This article appeared in the 25 September 2001 issue of the New York Times — exactly two weeks after the collapse of the twin towers of the World Trade Center in New York. In it, the views of various structural and building experts, as well as of seasoned specialists in the demolition of large structures, are gathered, in an attempt to analyse the manner in which these towers fell — and to study the role and interaction of two phenomena familiar from physical laws, namely the force of compressed air and the pull of gravity, in the final shape that the destruction of the towers took.
The laws of physics finished what terror had begun.
Nothing is left but rubble and dust. The scale of the destruction in the World Trade Center after the terrorist attack of 11 September is beyond comprehension. But understanding the nature of the natural forces that abetted the calamity can help to explain the dimensions of the destruction in the twin towers — to clarify why the towers came down in the manner they did, and what, in expert terms, drove out the vast clouds of dust that erupted from the site.
The two buildings contained more than two hundred thousand tons of steel, three hundred and twenty-five thousand cubic metres of concrete, and fifty-five thousand seven hundred and fifty square metres of glass in forty-three thousand window units. Each floor — a concrete slab on a metal deck supported by a grid of steel beams — measured 4,000 square metres and weighed 2,177 tons.
The general outline of how the towers fell was understood quickly after the terrorists' attack. Engineers hold that the force of the impact of the 767 jets, together with the intense heat from thousands of litres of burning jet fuel, weakened and warped the steel skeleton of the buildings, caused the upper floors to give way, and set off an unstoppable chain reaction along the line of gravitational force.
Since the moment the planes struck, engineers have been at work analysing the dimensions of the disaster. They say the buildings came down in much the way they would have if controlled-demolition methods had been used — with the difference that, in a controlled demolition, the work would of course have begun by placing small charges of explosive at chosen points.
Demolition engineers say that the size and weight of the concrete slabs and the manner in which they piled one upon the next help to clarify why the immense bulk of the towers was so finely pulverised. As soon as one floor crashed onto the floor below, the combined weight grew so much that it pulverised both those floors themselves and everything in their path, finer and finer, so that what eventually remained of them was nothing but dust and broken fragments.
Energy equivalent to 600 tons of TNT
Dr. Frank Moscatell, a physics professor at Swarthmore College in Pennsylvania, has calculated that the total energy released in the disaster was equivalent to the explosion of 600 tons of TNT. He says this quantity of energy — which includes the weight of the 767 jets, the effect of their speed, the jet-engine fuel, and the weight of the mass of the buildings — is roughly one-twentieth the power of the ten-kiloton atomic bomb dropped on Hiroshima.
Dr. Moscatell also says that most of this energy came from gravity and was released by the building's collapse. "Most people don't think a stationary object has any energy, but it does." The energy of a gravity-driven collapse is the same force that demolition specialists use to bring down buildings — and it is very powerful.
Dr. Moscatell says that as soon as the upper part of the building begins to fall, a momentum is generated whose magnitude is added to the force the lower portions of the building must bear. He says that if the rest of the building cannot bear that force, then everything begins to come down.
John D. Magnusson, president and managing director of Magnusson Klemencic in Seattle, who worked on the original design of the World Trade Center buildings, says that the immense energy released in the course of the collapse was enough to powderise both towers and their contents.
Air pressure and the eruption of dust
As soon as the buildings fell, vast clouds of dust generated under the structures emerged with great speed and an erupting force. Engineers said of this phenomenon that the air pressure inside the building drove all that dust into the air.
Jim Redyke, head of Decon Blasting, a building-demolition firm in Tulsa, Oklahoma, says that people don't realise it, but every building contains a great volume of air. "A building is like an accordion. When a building collapses, the air inside it has to go somewhere. The volumetric force of that air is so great that it can blow out into eruption all the dust from concrete, broken stone, and other items inside the building."
In a preliminary analysis of the World Trade Center disaster, published the previous week (17 September) by Hazard Management Inc. of Newark, California — which analyses natural events for insurance companies and other clients — it was estimated that the floors of the tower falling onto one another caused a roaring cloud of debris to form in the surrounding space. According to this analysis, the cloud spread initially at more than 80 kilometres per hour, hurling considerable quantities of dust over a radius of more than 800 metres, and small quantities of demolition debris more than 3 kilometres from the towers.
The plumb-line fall and the steel-tube design
Mr. Magnusson and several other experts say that they were initially astonished that the buildings came down vertically, without any lateral slip and without toppling — that they fell along a plumb line. Mr. Magnusson says the images of the moment of collapse suggest that the outer frames of the building's structure may have helped to direct the fall of the floors along the plumb line.
Structurally, these buildings used what is termed a 'steel tube design'. This kind of design consisted of hundreds of steel columns set at fixed intervals around the outer face of each tower. These columns lent strength to the whole skeleton and carried a major part of its weight, including the weight of the concrete slabs. This outer ring was tied by steel trusses to a set of metal columns in the central core. The central core bore the load of the building's middle elements — elevators, stairs, and part of the concrete slabs of each floor.
Mr. Magnusson says the tube-like network of steel columns on the outer face of the building — which on each tower comprised 61 columns on each of its four faces — apparently regulated the plumb-line direction of the concrete floors as they collapsed.
Mr. Magnusson says: "When you watch the videotape of the buildings' collapse, the firm form of the buildings' outer shell appears to be preserved before any change of shape, and is held until the falling floors pass through it. The reason the bodies of the buildings remained standing for some time after the floors had fallen straight through them was the firm tube-like behaviour of the towers' outer frames."