing speeds. However, in the September 11 events, the Boeing 767-200ER aircraft that hit both towers were considerably larger with significant higher weight, or mass, and traveling at substantially higher speeds. The Boeing 707 that was considered in the design of the towers was estimated to have a gross weight of 263,000 pounds and a flight speed of 180 mph as it approached an airport; the Boeing 767- 200ER aircraft that were used to attack the towers had an estimated gross weight of 274,000 pounds and flight speeds of 470 to 590 mph upon impact. (FEMA WTC BP Study, Chapter 1, 1-17)

Diesen Angaben folgend wurden in den damaligen Berechnungen zwei Parameter nicht berücksichtigt, die im Jahr 2001 von großer Bedeutung waren:

1. Die hohe Geschwindigkeit der Flugzeuge.

2. Die Brandenergie, die sich aus dem Flugzeug-Treibstoff ergibt.
 

Die Kontroverse um die Geschwindigkeit

Nachfolgend veranschaulicht eine vergleichende Darstellung die enormen Unterschiede der kinetischen Energie einer relativ langsamen Boeing 707 und einer deutlich schnelleren Boeing 767.
 

Quelle

Welche Einschlags-Geschwindigkeit tatsächlich bei den damaligen Berechnungen berücksichtigt wurde, ist Gegenstand bemerkenswerter Kontroversen:
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.
There were only two problems. The first, of course, was that 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. The second problem was that no one thought to take into account the fires that would inevitably break out when the jetliner's fuel exploded, exactly as the B-25's had. And if Wien was the trade center's Cassandra, fire protection would become its Achilles' heel. (New York Times, 08.09.2002)

Zusammengefasst:
Schriftliche Unterlagen zu den Berechnungen existieren nicht mehr.
1964 brachte ein Architekt der PANY die Behauptung in Umlauf, das Gebäude wäre auf einen hypothetischen Einschlag von 600 mph ausgelegt. Robertson verhielt sich vor 40 Jahren loyal gegenüber seinem Auftraggeber und schwieg - heute bestreitet er die Richtigkeit dieser Angabe.

Dieser Widerspruch wird schliesslich auch zum Thema im NIST-Report:
NIST continues to be interested in documents related to the ability of the WTC towers to withstand the abnormal load condition of a Boeing 707 aircraft impact that was considered in the original design (this assessment is part of Project #2). A property risk assessment report prepared for Silverstein Properties prior to leasing the WTC towers in 2001 identifies the scenario of an aircraft striking a tower as one of the “maximum foreseeable losses.” The assessment states: “This scenario is within the realm of the possible, but highly unlikely. … In the event [of] such an unlikely occurrence, what might result? The structural designers of the towers have publicly stated that in their opinion that either of the Towers could withstand such an impact from a large modern passenger aircraft. The ensuing fire would damage the ‘skin’ in this scenario, as the spilled fuel would fall to the Plaza level where it would have to be extinguished by the NYC Fire Department.” A three-page document from March 1964 contains calculations to estimate the “period of vibration due to plane crash at 80th floor,” but it provides no details on the ability of the WTC towers to withstand such an impact on the towers. NIST will continue to work with the Port Authority’s structural contractor to locate and obtain documents related to the WTC towers ability to withstand aircraft impact. The Port Authority and Silverstein Properties have previously informed NIST that many of the documents cited above were destroyed in the collapse of WTC 1, which housed documents for the Port Authority, and in the collapse of WTC 7 and WTC 1, which housed documents for Silverstein Properties.  (NIST Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center Disaster, May 2003,  S. 16, Archiv)

Im Abschlußbericht des NIST vom September 2005 findet sich schließlich folgende Darstellung: 
Finding 11: Documents from The Port Authority of New York and New Jersey indicated that the safety of the WTC towers and their occupants in an aircraft collision was a consideration in the original design. The documents indicate that a Boeing 707, the largest commercial aircraft at the time, flying at 600 mph was considered, and the analysis indicated 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. No documentary evidence of the aircraft impact analysis was available to review the criteria and methods used in the analysis of the aircraft impact into the WTC towers, or to provide details on the ability of the WTC towers to withstand such impacts.
A three-page white paper “Salient points with regard to the structural design of the World Trade Center towers”, February 1964, from the PANYNJ (see Appendix A) indicated that the impact of a Boeing 707 or DC 8 aircraft flying at a speed of 600 mph was analyzed during the design stage of the WTC towers. The paper also addressed the life safety considerations following such impact. The paper stated that “…The Buildings have been investigated and found to be safe in an assumed collision with a large jet airliner (Boeing 707 - DC 8) traveling at 600 miles per hour. Analysis indicates 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.”
The Federal Emergency Management Agency (FEMA) 403 (2002) report indicated that it was assumed in the 1960s design of the WTC towers that a Boeing 707 aircraft, lost in fog and seeking to land at a nearby airport, might strike the towers while low on fuel and at a landing speed of 180 mph.
A property risk assessment report, prepared for Silverstein Properties prior to leasing the WTC towers in 2001, identified the scenario of an aircraft striking a tower as one of the maximum foreseeable losses. The assessment states “This scenario is within the realm of the possible, but highly unlikely. In the event [of] such an unlikely occurrence, what might result? The structural designers of the towers have publicly stated that in their opinion that either of the Towers could withstand such an impact from a large modern passenger aircraft. (Quelle, , NIST NCSTAR1-2, S. 298)

Der NIST-Report widerspricht demnach dem FEMA-Report. Letzterer bezieht sich wiederum auf den Bauingenieur Leslie Robertson, der  in den 60er Jahren für die Statikberechnungen verantwortlich war.

Der Hintergrund dieser Diskrepanz lässt sich verstehen, wenn man den historische Kontext der ‘600 mph’-Behauptung näher betrachtet:
 

Die Rache der frühen Desinformation

Das Bauprojekt ‘World Trade Center’ war in den frühen 1960er Jahren ein revolutionäres Vorhaben -  Konstruktionsprinzip und Größe der Gebäude waren heftig umstritten.
Zu den Wortführern der Bedenkenträger gehörte Lawrence Wien, der seinerzeit in andere Bürogebäude investiert hatte. Insbesondere die Millioninvestition in das Prestigeobjekt und damals höchste Gebäude New Yorks, das Empire State Building, spielte eine nicht unwesentliche Rolle in diesem Konflikt:  
His groups held ownership or long-term leases on the Empire State Building, the Equitable Building, the Graybar Building, the Fisk Bulding, the Garment Capitol Building, the Fifth Avenue Building and the Lincoln Building, where he had his offices. (Quelle)
That tale began in 1961, when Malkin, father-in-law Lawrence Wien and Harry Helmsley devised what became a prototype for real estate syndication.
In a complex financing setup, the trio bought control of the skyscraper from Henry Crown. Then Malkin, Lawrence Wien and Helmsley leased the facility to Empire State Building Associates. (Quelle)

Die Auseinandersetzung aus der Sicht der Kontrahenten rekapitulierte im November 2002 der verantwortliche PANY-Bauleiter des kompletten Projektes, Guy F. Tozzoli in einem Interview:  
Interviewer: Die New York Times veröffentlichte 1968 fast hellseherisch ein Bild, auf dem zu sehen war, wie ein Flugzeug ins World Trade Center fliegt.
Tozzoli: Ich sag Ihnen jtzt mal, wie das wirklich war. Dieses Bild war eine Antiwerbung einer Gruppe, die sich "Komitee für ein angemessenes World Trade Center" nannte. Dieses Komitee war von Lawrence Wien ins Leben gerufen worden, dem Besitzer des Empire State Buildings. Er hat 500 000 Dollar investiert, nur um mich aufzuhalten. Ich hab mich mit ihm getroffen und gefragt: Larry, was ist ein angemessenes World Trade Center? Er sagte: Ein Haus mit 100 Etagen. Seins hatte 102 Etagen. Ist das nicht ein Witz?
Interviewer: Es gab einen Wettbewerb um das höchste Haus der Stadt?
Tozzoli: Natürlich, Wien hatte Angst, das Prestige zu verlieren, das höchste Haus in der Welt zu besitzen. Und außerdem hatte er Angst um seine Aussichtsplattform. Ich sagte, hör zu, sorge dich nicht. Du bist in Midtown, das ist wie eine eigene Stadt. Ich bin in Downtown, das ist eine andere Stadt. Es zeigte sich, dass es genauso werden sollte. Jeder von uns hatte etwa zwei Millionen Besucher im Jahr. (Quelle)

Die Journalisten James Glanz und Eric Lipton publizierten zahlreiche Artikel in der NEW YORK TIMES und veröffentlichten 2003 das Buch City in the Sky. Hier wird auch diese Konfrontation ausführlich beschrieben und deckt sich mit der Tozzoli-Version:
These barons of real estate had decided, first of all, that they did not like the idea of 10 million square feet of tax-free, government-sponsored real estate competing with their holdings. [...] In the weeks since Yamasaki had unveiled the twin towers design, Wien formed them into a group he called the Committee for a Reasonable World Trade Center and began attacking the project. (City in the Sky, S. 134)

Lawrence Wien zog aus dem Vergleich mit dem Empire State Building-Impact die Argumente gegen das WTC-Projekt:
The twin towers, Wien understood, would not have that kind of protection. Their facades would be covered with aluminium cladding, not sixteen inches of brick and limestone. Behind the aluminium, wind and gravity loads would be shared by sixty-one of Yamasaki’s delicate pinstripe columns, rather than the smaller number of massive columns in the Empire State Building. The plane could punch straight through, and once it was inside, with the perimeter columns sliced to pieces, there were no more columns to take up the extra loads that would have to be supported. At least that was how Wien saw it. And that was why Wien, Helmsley, and their real estate allies made the allegation that the twin towers were “unsafe in an explosion or if hit by an airplane”. (City in the Sky, S. 134f)

Die PANY reagierte umgehend mit einer Pressekonferenz, auf der u.a. eine Stellungnahme von Richard Roth publiziert wurde, der das renommierte Architektenbüro Emery Roth &Sons vertrat. Das Architektenbüro war gemeinsam mit Yamasaki für das Design des World Trade Center verantwortlich. In dieser Stellungnahmen heisst es u.a.:
Mr. Roth said that a structural analysis by the firm of Worthington, Skilling, Helle & Jackson had found that if a tower were hit by an airliner at 600 miles an hour, the damage to the tower would be only local and its occupants outside the immediate area of impact would not be endangered. (New York Times, 15. Februar 1964, Artikelkopie)

Das dreiseitige sogenannte “Roth-Telegramm” sollte die Kritiker widerlegen und den Bauträger des Projektes unterstützen - die Port Authority von New York.
Tatsächlich handelte es sich bei dieser Stellungnahme jedoch lediglich um die Reproduktion des dreiseitigen “White Paper” vom 03. Februar 1964 - vom PANY-Projektleiter Malcolm P. Levy höchstselbst diktiert:
What the real estate people did not know was that Levy already had his script prepared. Nor could they have known how far the Port Authority was wiling to go to protect its project. [...] To anyone who had not seen Mal levy’s white paper and though through its implications, such an endorsement from Roth must have closed the case: the twin towers would be safe in the most extreme circumstances conceivable. But to anyone who had read the white paper, something else was clear: Roth had been given Myl Levy’s script to transmit as an urgent and authoritative telegram from an expert outside the Port Authority. Point by point, it was the same, almost word for world, as the white paper. Roth was simply parroting Levy, not offering an external judgement. (City in the Sky, S. 135f, Hervorhebung von mir)

Damals wie heute zeigte die Doppel-Präsentation ihre Wirkung. Die Truthseeker-Site 911research.wtc7.net belegt es deutlich - das ‘PANY-White Paper’ und das Roth Telegramm werden als getrennte Quellen für die 600 mph Angabe angeführt.
Eine typische ‘Truthseeker’-Methode treibt die Präsentation auf die Spitze: Als Quelle wird das Buch City in the Sky angegeben, aber die Hälfte der Story wird verschwiegen - ebenso die PANY-Verbindung zum ‘White Paper’.

Das PANY-Dokument aus dem Jahr 1964 wurde innerhalb des NIST-Untersuchungsberichtes publiziert (zum Vergrößern bitte anklicken):
 

Aber auch der PANY-Bauleiter Guy F. Tozzoli widerspricht dieser Version:
Wir haben damals eine Studie gemacht und ausgerechnet, was passieren würde, wenn das größte Flugzeug, eine 707, einen Turm trifft. Wir sind davon ausgegangen, dass Flugzeuge über New York ihre Geschwindigkeit auf 200 Meilen pro Stunde drosseln müssen. Und dass sie auch nicht voll getankt sein würden, sondern sich im Landeanflug befänden. Das Ergebnis der Studie war: Ein Flugzeug würde sieben Etagen auf einer Seite raus hauen, aber der Turm würde stehen. (Quelle)

Dieses Szenario macht Sinn - damals wie heute.
In den 60er Jahren war hingegen ein Flugzeug mit Höchstgeschwindigkeit in einer Höhe von 300 Metern überhaupt keine denkbare Bedrohung und damit eine völlig überzogende Sicherheitsreserve bei den Planungen. Es ging 1964 primär um überzeugendes Marketing, wie das Schluss-Statement des Roth-Telegramms - alias PANY-Pressemeldung - zeigt:
The Towers may be said to be the first buildings of the 21st century and the design concepts which they embody will be incorporated in some measure in evrey future high rise building ever built.
 

Zwischenfazit:

Es gibt nur eine Quelle für die ‘600 mph’ Behauptung - PANY-Projektleiter Malcolm  Levy und dessen ‘White Paper’.
Sowohl Tozzoli als auch Robertson behaupten hingegen, die berücksichtigte Impact-Geschwindigkeit lag bei ca. 180 mph.
Es gibt keine einzige schriftliche Quelle für die Berechnungen, wie zahlreiche Beteiligte bereits zugestehen mussten.
Realistisch erscheint der niedrigere Wert, gleichzeitig lässt sich die Levy-Angabe erklären.
Es bleibt letztlich eine Glaubensfrage, welche Version man bevorzugt.

Abseits dieser Glaubensfrage bleibt jedoch der zweite, wesentlich bedeutendere Impact-Faktor.
 

Die Kontroverse ums Feuer

The second problem was that no one thought to take into account the fires that would inevitably break out when the jetliner's fuel exploded, [...] (New York Times, 08.09.2002)

In ihrem Buch formulieren die Journalisten den Sachverhalt etwas härter:
In fact, the second flaw was that the Port Authority and its engineers did not think very hard about how their structure would behave in a fire, especially an intense, violent fire. (City in the Sky, S. 138)

Man mag seinerzeit vielleicht daran gedacht haben, aber das Problem war ein ganz profanes:
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. (Quelle)

Dieses Statement stammt wieder vom involvierten Leslie Robertson, daher sei auf einen weiteren Experten hingewiesen:
Das World Trade Center, errichtet 1970-76, ist zudem in einer Zeit entstanden, in der hochrangige Experten wie der Braunschweiger Professor für Stahlbeton und Massivbau, Karl Kordina, freimütig einräumten: "Bei großen ausgedehnten Bauwerken ... [kann] das Brandverhalten des Gesamtbauwerks nicht hinreichend genau vorhergesagt werden” (Kordina im Oktober 1975 auf der Deutschen Konferenz Hochhäuser in Mainz). (Quelle)

Auch für die Auswirkungen eines Kerosin-Feuers scheinen keine Unterlagen vorzuliegen, daher verweist selbst der abschliessende NIST-Report nur auf widersprüchliche Interviews-Statements der Gebäudeplaner: 
While the documents from the PANYNJ indicated that aircraft impact was considered in the design, there were two views expressed by the building designers during media interviews on whether the effects of the subsequent fires and the implications on life safety were a consideration in the original design. One view suggested that an analysis was done indicating that the biggest problem would be the fact that all the fuel would dump into the building and there would be horrendous fire. For implications on life safety, this view suggested that a lot of people would be killed, but the building structure would still be there. The other view suggested that the fuel load and the subsequent fire damage may not have been considered in the design stage. (NIST NCSTAR1-2, S.4)

Selbst in einem Bericht zur Risikoabschätzung für den neuen Eigentümer ‘Silverstein Properties’ wurde nur auf öffentliche Statements verwiesen:
A property risk assessment report, prepared for Silverstein Properties prior to leasing the WTC towers in 2001, identified the scenario of an aircraft striking a tower as one of the maximum foreseeable losses. The assessment states “This scenario is within the realm of the possible, but highly unlikely. In the event [of] such an unlikely occurrence, what might result? The structural designers of the towers have publicly stated that in their opinion that either of the Towers could withstand such an impact from a large modern passenger aircraft. The ensuing fire would damage the skin in this scenario, as the spilled fuel would fall to the Plaza level where it would have to be extinguished by the NYC Fire Department.” (NIST NCSTAR1-2, S. 298, bzw. Original-Dokument)

Es ist daher nicht erstaunlich, daß auch der Leiter der NIST-Untersuchungen die Berücksichtigung dieses Faktors in Frage stellt:
Potentially challenging other statements by Port Authority engineers, Dr. Sunder 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. [...] (Quelle)

Einen negierenden Hinweis findet man in einem Artikel im Journal of Engineering Mechanics:
What was not considered in design was the temperature that can develop in the ensuing fire.

Der Vorstand des Instituts für Baustatik an der TU Wien, Prof. Helmut Rubin:
Sei ein Hochhaus statisch starr oder flexibel gebaut - es gebe für Hochhäuser keine Modellrechnungen, in denen (wie etwa bei AKW) die Auswirkungen eines Flugzeugabsturzes berechnet werden. (Quelle)

Auch Prof. Kausel vom MIT bezweifelt die Berücksichtigung des Faktors Kerosin:
“[...] but the designers never considered the fuel load and inferno that would surely ensue.” (PDF-Quelle)

Kausel fügt etwas interessantes hinzu:
Thus, the suggestion that the buildings were designed for the crash of an aircraft is ultimately self-delusion and perhaps also public relations on the part of the design team, because not all aspects of a crash, i.e. the explosion and fire, were taken into account, perhaps because the probability of such an occurrence was deemed insignificant. [Hervorhebung von mir]

“Public relations” - ein Begriff, der sich öfter aufdrängt, wenn man die Historie des WTC betrachtet. Oben ist in diesem Zusammenhang Malcolm P. Levy erwähnt, aber auch der TV-Auftritt eines späteren WTC-Architekten drängt sich in diesem Zusammenhang auf.

Man denke an die ‘unsinkbaren’ Titanic und Pamir oder  das ‘sicherste Flugzeug der Welt’, die Concorde.

“Public relations” könnte auch für ein Statement gelten, das einer der beiden Chefkonstrukteure des WTC, John Skilling, unmittelbar nach dem Bombenanschlag von 1993 äußerte:
Our analysis indicated the biggest problem would be the fact that all the fuel (from the airplane) would dump into the building. There would be a horrendous fire. A lot of people would be killed," he said. "The building structure would still be there. (Quelle, 27.02.1993)

“Public relations” könnte eine naheliegende Erklärung für die Statements sein, die vor dem Eintritt des ‘Ernstfalles’ abgegeben wurden und die Berücksichtigung sämtlicher Faktoren suggerieren, obwohl dafür keine Unterlagen und Belege existieren und obwohl unabhängige Fachleute die Kalkulation solcher Faktoren für die 60er Jahre als unwahrscheinlich ansehen.

Die potentielle Brandenergie aus dem Flugzeugtreibstoff wird im Vergleich folgendermaßen dargestellt
 

Quelle

Die Größenordnung dieses Faktors:
[...] it is estimated that, at the time of impact, each aircraft had approximately 10.000 gallons of unused fuel on board (FEMA WTC Study, S. 2-15 bzw NIST NCSTAR1-2, S.138)

10.000 Gallonen sind etwa 38.000 Liter. Nach den Berechnungen von Prof. Jean-Baptiste Schleich wurde schlagartig eine Energiefreisetzungsrate von über 4.000 MW erreicht:
Ganz anders als bei einem herkömmlichen Bürobrand von 100 bis 200 MW und Gastemperaturen von 500 bis 800 Grad Celsius entsteht jetzt hier ein 4000 MW Brand. (Quelle)
 

Quelle

Der Brandexperte Professsor Forman Williams dazu:
He says that while the jet fuel was the catalyst for the WTC fires, the resulting inferno was intensified by the combustible material inside the buildings, including rugs, curtains, furniture and paper.
"The jet fuel was the ignition source," Williams tells PM. "It burned for maybe 10 minutes, and [the towers] were still standing in 10 minutes. It was the rest of the stuff burning afterward that was responsible for the heat transfer that eventually brought them down." (Quelle) 

Die Rolle des Flugzeugbenzins für das Brandgeschehen wird demnach oftmals falsch interpretiert. Tatsächlich trägt das Kerosin lediglich ca. 20% zur Brandenergie bei, dient gleichzeitig jedoch über mehrere Stockwerke hinweg als Brandbeschleuniger für die Bürobrandlast - welche ihrerseits eindeutig den Hauptlieferanten für die Brandenergie bildet. 
 

Fazit:

Die Auswirkungen des Feuers sind in den Statik-Berechnungen  der WTC-Architekten wahrscheinlich  nicht berücksichtigt worden, weil es seinerzeit noch keine validierten Modellrechnungen für solche Situationen gab.
Es gibt weitgehende Übereinstimmung in der Fachwelt darüber, daß insbesondere die nachfolgenden Brände für den Einsturz der vorgeschwächten Gebäude verantwortlich waren.

Jon Magnusson, Chef des Bau-Ingenieurbüros Magnusson Klemencic Associates beschreibt die Kombination der Faktoren so:
If the buildings had not been damaged by airplanes, they could have lost strength and still stood,” says structural engineer Magnusson. “But the airplanes did two things in terms of the buildings survivability:
First, they damaged the structure, so they took out the towers redundancy, their ability to balance overload. The structure load went way up. Then, the impact struck out sprinklers and fireproofing, and the fire elevated the temperature of steel. Then you start to weaken the steel by heating it up. And it was only those two things in combination that were enough to bring the buildings down. (Debunking 9/11 Myths, S. 40)

Angesichts der Kombination verheerender Faktoren und der temporären Widerstandskraft der Gebäude dreht der Chef des Ingenieurbüros, dessen Vorgänger für die WTC-Konstruktion verantwortlich war, die Fragestellung einfach um:
On Sept. 11, 2001, national news anchor Peter Jennings asked Magnusson, on live television, why the 110-story twin towers fell. Magnusson replied: "You're asking the wrong question. You should be asking: 'How were the buildings able to stand up?' (Quelle).

Das Argument des ‘einkalkulierten 707-Impactes’ ist demnach nur die halbe Wahrheit und taugt nicht dazu, die Einsturz-Ursache in Frage zu stellen.

Abschliessend sei eine unvollständige Auflistung diverser Publikationen von Experten genannt, die sich mit dem Thema weltweit auseinandergesetzt haben  - Publikationen von Theologen, Politiker und Physiker finden in dieser Zusammenstellung bewusst keine Berücksichtigung:

Eager & Musso: Why Did the World Trade Center Collapse? Science, Engineering, and Speculation

Bazant & Zhou: Why Did the World Trade Center Collapse?—Simple Analysis

Clifton: Collapse of the World Trade Centre Towers

Clifton: Elaboration on Aspects of The Postulated Collapse Of the World Trade Centre Twin Towers

Wilkinson: Why Did It Collapse?

Wilkinson: The World Trade Center and 9/11: A Discussion on Some Engineering Design Issues

Ghoniem: The Towers Lost and Beyond - The Fire

Buyukozturk & Ulm: The Towers Lost and Beyond - Materials and Structures

Graubner: Zum Einsturz der Türme des World-Trade-Center

Hauser: Bauen mit Stahl

Shwartz: Structural engineer describes collapse of the World Trade Center towers

Corley: Seeking Structural Lessons

Abolhassan Astaneh-Asl: World Trade Center Collapse, Field Investigations and Analyses

Scheuerman: It was the Fire, caused the Twin Tower Collapse

Dunn: Why the World Trade Center Buildings Collapsed - A Fire Chief ’s Assessment

Civil Engineering Magazine: Dissecting The Collapses

Kodur: Role of fire resistance issues in the collapse of the Twin Towers

Jeremy Kirk: The World Trade Center Disaster: Analysis and Recommendations

Mehdi Zarghamee: Computer Modeling of the Collapse of the World Trade Center Towers

A High-Quality Physically-Accurate Visualization of the September 11 Attack on the World Trade Center

Bazant, Greening, Benson: Collapse of the World Trade Center Towers: What caused it and what didn't

Bazant: Mechanics of Progressive Collapse: Learning from World Trade Center and Building Demolitions

Karim: Impact of the Boeing 767 Aircraft into the World Trade Center

Baum: A simple model of the World Trade Center fireball dynamics.

Petroski - The Fall of Skyscrapers

Choi et al.: The behaviour of lightweight composite floor trusses in fire

Sakumoto et al.: Fire resistance of steel frames

Weidlinger: World Trade Center - Structural Engineering Investigation

Conference Report: Fire and Structures: The Implications of the WTC Disaster
 

Einige Publikationen aus der realen Offline-Welt:

A suggested cause of the fire-induced collapse of the World Trade Towers. By: Quintiere, J.G.; di Marzo, M.; Becker, R.. Fire Safety Journal, Oct2002, Vol. 37 Issue 7, p707, 10p.

Could the world trade center have been modified to prevent its collapse?; Newland, D. E.; Cebon, D. Journal of Engineering Mechanics; 2002 Vol. 128 Issue 7, p795-800, 6p.

How did the WTC towers collapse? A new theory; Usmani, A. S.; Chung, Y. C.; Torero, J. L. Fire Safety Journal; 2003 Vol. 38, p501-533, 33p.

How the airplane wing cut through the exterior columns of the World Trade Center; Wierzbicki, T.; Teng, X. International Journal of Impact Engineering; 2003 Vol. 28, p601-625, 25p

Stability of the World Trade Center Twin Towers Structural Frame in Multiple Floor Fires. By: Usmani, A. S.. Journal of Engineering Mechanics, Jun2005, Vol. 131 Issue 6, p654-657.

Effect of insulation on the fire behaviour of steel floor trusses. Fire and Materials, 29:4, July/August 2005. pp. 181 - 194. Chang, Jeremy; Buchanan, Andrew H.; Moss, Peter J.

Reconnaissance and preliminary assessment of a damaged high-rise building near Ground Zero. The Structural Design of Tall and Special Buildings. 12 :5, 15 December 2003. pp. 371 - 391. Warn, Gordon; Berman, Jeffrey; Whittaker, Andrew; Bruneau, Michel

Structural Responses of World Trade Center under Aircraft Attacks. Omika, Yukihiro.; Fukuzawa, Eiji.; Koshika, Norihide. Journal of Structural Engineering v. 131 no1 (January 2005) p. 6-15

The Structural Steel of the World Trade Center Towers. Gayle, Frank W.; Banovic, Stephen W.; Foecke, Tim. Advanced Materials & Processes v. 162 no10 (October 2004) p. 37-9

WTC Findings Uphold Structural Design. Post, Nadine M. ENR v. 253 no17 (November 1 2004) p. 10-11

Siehe auch unter Bibliography Of Scientific Literature On The World Trade Center Collapse .

Dr. Seffen, Dozent am Department of Engineering der Universität Cambridge stellte Berechnungen zum Einsturzverlauf an, die im Februar 2008 in einem Artikel im Journal of Engineering Mechanics veröffentlicht wurden (Artikel-Draft):
Dr Seffen was able to calculate the "residual capacity" of the undamaged building: that is, simply speaking, the ability of the undamaged structure to resist or comply with collapse.
His calculations suggest the residual capacity of the north and south towers was limited, and that once the collapse was set in motion, it would take only nine seconds for the building to go down.
This is just a little longer than a free-falling coin, dropped from the top of either tower, would take to reach the ground.
The University of Cambridge engineer said his results therefore suggested progressive collapse was "a fair assumption in terms of how the building fell".
"One thing that confounded engineers was how falling parts of the structure ploughed through undamaged building beneath and brought the towers down so quickly," said Dr Seffen.
He added that his calculations showed this was a "very ordinary thing to happen" and that no other intervention, such as explosive charges laid inside the building, was needed to explain the behaviour of the buildings. (Quelle)

Zuletzt sei auf die FAQ des National Institute of Standards and Technology (NIST) hingewiesen, die Ergebnisse des meterdicken Untersuchungsberichtes extrahiert:
Question: If the World Trade Center (WTC) towers were designed to withstand multiple impacts by Boeing 707 aircraft, why did the impact of individual 767s cause so much damage?
Answer: As stated in Section 5.3.2 of NIST NCSTAR 1, a document from the Port Authority of New York and New Jersey (PANYNJ) indicated that the impact of a [single, not multiple] Boeing 707 aircraft was analyzed during the design stage of the WTC towers. However, NIST investigators were unable to locate any documentation of the criteria and method used in the impact analysis and, therefore, were 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.…”
The capability to conduct rigorous simulations of the aircraft impact, the growth and spread of the ensuing fires, and the effects of fires on the structure is a recent development. Since the approach to structural modeling was developed for the NIST WTC investigation, the technical capability available to the PANYNJ and its consultants and contactors to perform such analyses in the 1960s would have been quite limited in comparison to the capabilities brought to bear in the NIST investigation.
The damage from the impact of a Boeing 767 aircraft (which is about 20 percent bigger than a Boeing 707) into each tower is well documented in NCSTAR 1-2. The massive damage was caused by the large mass of the aircraft, their high speed and momentum, which severed the relatively light steel of the exterior columns on the impact floors. The results of the NIST impact analyses matched well with observations (from photos and videos and analysis of recovered WTC steel) of exterior damage and of the amount and location of debris exiting from the buildings. This agreement supports the premise that the structural damage to the towers was due to the aircraft impact and not to any alternative forces.

sowie

Question: How could the WTC towers have collapsed without a controlled demolition since no steel-frame, high-rise buildings have ever before or since been brought down due to fires? Temperatures due to fire don't get hot enough for buildings to collapse.
Answer: The collapse of the WTC towers was not caused either by a conventional building fire or even solely by the concurrent multi-floor fires that day. Instead, NIST concluded that the WTC towers collapsed because: (1) the impact of the planes severed and damaged support columns, dislodged fireproofing insulation coating the steel floor trusses and steel columns, and widely dispersed jet fuel over multiple floors; and (2) the subsequent unusually large, jet-fuel ignited multi-floor fires weakened the now susceptible structural steel. No building in the United States has ever been subjected to the massive structural damage and concurrent multi-floor fires that the towers experienced on Sept. 11, 2001.
 

Im Flug zur Truth

Eine winzige Anekdote über Informationsfluss und Wahrnehmungsdefizite:
Das Freizeitprojekt WERBOOM (Vorgänger-Domain von MOSAIK911) thematisierte Anfang August 2006 den Sachverhalt ‘707 vs 767’ (siehe Update-Liste).
Seit November 2006 präsentiert die Organisation Pilots for 9/11 Truth das PANY-White Paper  - das Resultat von bemerkenswertem ‘Truther’-Theater.

Neben den dokumentierten Zitaten und Quellenverweisen zum NIST-Report hat WERBOOM (=MOSAIK911) explizit die Abbildungen des 3-seitigen PANY-Dokumentes eingefügt, um einen direkten Zugriff zu gewährleisten.
Das Dokument ist Bestandteil des NIST-Reportes - auch dieser Hinweis findet sich in einem der obigen Zitate:
A three-page white paper “Salient points with regard to the structural design of the World Trade Center towers”, February 1964, from the PANYNJ (see Appendix A) [...] [Hervorhebung von mir]

Am 30.10.2006 tauchte im Forum der Pilots for 9/11 Truth ein bemerkenswertes Posting auf: (link zum Forum)
I found photostat images of WTC designer John Skilling's 3 page 1964 White Paper on a German website and posted the links over on the UK911 board.
5 days later, having quoted them and posted the links there only 3 days ago
(they specifically refer to the successful calculations showing the buildings could withstand impact by Boeing 707 - DC-8 class aircraft at 600mph - but not the calculations themselves - just a summary) the images on the links mysteriously degraded from about 79kB to less than 7kB in size, thus becoming unreadable.
Lucky I saved them, and I'd urge anybody interested to do so too for posterity if nothing else.
For comparison, here are the links I saved the original images from, less than a week ago:
[....]
http://www.werboom.de/vt/assets/images/aut...ns_-_Seite1.jpg
http://www.werboom.de/vt/assets/images/aut...ns_-_Seite2.jpg
http://www.werboom.de/vt/assets/images/aut...ns_-_Seite3.jpg
It's enough to make anybody believe in conspiracy theories. :)

Ein Musterbeispiel wie Verschwörungstheorien gebastelt werden. ‘Truther’ lesen nur die Hälfte, schauen sich Bildchen an, verweisen auf falsche Links, unterschlagen den kompletten Kontext und schreien zur Krönung: Verschwörung, Vertuschung, Verdummung.

Der Administrator des Pilots4Truth-Forums (und Mitbegründer der Organisation) erwidert daraufhin:
great work chek. I tried to fix your links.. but it seems you may have copied an abbreviated link and pasted it here. You may need to get the links again from the exact page. Again..excellent work!

Bilder von einer verhassten ‘Debunker’-Website kopieren und falsche Links anführen = “großartige Arbeit”?

Der Admin zeigt sich trotzdem begeistert von dem ‘Sensationsfund’:
Wow! what a difference in the images. Spread it everywhere.. tell people to download it.. I saved it in several places. (Quelle)

Auf der Suche nach der ‘Wahrheit’ hätten die ‘Truthseeker’ nur in die NIST-Publikationen blicken müssen. Das fiel dem Initiator des Threads dann wohl plötzlich auch auf:
Panic over - they can also be found in NISTNCSTAR1-2Draft.pdf [...]

“Panic over”? Wieso versetzt nahezu jede eigens konstruierte ‘Merkwürdigkeit’ diese Herrschaften regelmäßig in Panik?

Nun ziert der ‘Sensationsfund’, der laut sinnloser VT-Logik von WERBOOM nachträglich manipuliert wurde, als ein ‘Truthseeker’ darauf aufmerksam wurde ([...] mysteriously degraded from about 79kB to less than 7kB in size, thus becoming unreadable) eine P4Truth-Seite.

Rot unterstrichen: “600 miles per hour” und “not cause collaps”.

Die konsequente Selektivität reicht jedoch noch immer nicht. Zusätzlich findet sich folgende Falschinformation:
Please note that official reports you see in mainstream media all claim that the WTC was designed to withstand an impact at low speed. The following reports contradict that claim.

Dieser widersprechende ‘Report’ findet sich im NIST-Untersuchungsbericht - das ist DER offizielle Untersuchungsbericht zu diesem Thema.
Oben sind die Hintergründe der unterschiedlichen Angaben ausführlich dokumentiert.