Opinioni esperte sul crollo delle Torri Gemelle: Matthys Levy

di Paolo Attivissimo

Durante il mio recente dibattito video con un sostenitore delle tesi alternative, Massimo Mazzucco, ho segnalato e mostrato il libro Why Buildings Fall Down, di Matthys Levy e Mario Salvadori, come esempio di esperti in strutture che non trovano nulla di anomalo nella dinamica dei crolli delle Torri Gemelle e confermano quella che viene impropriamente definita “versione ufficiale”.

Mario Salvadori (1907-1997) era ingegnere strutturista e professore d'ingegneria civile e architettura presso la Columbia University. Autore di dieci libri sulle strutture architettoniche e di cinque testi di matematica applicata (per esempio Structural Design in Architecture, 1967, e Numerical Methods in Engineering, 1953), aveva lavorato per la società d'ingegneria Weidlinger Associates, Inc. come consulente, diventandone poi membro e infine presidente onorario. Era stato insignito del Founders Award della National Academy of Engineering nel 1997 e della Hoover Medal da parte della American Society of Civil Engineers (ASCE) nel 1993.

Matthys Levy è da oltre cinquant'anni ingegnere civile e consulente in strutture; attualmente è anche presidente onorario della medesima Weidlinger Associates. Fra i suoi numerosi riconoscimenti professionali si può citare il premio Innovation in Civil Engineering dell'ASCE (1994). Levy ha anche pubblicato due articoli tecnici sul crollo delle torri: The Anatomy of the World Trade Center Collapses, A Structural Engineering Investigation, con N. Abboud et al., (Proc. Of the Third Forensic Engineering Congress, ASCE, San Diego, ottobre 2003) e Anatomy of a Disaster: A Structural Investigation of the World Trade Center Collapses (SFPE-SEI Proc. Conference on Designing Structures for Fire, Baltimore, settembre 2003). È progettista di grandi strutture come il Georgia Dome Stadium di Atlanta (70.000 posti coperti da quella che fino al 1999 è stata la cupola più grande del mondo), il Rose Center for Earth and Space a New York e lo stadio La Plata in Argentina.

Why Buildings Fall Down, scritto originariamente nel 1992 per esaminare i grandi crolli di edifici nella storia antica e contemporanea, è stato aggiornato ed ampliato da Matthys Levy dopo la morte di Salvadori per includere gli eventi dell'11/9. Questa è la trascrizione della parte saliente della sezione dedicata all'11/9 nel libro (da pagina 263 a pagina 268), specificamente nella sua edizione del 2002 (ISBN 0-393-311525).

Qualora vi fosse interesse sufficiente in una traduzione in italiano di questo brano, Undicisettembre si adopererà per fornirla.

As the towers fell, the piston effect of the collapse hurled debris and flaming embers violently against adjacent buildings, damaging many and causing some to burn and collapse. The roof of a subway tunnel directly under the complex collapsed under the weight of the debris, as did some of the Trade Center's underground levels. In all, 12 million sq. ft. (1.1 million m²) of the magnificent Center were reduced to 1.2 million tons of debris: ash, rubble, and twisted steel. Entombed in this rubble were more than three thousand people who either had been trapped on the floors above the impact or did not have sufficient time to escape the inferno before the towers collapsed. It was the most devastating event ever witnessed and the greatest domestic tragedy since the Galveston flood of 1900, but unlike that natural event, this one was the direct result of terrorism – the destruction of a symbol of America and the city of New York.

The two planes that smashed into the World Trade Center had been hijacked by a group of terrorists that also hijacked two other planes. One of these dove into the Pentagon, the headquarters of the United States military establishment and symbol of American military might. The fully fueled plane, a Boeing 757, flying diagonal to one face of the building at an estimated 345 mph (550 km/hr), destroyed a large section of one corner. The force of the impact demolished the plane, sending parts penetrating through three outer rings of the structure. The building, a reinforced concrete structure built at the start of the Second World War, had recently been partly renovated, reinforcing the two foot (600 mm) thick concrete and masonry outer wall with a grid of steel members. Windows had also been made blast resistant and a layer of Kevlar was introduced on the inner face of the outer wall to catch debris. All these measures, which were virtually complete in the affected section of the building, contributed to the survival of the structure for a half hour before it collapsed, allowing survivors time to escape. The major damage to the adjacent sections of the complex was caused by the ensuing fire, smoke, and water used to extinguish the fire. In spite of it all, the Pentagon stayed open and the affected section of the building was soon stabilized and isolated.

The fourth plane crashed in a field in Pennsylvania, apparently as a result of passengers overwhelming the terrorists. A total of nineteen terrorists, some of whom were trained as pilots, undertook this coordinated attack. It is believed that these perpetrators were connected with and financed by Osama bin Laden, who had also directed the early attacks on the U.S. embassies in Africa and on the U.S. cruiser Cole in the port of Aden.

After the fall

How could two such seemingly sturdy and powerful structures collapse? The 110-story towers, which were completed in the 1970s, consisted of a steel column-supported core housing elevators and services surrounded by a clear span office space supported along the perimeter by a ring of closely spaced steel columns. These perimeter columns were rigidly joined to short and deep steel beams, thus forming a stiff tubular structure designed to resist the effect of wind and giving the buildings their characteristic lace-like appearance. Floors consisted of concrete topped light steel trusses spanning between the core and exterior. The building's structural engineer, Leslie Robertson, had developed a unique device using a viscoelastic material to connect the trusses to the columns. This device responded like soft rubber to slowly applied loads and like hard rubber to rapidly applied loads. In this way, the connector was able to damp-out uncomfortable vibrations due to the lateral sway of the slender towers under wind while not supporting loads from the floors.

The plane's momentum, when it struck a tower, carried it into the building, with parts penetrating the core and some parts exiting out the other side (see Chapter 2). The force of the collision was less than one-quarter of the towers wind-resisting capability, which is why it was able to survive the impact. The plane spilled its fuel, resulting in a fireball that enveloped several floors in the impact zone. The temperature of the fire was estimated to be 1000-2000°F (550-1100°C). It can be surmised that as the plane penetrated the structure, it scraped and destroyed much of the fireproofing material that protects steel from the damaging effects of heat. The steel columns in the core were then intensely heated; as they reached temperatures of 1100°F (600°C) they had lost half their strength and began to flow and buckle (Fig. 18.3). Floors in the impact zone also undoubtedly failed, pulling against the weakened columns and further hastening their tendency to buckle. The weight of the floors above the impact zone then caused the columns to fail, dropping the undamaged upper structure onto the lower one (Fig. 18.4). This impact force, estimated to have been thirty times the weight of the upper structure, caused the lower floors to fail in rapid sequence, dropping the tower vertically (similar to what happened at L'Ambiance Plaza [see Chapter 12]). This analysis assumes that the perimeter columns did not initially fail due to the fire because of the ventilating effect of wind on the exterior face of the building and only peeled off the tower as it fell. The second tower to be hit, which was the first to fail, actually tilted above the impact zone as it began to crumble, but then continued to drop vertically. This initial tilt was undoubtedly due to the fact that this tower suffered substantial damage on one face near a corner when the plane struck. Although the towers were designed and built as structurally efficient buildings and complied with existing regulations, they could not survive such an unanticipated, horrendous attack.

Other failure scenarios will undoubtedly be proposed and debated as investigations into the catastrophe take place over many years. Yet we believe that the basic outline of the failure is clear.

Can skyscrapers be designed to safely survive another such terrorist attack? The answer to this question is necessarily guarded. In both cases of planes crashing against skyscrapers – the Empire State building and the World Trade Center – the structure survived the initial impact. The redundancy in the rigidly joined beam and column outer tube of the World Trade Center served it well by redistributing column loads away from the damaged region. What doomed it was the fire. A more robust core structure, perhaps of reinforced concrete or an as yet undeveloped material and a more advanced fire suppression system might provide a more terrorist-resistant structure. But even the masonry and concrete wall of the Pentagon could not prevent a plane from penetrating the building. Heavy concrete bunkers can be designed to survive also almost any explosive event, but we do not want to live and work in such oppressive bunkers. A compromise between the bunker and the glass box will have to be accepted to reduce with the risk of future catastrophic structural collapses.