Unthinkable nightmare

Terrorist attacks in the United States bring theatrics of the big screen to life

October 01, 2001 |

Ron Klemencic is chairman of the Council on Tall Buildings and Urban Habitat, as well as president of Seattle-based structural engineer Skilling Ward Magnusson Barkshire. Like almost everyone else, he experienced horror as he watched the World Trade Center's twin towers collapse on Sept. 11.

"My initial reaction was total disbelief," Klemencic says. But as he watched the video replays of the collapse of first one tower and then the other, he adds: "It was pretty clear why they collapsed."

Larry Griffis is another structural engineer who finds, in retrospect, that the collapse of the World Trade Center towers was not surprising. He is senior principal with Houston-based structural engineer Walter P. Moore and Associates.

Klemencic says he and his colleagues were amazed that the towers stood for as long as they did. Griffis says they survived the aircraft impacts remarkably well, despite gaping holes that may have severed more than 20 percent of their columns. The towers had perimeter columns spaced only 3 feet, 3 inches apart. This provided a redundancy that enabled columns still in place to carry the loads of severed columns.

At the towers' third level, the columns transitioned in an arch-like fashion to a 10-ft. spacing. The 208-ft.-by-208-ft. floors were supported by 60-ft.-long trusses that spanned from the core to the perimeter.

Steel-framed buildings such as the World Trade Center towers are encased in cementitious fireproofing to protect framing members — typically to provide structural integrity for up to three hours. But the fire loads that govern the amount of applied fireproofing are based on requirements for fires fed by materials found in offices (e.g., paper, furniture and curtains), and not by volatile jet fuel.

Griffis says the inferno ultimately destroyed the fireproofing and the steel framing was heated to a temperature at which it lost its structural properties. Columns buckled, initiating a progressive collapse in which one floor banged down on the floor below, with the increasing weight of higher floors becoming an impact load.

Klemencic says that if the sprinkler systems were initially operational, the resulting water discharge may have helped to spread the jet fuel. A foam-based fire protection system is not practical for a major building, he notes.

Hyman Brown, now an engineering professor at the University of Colorado, supervised work on the towers for Tishman Realty and Construction when they were under construction in the early 1970s. He says the towers could have withstood the impact of the aircraft, "but not the 24,000 gallons of aircraft fuel" that fed the resulting fire.

Are tall buildings passé?

Neither Klemencic nor Griffis believes the collapse of the World Trade Center towers signals the end of tall building development. "The World Trade Center wasn't targeted necessarily because it was tall, but probably because it was the symbol of economic might," Klemencic says. Griffis adds: "In five years, while this event won't be forgotten, tall building design will have moved on."

The catastrophe may result in new ground rules for fire and rescue crews that face a similar situation. John Knapton, professor of structural engineering at Newcastle University in the United Kingdom, says firefighters and police officers should have received an order to withdraw within an hour after the planes struck, because the towers could be expected to collapse within two hours. They "simply should not have been there," he declares. "If they did decide to take the risk, they should have been pulled out after an hour."

Recreate the original?

Chicago developer Steven Fifield, interviewed by the Chicago Tribune, speculates that it would cost $4.5 billion to replace the World Trade Center's twin towers. He figures it would be about $1 billion less expensive to house their 10.6 million square feet of space in a series of buildings of 50 stories or less. A rent premium of about $100 million a year — or $10 per square foot more than the prevailing $35 rate — would be necessary to make a new World Trade Center financially feasible, he says.

Phillip Waier, principal engineer with Kingston, Mass.-based R.S. Means Co., a supplier of construction cost information, concurs with Fifield's general cost estimate. By applying his company's historical construction index to 1973 cost data, the year the twin towers were completed, Waier calculated the cost to rebuild the towers to be approximately $4 billion.

The World Trade Center's demise, not surprisingly, has led to questions such as this: The headline in a suburban newspaper asks, "Do skyscrapers make sense in today's world?" The story questions whether Donald Trump's planned world's tallest building in Chicago would constitute the world's tallest target.

The World Trade Center debacle will be the subject of much scrutiny. An American Institute of Steel Construction task force will analyze the factors that contributed to the collapses, and make recommendations to AISC's Specification, Blast and Fire committees. A forum sponsored by Georgia Tech's College of Architecture featured presentations by professors and practitioners, concluding with a discussion of what the disaster implies for the future of high-rise design.

A preliminary report by real estate firm Grubb & Ellis indicates that about 20 percent of downtown Manhattan office space was destroyed. The firm said that of the 15.5 million square feet of space in the World Trade Center area, about 97 percent was leased.

The Council on Tall Buildings and Urban Habitat at Lehigh University in Bethlehem, Pa., is an international organization sponsored by architectural, engineering, planning and construction professionals. Following the 1993 World Trade Center bombing, the council focused more sharply on what could be done to enhance building security — a topic whose importance was reinforced by the bombing of the Oklahoma City federal building two years later. A number of recommendations resulted.

For example, it was common to place building security and fire control centers close to each other and adjacent to the lobby. While this arrangement provides a convenient location from which security personnel can provide surveillance, it made these facilities vulnerable. Designers are now encouraged to encapsulate them within the building.

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