The present article was published in the September 25, 2001 issue of the New York Times, just two weeks after the collapse of the twin towers of the World Trade Center in New York. In this article, by gathering expert opinions from various building and structural specialists, as well as statements from experienced professionals in the demolition of large buildings, an effort has been made to analyze the manner of collapse of these towers, and the role and influence of well-known phenomena in the laws of physics — namely compressed air force and gravitational force — in the final shaping of the manner of destruction has been examined and studied.
The laws of physics finished what the terror had started. Nothing remains but rubble and dust. The scale of destruction at the World Trade Center buildings after the terrorist attack on September 11 is beyond comprehension. But understanding the nature of the forces of nature that facilitated this catastrophe can help explain the extent of the twin towers' destruction and clarify why these towers collapsed in the manner they did, and from a technical standpoint, what happened that caused massive clouds of dust to erupt.
These two buildings contained more than two hundred thousand tons of steel, three hundred and twenty-five thousand cubic meters of concrete, and fifty-five thousand seven hundred and fifty square meters of glass in the form of forty-three thousand window units. Each floor, constructed from a concrete slab on a metal surface supported by a network of steel beams, had 4,000 square meters of area and weighed two thousand one hundred and seventy-seven tons.
The general outlines of the towers' collapse became known immediately upon the terrorist attack. Engineers believe that the intensity of the force from the impact of a 767 jet and the intense heat from thousands of gallons of burning jet engine fuel weakened, twisted, and caused the steel frame of the building to buckle, causing the upper floors to collapse and creating an unstoppable chain reaction along the line of gravitational force.
Since the time the planes struck, engineers have been continuously analyzing the disaster. Mr. Magnusson and other experts say they were initially surprised that the buildings collapsed vertically, without any lateral slippage and without toppling, straight down along the plumb line. Mr. Magnusson says the footage of the moment of collapse shows that the exterior braces of the building's structure may have helped direct the collapse of the floors in the vertical direction.
They say the buildings essentially collapsed in much the same manner they would have in a controlled demolition — with the difference that in controlled demolition, the work program obviously begins with the placement of explosives in small quantities.
Demolition expert engineers say the size and weight of the concrete slabs and the manner in which each one crashed onto the next help explain how these massive constituent volumes were so thoroughly crushed. As soon as each floor crashed onto the floor below, the cumulative weight increased such that it crushed everything in its path, making them finer and finer until ultimately what remained of them was nothing but dust and small fragments.
Dr. Frank Moscatelli, a physics professor at Swarthmore College in the state of Pennsylvania, has calculated that the total energy released in this disaster was equivalent to the explosion of 600 tons of TNT. He says this energy — including the weight of the 767 aircraft, the effect of their speed, jet engine fuel, and the weight of the building mass — amounts to approximately one-twentieth of the atomic power of the ten-kiloton bomb dropped on Hiroshima.
Dr. Moscatelli also says that most of this energy derived from gravitational force and resulted from the collapse of floors. He explains that once the mass of the upper portion of the building begins to fall, a momentum is generated whose magnitude adds to the force that the lower sections of the building must bear. He says if the building sections cannot withstand this force, then everything starts to collapse.
John D. Magnusson, president and principal of the Magnusson Barkshire firm in Seattle, who worked on the original design of the World Trade Center building, says the enormous energy released during the building's collapse was enough to pulverize the twin towers and their contents.
Engineers stated regarding this phenomenon that the air pressure created beneath the collapsing structure had pushed all this dust into the air.
James Redyke, head of Dykon Blasting, a demolition firm in the city of Tulsa, Oklahoma, says people do not even realize it, but in every building there is an enormous volume of air. A building is like an accordion. When a building collapses, the air inside it must go somewhere. The volumetric force of this air is so great that it can erupt all the dust from concrete, crushed stone, and other objects inside the building.
In a preliminary analysis of the World Trade Center disaster, published the previous week (September 17), it was estimated that the collapse of the tower floors onto one another caused a surging massive cloud of debris fragments to form. This surging cloud, as indicated in the analysis, began expanding at an initial speed of more than 100 kilometers per hour, projecting a considerable amount of dust masses to a radius of more than 800 meters, and a smaller quantity of demolished fragments to more than 3 kilometers away.
Structural Design
Why were these buildings designed in what is technically called a "tube structure" design? This type of design consists of hundreds of steel columns placed at close intervals around the exterior surface of each of the towers. These columns gave the structural frame its strength and bore a major portion of its weight, including the weight of the concrete slabs. This exterior ring was connected by steel trusses to the assembly of steel columns in the central core. The central core bore the weight of the intermediate elements such as elevators, stairs, and also a portion of the concrete slabs of each floor. Mr. Magnusson says the tube-like assembly of steel columns on the exterior surface, which in each tower consisted of 61 columns on each of the four facades, apparently helped regulate the vertical direction of the concrete floor collapse during the fall.
Mr. Magnusson says when you watch the videotape of the buildings' collapse, it appears as though the floors pass through them. They are preserved, and the reason the building shells remained standing for a while after the collapse is that this was the exterior shell of the towers.