Fysik-Professor: Tekniken bakom WTC-kollapsen

We will go into greater detail documenting the collapse of the World Trade Center in a future article, but a number of factors explain why the Empire State Building suffered relatively minor damage while the twin towers were catastrophically destroyed. First, the energy of impact sustained by the buildings differed by orders of magnitude. The B-25 that struck the Empire State Building weighed approximately 21,500 lb (9,760 kg) and was traveling around 200 mph (320 km/h). The kinetic energy it created in the collision was about 30 million ft-lb (40 million Joules).

The twin towers of the World Trade Center, by comparison, were struck by Boeing 767 airliners traveling over twice as fast and weighing nearly 15 times as much as a B-25. The energy of impact for the two planes ranged from 2 billion ft-lb (2.6 billion Joules) to 3 billion ft-lb (4.1 billion Joules), some 60 to 100 times greater than that absorbed by the Empire State Building. This estimate is also conservative since it does not account for the energy released by the exploding jet fuel, which greatly exceeded the energy released by the much smaller B-25 fuel supply as well. The greater kinetic energy allowed the 767 aircraft to penetrate much further into the twin towers than the B-25 was able to do at the Empire State Building. Most of the B-25 impact was absorbed by the building's exterior wall leaving very little to damage the interior structure. The 767 impacts, however, not only produced gaping holes in the WTC exterior but also destroyed much of the structural core at the center of each tower.

Even so, the impact alone does not fully explain what doomed the World Trade Center towers. A fatal contributing factor was the fires ignited by the exploding fuel tanks. A 767 has a maximum fuel capacity 35 times greater than that of a B-25D. The aircraft that struck the Empire State Building was nearly out of fuel when it crashed while each 767 still carried approximately half of its maximum fuel load at impact. The Empire State Building fire exhausted its supply of fuel rapidly while that at the World Trade Center ignited the office contents across several floors and burned much longer. The type of fuel carried may also be a significant factor. The B-25 burned avgas, a high-octane version of gasoline still used aboard piston engine aircraft today. The 767 instead uses Jet-A, a derivative of kerosene that fuels all commercial jetliners. Jet fuel tends to reach higher temperatures than gasoline causing the fires in the WTC to burn more intensely than that in the Empire State Building.

Aggravating the situation further was the size of the holes torn in the building exteriors. Fires in office buildings generally consume the oxygen available in the enclosed space rapidly limiting the growth and strength of the fire. The exterior holes, however, allowed fresh air to be pulled into the buildings helping the fires to move through the building and consume additional combustible material. Since the damage to the facades of both WTC towers was far more extensive than at the Empire State Building, more air was available to encourage the fires. The air at the Empire State Building was also damp because of the foggy conditions and may have played a role in limiting the extent of the fires in that structure.

Furthermore, the Empire State Building is a reinforced masonry structure in which the structural steel beams are encased within limestone walls or slabs of concrete 8 inches (20 cm) thick. This heavy mass provides exceptional fire protection that insulates the steel within from excessive heating. Many modern skyscrapers like the WTC towers have eliminated this extensive use of stone and concrete to reduce cost. The World Trade Center instead relied on lightweight spay-on coatings for insulation. This insulation was simply blown off the WTC structure by the 767 collisions exposing the steel beams and floor trusses to the raging fire.

The Empire State Building is also a heavily compartmented structure. Each floor is self contained with its own independent heating and cooling ducts, elevator and utility shafts are surrounded by thick masonry walls, fire partitions separate each floor and rooms within each floor, and the fireproof stairway prevents smoke from rising to upper stories. These features make it very difficult for fire to spread beyond a limited area. The World Trade Center instead offered vast open floor spaces that appealed to tenants but allowed fires to spread far more easily. Moreover, the fire suppression system in both towers lacked redundancy and the 767 collisions cut off the water supply to the sprinklers. For these reasons, the Empire State Building is still considered one of the world's safest skyscrapers in a fire.

The Empire State Building crash of 1945 also offers insights into the Pentagon attack on September 11. Both buildings are reinforced masonry structures built using similar methods and materials, although the Pentagon has been considerably upgraded to survive impact damage. One topic often used to promote conspiracy theories is the size of the hole in the exterior wall of the Pentagon created by the Boeing 757 that struck it. The 757 has a wingspan of almost 125 ft (38 m), yet most conspiracy sites suggest the impact hole is only 15 to 65 ft (4.5 to 20 m) wide. The same can be said of the Empire State Building where a plane with a wingspan greater than 67 ft (20.5 m) created a hole no more than 20 ft (6 m) across.

Both aircraft caused damage consistent with the size of the plane and the structural materials used in the facade. Most of the mass of a plane is contained within the fuselage, inner wing structure, and engine nacelles. These portions of the aircraft have the greatest power to penetrate a wall upon impact, and the sizes of the impact holes at both the Empire State Building and the Pentagon are consistent with the dimensions of the fuselage and nacelles of the B-25 and 757, respectively. The outer wings and tail surfaces are much lighter structures consisting mostly of a thin skin enclosing empty space. Upon colliding a thick wall composed of a dense material like stone or concrete, these light aerodynamic structures simply disintegrate. The impact often produces surface gouging and perhaps small, localized holes, but the lighter aircraft structures generally cannot penetrate a reinforced masonry wall. Close examination of both buildings shows gouges extending outward from the central impact hole as would be expected from the collision of wings.