Designing Buildings to Avoid Progressive Collapse

A symposium sponsored by the American Institute of Steel Construction and The Steel Institute of New York discusses ways engineers can better design buildings to avoid the type of progressive collapse seen in recent years as the result of explosions and terrorist activity.

By Natalie Keith

Engineers may disagree on the best way to design buildings that don't collapse after an explosion, but they can agree on one point: The industry needs better guidelines to address blast resistance in the post-Sept. 11 world.

"The attacks on the World Trade Center and the Pentagon have put the issue into focus," said Gary Higbee, director of industry development of the Steel Institute of New York. "The steel industry has undergone much scrutiny since Sept. 11."

The issue was the subject of a symposium called "Blast and Progressive Collapse Resistance" sponsored by the American Institute of Steel Construction Inc. and The Steel Institute of New York Dec. 4 and 5 in New York City.

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Progressive collapse - whereby all or part of a structure collapses after being damaged by an explosion or other accident - is not new to the industry. Engineers have been studying the issue since 1968, when an explosion on the 18th floor of the 22-story Ronan Point apartment tower in London caused an entire section of the building to collapse, killing four people.

The Ronan Point collapse is noteworthy because a relatively small blast - a woman struck a match in her kitchen and set off a gas explosion - caused an extraordinary amount of damage. After the explosion knocked out a load-bearing precast concrete panel near the corner of the building, the loss of support caused the floors above it to collapse. The impact of the collapses set off a chain reaction of collapses all the way to the ground.

Among goals of the industry is to prevent the type of disproportionate damage that was suffered during the Ronan Point collapse.

Accidental explosions are still the most common cause of building collapse. But recent events involving terrorism - while comparatively rare - have caused the industry to consider the issue in a new light.

Among events given much of the attention are the 1995 bombing of the Murrah Federal Office Building in Oklahoma City, the 1993 bombing of the World Trade Center and the Sept. 11 attacks.

Dr. Anatol Longinow, an adjunct professor of civil engineering at Valparaiso University in Valparaiso, Ind., said one of the best ways to learn about blast resistance is to document damage after an explosion. In the seminar, Longinow shared pictures and information about the many accident sites he has studied over the past few years.

"There is little research on what happens to buildings when they explode," he said. "There are computer programs to calculate the effects of a blast on a building, but we need real-life data."

Specific design requirements to mitigate blast resistance do not exist, but since the Ronan Point collapse, the industry has sought to develop guidelines to address the issue.

Among those are the American Society of Civil Engineers' Minimum Design Loads for Buildings and Other Structures and the American Concrete Institute's Building Code Requirements for Structural Concrete. There's also the U.S. General Services Administration's Facilities Standards for the Public Buildings Service and Progressive Collapse Analysis and its Design Guidelines for New Federal Office Buildings and Major Modernization Projects.

R. Shankar Nair, principal and senior vice president of Teng & Associates Inc. in Chicago, said some aspects of the guidelines have shifted over the years. First instance, a 2003 guideline called for applying design principles to buildings more prone to attack. An earlier version of the guideline published in 2000 called for applying the principles to all buildings regardless of perceived threat.

"Even within an organization like the GSA, there does not seem to be any clear guidelines," Nair added.

However, the industry has identified three approaches to designing structures to reduce susceptibility to collapse: Redundancy or alternate load paths, local resistance and interconnection or continuity.

Redundancy involves designing a structure so that if one component fails, alternate paths are available to accommodate the load. Local resistance involves providing components that might be subject to attack with additional resistance and interconnection whereby structural components are interconnected more effectively.

"We don't have the answer yet with how to avoid progressive collapse," Nair said. "The focus of the profession seems to be on redundancy, but a redundancy requirement can be useless or counterproductive."

Jon Magnusson, chairman and CEO of Magnusson Klemencic Associates in Seattle and Chicago, said the World Trade Center attacks have spawned many code changes, but not all provide useful direction on the issue. He criticized engineers who have made public statements claiming that buildings can be constructed to withstand terrorist attacks like those on Sept. 11.

To make some of these ideas work, "you would have to literally repeal the laws of physics," he said. "The idea that we can outbuild a determined terrorist is ridiculous. If we don't speak up about what we know as engineers, we are failing our profession."

A common approach to mitigating collapse is to design a building so it can withstand the removal of one column, but that can have limited impact. Whatever the approach, engineers must discuss the issue with the building owner, architect and building officials.

"Engineers who understand these concepts need to speak loudly so our policy makers don't divert precious resources away from things that create real safety," Magnusson said.

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