IN THE OKLAHOMA CITY BOMBING, 35 PERCENT OF THE SURVIVORS SUSTAINED AUDITORY DAMAGE. A LARGE PERCENTAGE OF THEM WERE NOT RESOLVED AS LATE AS NINE MONTHS LATER.
BY NORMAN J. GLOVER, P.E., R.A., FASCE FIStructE
Among architects and engineers, mitigating the effects of terrorist attacks has centered on mitigating the effects on buildings and infrastructure. Perhaps this is because of professional interest or because considering the injurious effects of violence on the human body is too intimidating. However, the real objective of such study is and should be protecting people from injury and death.
The principal explosion-related injuries that affect building occupants and those in the building’s vicinity result from the following explosion effects.
Blast. The blast produces overpressure (i.e., pressure generated by explosion above normal atmospheric pressure) and a dynamic impulse (overpressure’s duration over time); injury severity depends on the duration of the overpressure.
Fireball. Fireball-related injuries result from engulfment and thermal radiation; persons engulfed in a fireball are presumed dead.
Missiles. Primary missiles from the explosion include fragments from the explosive device casing or vehicle, in the case of a car bomb. Secondary missiles include fragments from the crater created by explosion and fragments the blast sets in motion.
Additional causes of death and injury include building damage (i.e., falling debris and shattered glass), building collapse, and the crater created by the explosion. Hot fragments from the blast may ignite secondary fires that may also result in secondary explosions.
Based on wartime and terrorism experience, in a built-up area, deaths and injuries will be proportional to the effective charge weight (see Figure 1). A British study found a good correlation between charge weight and bodily injury and between charge weight/impulse and building damage. Although this is based on a housing damage model, analysis of air raid damage during World War II showed a similar probability of injury. Battle wounds unrelated to explosion in World War II, the Korea War, and Vietnam can be divided into three categories of typically equal numbers: one-third moderate, one-third severe, and one-third fatal.
Tables 1 and 2 show the range of direct blast effects of explosions and human injury in proximity to a small bomb.
Note that the blast from a relatively small bomb can cause death or serious injury at close range. However, blast pressures decay rapidly with distance. As has often been stated, standoff distance is the best defense to blast effects. Even a 100-pound (TNT equivalent) bomb is survivable at a distance of 100 feet, while the blast pressure of a 500-pound bomb at 100 feet will likely cause serious injury or even death.
Ear. The healthy ear responds well to sounds over a wide range of frequency at low energy levels. It is unable to respond to a pulse of less than 0.3 milliseconds, and the attempt to do so causes rupture. The response is governed by overpressure duration, peak, and rapidity of rise. In the Oklahoma City Bombing, 35 percent of the survivors sustained auditory damage. A large percentage of these cases were not resolved as late as nine months after the event and must be considered permanent.
Lungs. Hemorrhage is the primary blast effect on lungs because of overpressure and overpressure duration. The lungs are highly susceptible to serious damage, and injury is more severe if the body is oriented normal to the blast wind or located so that blast pressures are reflected or concentrated by hard surfaces (see Figures 2, 3).
Violent movement. A third cause of injury is the violent bodily movement of the victim by the blast wave. Figure 4 respresents the probability of victim survival in this situation. Injury is apt to result not only from the initial impact of the wave but also from the collision with a hard impact of the victim’s body with an object in the victim’s path-for example, a fatal blow or fracture to the head.
Thermal burns. In the Oklahoma City Bombing, nine persons had thermal burns covering up to 70 percent of their body surface; seven of those were near the point of detonation.
The susceptibility of the body to wounding or other medical complications is illustrated in Table 3.
As shown, the head, neck, thorax, and abdomen are the most vulnerable parts of the body. These account for 39 percent of the projected area of the body.
OKLAHOMA CITY BOMBING
In the Oklahoma City Bombing, 759 people sustained injuries, and 168 died. Victims in the collapsed portion of the building were significantly more likely to die (87 percent) than those in other parts of the building (5 percent). The most frequent cause of death was multiple injuries. The Oklahoma State Department of Health noted that most fatalities resulted from “blunt trauma and crushing” (probably as a result of the building collapse).
The explosive charge in Oklahoma City was so large that collapse was inevitable. However, in November 1997 at a meeting in Minneapolis, the Federal Emergency Management Agency/American Society of Civil Engineers (FEMA/ASCE) investigating team reported that a modest investment of $70,000 (1997 dollars) in additional reinforcing steel at the time of original construction could have reduced the extent of the collapse and presumable fatalities by 50 to 85 percent. With no architectural changes, 140 lives might have been saved for $500 each.
This coincides with a statement made in London at a joint meeting of the Institution of Civil Engineers and the Institution of Structural Engineers in October 1993. It said, in effect, that for approximately 0.9 percent of new construction cost, commercial buildings could be made reasonably resistant to blast-induced collapse. Unfortunately, a further statement at the time said that a similar level of protection against blast-generated shattered glass would cost at least 2 percent of building cost. Most practitioners feel this estimate is too low, while acknowledging that 65 to 80 percent of blast injuries are generated by flying glass.
Of the 1,787 nonfatal injuries at Oklahoma City, 516 were attributed to unknown causes. Identified causes included glass (light fixtures, window frames, and venetian blinds), 699, and unidentified flying debris, 155.
Other injuries were attributed to furnishings and finishes including ceiling material, 90; lighting fixtures, 45; furniture, 31; partitions, 5; and ductwork, 3.
Of the 29 survivors with brain injuries, the identified causes were flying/falling debris, 12; violent body movement, 5; unknown, 4; ceiling, 4; other, 2; and furniture, 1. A total of 80 persons were diagnosed with head injuries.
This establishes that glazing and interior building finishes are significant casualty generators. Falling ceilings, light fixtures, or debris account for 43 percent of the nonfatal head injuries. It appears from FEMA/ASCE team findings that at least in new construction, great strides have been made to mitigate the effects of terrorist violence through preventing progressive collapse. A great deal of effort has been mobilized to reduce casualties resulting from shattered glass. The next fruitful target is specifying safer building interiors. The Architectural Engineering Institute of the ASCE has formed a committee to codify procurement, specification, and installation of architectural building systems that are normally not covered by engineering standards.
NORMAN J. GLOVER, P.E., R.A., FASCE FIStructE, is executive director of the AEGIS Institute, a nonprofit foundation devoted to anti- and counter-terrorism technology, and past president/chairman of the Board of Governors of the Architectural Engineering Institute of the American Society of Civil Engineers. He previously chaired the Facilities Planning and Program Management Committees and Working Group on the Mitigation of the Effects of Terrorism of the Architectural Engineering Division of the American Society of Civil Engineers. A civil engineering graduate of Columbia University, Glover has served for more than 25 years on the School of Engineering and Applied Science Advisory Council. He has done graduate work at Columbia and Harvard Universities, the Naval War College, and the City University of New York. Glover is an adjunct professor of international hotel development at New York University.
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