Explosion and Collapse: Disaster in Milliseconds
BY EVE E. HINMAN, Eng.Sc.D.
This article is excerpted from the white paper “The Bombing of the Oklahoma City Federal Building: A Failure Analysis,” Failure Analysis Associates, Engineers and Scientists, Menlo Park, California.
At 9:02 a.m. on April 19, 1995, a 4,800-pound bomb, concealed in a rented truck, exploded 20 feet from the Alfred P. Murrah Federal Building in downtown Oklahoma City. The explosion and partial collapse of the nine-story building killed 169 people, injured hundreds more, and resulted in millions of dollars of losses.
Following the disaster, specialists from Failure Analysis Associates (FaAA) were called to the site to assist in the rescue effort to determine how to safely remove debris from the remaining structure. A second goal of the FaAA team included verifying concepts used in designing blast-resistant buildings so lessons learned from incidents such as this can form the basis for a set of design principles to minimize this type of threat in the future.
Let`s examine what probably occurred at ground zero at the moment of detonation and in the seconds immediately following the blast.
In the first instant of the blast in Oklahoma City, a small amount of explosive is detonated. This triggering explosion, in close proximity to a huge, carefully arranged mass of common fertilizer and fuel oil, initiates an exothermic chemical reaction between the nitrogen compounds in the fertilizer and the hydrocarbons in the fuel oil. The energy expelled during the rapid chain reaction generates heat, an audible blast, and a wave that propagates radially outward at supersonic velocities. Of all the forces unleashed from the bomb, it is the highly compressed particles of air from the shock wave that will cause most of the damage in Oklahoma City. The walls of the rental truck are shredded nearly instantaneously.
Continuing outward from the truck, the tremendous pressures exerted from the wave are decaying with the cube of the distance from the truck. At this rate the 4,800-pound ANFO device will have little effect on a building one-quarter mile away.
However, at a mere 20 feet, the wave slams into the federal building with a pressure of nearly 6,000 pounds per square inch.
The face of the building immediately in front of the explosion is hit worst by the shock wave, not only because it is closest to the explosion but because the wave is reflected backward and then amplified. Just as a seawall causes an onrushing ocean swell to reverse direction and then jump higher on the wall, so does the building momentarily exert this effect on the shock wave. On the walls of the building, pressures increase by up to eight times the original force. Glass windows, not a structural element and the weakest part of the building, comprise 50 percent of the outer facade. The air blast enters the building, carrying with it thousands of pounds of glass shrapnel.
Back at ground zero, the air rushes in to fill the partial vacuum created behind the pressure wave. This rushing air mass, the drag pressure, generates a force on objects 1,000 times stronger than the strongest hurricane. It is this wind that carries debris far from the area near the explosion.
Part of the energy of the explosion is also directed downward into the ground. More than 6,000 cubic feet of soil are blown away, creating a 30-foot-wide, eight-foot-deep crater, and generating a short-duration/high-intensity underground shock wave. The building sways as it would during an earthquake measuring 5 on the Richter scale.
Inside the building, as the blast enters the window spaces and collapses portions of the outer walls, it pushes upward on the concrete floor slabs. Since the floors are designed only for downward load, the upward pressure pushes the floors in the opposite direction and tears the slabs from their supporting columns, lifting them into the air. As the floors crash back down, the columns, destabilized by debris, are knocked over like bowling pins. The blast flows over and around the building, exerting massive pressures on all surfaces it encounters. Windows are blown out 1,000 feet away.
As the floor slabs and columns fail on lower levels, the structure above loses its support and begins to collapse, ripping 34-inch-diameter steel reinforcing bars from solid concrete. The collapse also propagates laterally as other floor slabs are destabilized by the removal of adjacent supporting members. Progressive collapse, the domino effect, threatens the entire structure. Nine floors of the building, one bay deep, fall–carrying concrete, wiring, sprinkler systems, office furniture, and occupants to ground level. Debris is piled 26 feet deep. The entire collapse takes less than a minute. n
References on Bomb-Resistant Design (Publications by Dr. Eve E. Hinman)
“Defensive Architecture.” Skylines, May 1995, Building Owners and Managers Association, pp. 14-15, Washington, D.C. and AILArchitect, May 1995, American Institute of Architects, p. 17, Washington, D.C.
With M. Levy, “Protecting Buildings Against Terrorism.” Fire Engineering, December 1993, pp. 105-106.
“Protective Design Approach for Civilian Structures.” March 1989, Proceedings, ASCE Specialty Conference on Structures for Enhanced Safety and Physical Security, Arlington, Virginia, pp. 191-202.
“Upgrading Facilities for Changing Threats.” May 1987, Presentation, Protective Design Conference, Institute of Security Design, Washington, D.C.
“Hardening Buildings Against Blast.” May 1986, Proceedings, Securing Installations Against Car-Bomb Attack Symposium, Defense Research Institute, Washington, D.C., pp. 1015-1027.
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EVE E. HINMAN, Eng.Sc.D., a senior staff engineer with Failure Analysis Associates, is a specialist in designing structures for dynamic loads, particularly those caused by explosions. Her experience includes the design of nuclear missile silos, NATO military facilities, industrial buildings subject to accidental explosion, and civilian buildings vulnerable to terrorist attack. She has worked with the U.S. Department of State on the development and implementation of structural guidelines to provide for the blast-resistant design of nearly a dozen U.S. embassy buildings. She also has worked with the Port Authority of New York and New Jersey to upgrade protection provided at the World Trade Center.
