Science of a destroyed tower

[Ninotchka]

The Physics of Turning a Tower Into a Cloud of Dust and Rubble
Warren E. Leary New York Times Service

NEW YORK Little is left but rubble and dust. The degree of destruction at the World Trade Center after the Sept. 11 attacks seems almost beyond comprehension.

But understanding the natural forces that contributed to the disaster can help explain the destruction of the twin towers, why they came crashing down the way they did and why enormous clouds of dust erupted from the site, experts say.

The buildings contained more than 200,000 tons of steel, 425,000 cubic yards of concrete and 600,000 square feet of glass in 43,000 windows. Each floor, a reinforced concrete pad on a metal deck supported by steel cross beams, was about one acre in area and weighed about 4.8 million pounds (2.2 million kilograms). Experts said the size and weight of the concrete floor pads, and the way in which they collapsed onto one another, contributed to so much of the mass of each building being smashed into small pieces and to the formation of huge clouds of dust and ash.

Demolition engineers said that as each concrete floor in a falling building pancakes into the one below it, their combined weight grows so large that they and everything in their path break into pieces, and the pieces are pounded smaller and smaller until mostly dust and small rubble remain.

Frank Moscatelli, a professor of physics at Swarthmore College in Pennsylvania, calculated that the total energy released in the disaster was equivalent to the explosion of 600 tons of TNT. That energy discharge, which takes into account the weight of the airplanes that struck the buildings, their speed at impact, the jet fuel and the mass of the buildings, is about one-twentieth of that released by the 10 kiloton atom bomb dropped on Hiroshima, he said.

"And most of this energy was supplied by gravitational collapse," Mr. Moscatelli said. "People don't think of an item sitting still as having energy, but it does. The energy from gravitational collapse is what the controlled demolition people use to take down structures, and it is very powerful."

Engineers said that the steel framework of the buildings buckled from the heat of the fire, causing the upper floors to collapse, which initiated an unstoppable chain reaction like the process often used intentionally by demolition experts who bring down buildings with the strategic placement of small explosive charges. Once the mass of the upper part of a building begins to fall, Mr. Moscatelli said, it gains momentum that adds to the force the structure below has to resist.

Jon Magnusson, chairman of Skilling Ward Magnusson Barkshire, a structural engineering company that worked out the original World Trade Center design, said the enormous energy released in the collapse was enough to pulverize the 110-story towers and their contents.

As the buildings fell, large plumes of dust billowed from the structures in high-speed jets. Engineers said compression of the buildings' air propelled the dust. James Redyke, president of Dykon Blasting, a demolition company, said: "A building is like an accordion, and when it collapses the air has to exit someplace. It exits at a pretty good volume, carrying all the dust from concrete, sheetrock and other things inside."

An initial analysis of the World Trade Center disaster released last week by Risk Management Solutions of Newark, California, a company that analyzes catastrophes for the insurance industry and others, estimated that the pancake collapse of the tower floors produced a major airborne "debris surge." That cloud dumped significant deposits of dust more than a half mile from the site and small amounts up to two miles away.

Mr. Magnusson and others said they were initially surprised that the buildings went straight down without sliding to the side or toppling, which would have greatly increased the loss of life and damage to the area. Pictures of the collapse indicate that the buildings' outer support structures may have helped guide the falling floors, Mr. Magnusson said.

The buildings had a so-called metal tube structure design, which consisted of hundreds of steel columns spaced around the outer face of each tower to give them stiffness and to support much of their weight. This outer ring was connected by steel trusses to a cluster of metal columns in the center that supported core components such as elevators and stairs, as well as part of each concrete floor.

"As you look at the videotape, it seems that as the building frame peels back, it stays long enough for the floors to pass," Mr. Magnusson said. "The reason the buildings stood and the reason they went straight down was that strong exterior tube."