Page 7

2
Terrorism: Its Motives, Methods, and Immediate Results

Two years before the April 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, the February 1993 World Trade Center bombing had already marked a watershed in America's perceived vulnerability to terrorism1 on its own soil. Prior to the World Trade Center bombing, modern terrorism was generally regarded as something that happened elsewhere: a problem of the unsettled Middle East and Latin America, which on occasion spilled over into the streets of London or other major cities. While Americans have often been targeted by terrorists abroad, this attack demonstrated that they can no longer believe themselves immune to terrorist violence within their own borders. The Oklahoma City bombing showed that the United States continues to be vulnerable to terrorist attacks.

Three bombings in July 1994 also underscored the attractiveness and vulnerability of civilian buildings as terrorist targets. On July 18, 1994, a massive car bomb destroyed the Jewish Community Center in Buenos Aires. One week later, another car bomb exploded outside a London apartment adjacent to the heavily guarded Israeli Embassy. Less than 12 hours later, a London building housing a number of Jewish community organizations was the target of still another car bomb.

This chapter presents a background on the motives, methods, and immediate results of terrorist activities, including such topics as statistical patterns of recent

1The State Department defines ''terrorism'' as premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents, usually intended to influence an audience.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 7
Page 7 2 Terrorism: Its Motives, Methods, and Immediate Results Two years before the April 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City, the February 1993 World Trade Center bombing had already marked a watershed in America's perceived vulnerability to terrorism1 on its own soil. Prior to the World Trade Center bombing, modern terrorism was generally regarded as something that happened elsewhere: a problem of the unsettled Middle East and Latin America, which on occasion spilled over into the streets of London or other major cities. While Americans have often been targeted by terrorists abroad, this attack demonstrated that they can no longer believe themselves immune to terrorist violence within their own borders. The Oklahoma City bombing showed that the United States continues to be vulnerable to terrorist attacks. Three bombings in July 1994 also underscored the attractiveness and vulnerability of civilian buildings as terrorist targets. On July 18, 1994, a massive car bomb destroyed the Jewish Community Center in Buenos Aires. One week later, another car bomb exploded outside a London apartment adjacent to the heavily guarded Israeli Embassy. Less than 12 hours later, a London building housing a number of Jewish community organizations was the target of still another car bomb. This chapter presents a background on the motives, methods, and immediate results of terrorist activities, including such topics as statistical patterns of recent 1The State Department defines ''terrorism'' as premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents, usually intended to influence an audience.

OCR for page 7
Page 8 terrorist acts and the types of damage to structures and critical building systems, and of injuries that can be expected after a bomb detonates. Motives for Terrorist Attacks Terrorists' specific motives for attacking buildings are diverse, but they might be grouped into the following categories: • to obtain publicity for the terrorist group and its cause, • to exert political pressure or make a symbolic statement, • to destroy some asset within or part of the building, • to achieve financial gain through ransom or extortion, • to advance a religious imperative, • to kill or injure occupants, and/or • to seek vengeance or revenge. Publicity appears to be the most common objective, although any particular terrorist attack often combines several of the above-listed motives. While the actual intent of the World Trade Center bombers remains unclear, a major motive was no doubt publicity, together with a protest of U.S. support for conservative Arab governments and Israel, as the bombers explained in a letter they later sent to the New York Times claiming credit for the bombing. In addition to a quest for publicity, a major motive for a terrorist attack is to exert political pressure or to make a symbolic statement. For this reason, buildings are often the objects of terrorist attacks because of their specific occupants. Targeted tenants in the U.S. and worldwide have included governmental agencies, such as in the Oklahoma City bombing, storefront military recruiting stations, diplomatic and consular facilities, post offices, defense contractors, banks, corporate offices, and commercial establishments. Terrorists rarely appear to target a building simply to destroy the building itself or to damage some asset inside. While there is some indication that the World Trade Center bombers actually intended to bring down one of the twin towers (Wald, 1993), and the suicide bombers who attacked the U.S. facilities in West and East Beirut in the early 1980s sought—among other objectives—to destroy those structures, the damage sought in these and similar cases generally appears to have been a secondary goal, the primary goal being publicity and the exercise of political pressure. Perhaps the first well-publicized incident in which terrorists sought to destroy some specific asset inside a building was the 1946 bombing of Jerusalem's King David Hotel in what was then British-ruled Palestine (Hoffman, 1983). Jewish terrorists targeted the hotel, where the headquarters of the British administration and military command for Palestine was located, to destroy vital Jewish documents that had been seized by the British Army a month earlier and were believed to be stored on two floors of the west wing.

OCR for page 7
Page 9 Terrorists as well as nonpolitically motivated individuals have also targeted commercial buildings purely for economic gain. Terrorist organizations and criminal gangs in Latin America and Northern Ireland, for example, regularly extort protection money from building contractors, owners, and tenants (Adams, 1986). In some cases, the extortion amounts demanded, and the damage caused to the property of those who refused to comply, have been extraordinarily high. In August 1980, for instance, after a Nevada casino did not respond to extortion demands, a homemade bomb caused more than $12 million in damages to the casino (Center for the Study of Terrorism and Political Violence, n.d.). In many bombing incidents, it is often difficult to single out a specific motive for the attack. Although terrorist attacks on civilian buildings do not generally appear to have the sole intention of killing or injuring the occupants, in several instances, such as the bombings of the Alfred P. Murrah Federal Building in Oklahoma City and the Jewish Community Center in Buenos Aires, that was certainly a result. Similarly, attacks with apparent religious or vengeance overtones such as the World Trade Center bombing may have other motives as well. For example, suspicions have been raised that the World Trade Center bombing was masterminded by politically motivated agents who have little in common with the religious movement to which the convicted bombers belonged. Should this be the case, it would suggest that one organization with a particular agenda may manipulate another group with different objectives and motivations to carry out the act. Patterns of Terrorist Attacks Bombing has long been the most common terrorist tactic. Bombings account for nearly half (46 percent) of all international terrorist attacks carried out since 1968, a proportion that has rarely fallen below 40 percent or exceeded 50 percent in any year.2 Terrorists' reliance on bombing is not surprising, given that bombs provide a dramatic, yet fairly easy and often relatively risk-free (to the perpetrator) means of drawing attention to the terrorist cause. Few skills are required to manufacture a crude bomb, surreptitiously plant it, and then be miles away when it explodes. Terrorist attacks typically involve only one or two persons and, in general, do not require the same organizational expertise, logistics, and knowledge required of more complicated or sophisticated operations such as kidnapping, barricade and hostage situations, assassination, or assaults against defended targets. Armed attacks on buildings (including assaults with automatic weapons as well as hand grenades, bazookas, and rocket-propelled grenades, drive-by 2Forty-four percent of all terrorist attacks between 1968 and 1969 involved bombings, 53 percent involved bombings in the 1970s, 49 percent in the 1980s, and 39.5 percent between 1990 and 1993 (Center for the Study of Terrorism and Political Violence, n.d.)

OCR for page 7
Page 10 shootings, arson, vandalism, and sabotage other than bombing) represent a distant second to bombing, accounting for 22 percent of all terrorist operations since 1968.3 Not surprisingly, the frequency of various types of terrorist attacks tends to decrease in proportion to the complexity or sophistication they entail. Hijackings are the third most common tactic, accounting for 12 percent of terrorist attacks,4 followed by assassination (6 percent),5 and kidnapping (1 percent).6 The Sarin gas attack in Tokyo in 1995 represents a new method of attack, but it is too early to assess the likelihood of an increase in that mode of terrorism. The choice of bombing as the preferred terrorist tactic in the United States is evidenced by Federal Bureau of Investigation statistics. Between 1982 and 1992, bombings accounted for 77.5 percent of all terrorist incidents in this country.7 Most of the targets were commercial establishments (36 percent of all incidents), followed by military personnel and facilities (20 percent), federal and state government office buildings and property (19 percent), private residences (11 percent), diplomatic establishments (10 percent), and educational establishments (4 percent) (FBI, 1993). Explosive-incident statistics compiled from the latest report by the U.S. Department of Treasury's Bureau of Alcohol, Tobacco and Firearms (ATF) are shown in Table 2-1 by type of target (ATF, 1993). These figures augment the FBI statistics by including bombing attempts in addition to actual bombings (explosive and incendiary) by both terrorist and nonterrorist perpetrators for the period 1989–1993.8 Bombings of commercial buildings figure prominently in these statistics. Ease of access, entry, and escape figure prominently in terrorists' selec- 3Armed attacks on buildings accounted for 18 percent of all terrorist attacks between both 1968 and 1969 and during the 1970s, 19 percent in the 1980s, and 32 percent between 1990 and 1993 (Center for the Study of Terrorism and Political Violence, n.d.). 4Hijackings accounted for 33 percent of all terrorist attacks between 1968 and 1969, 7 percent in the 1970s, 4 percent in the 1980s, and 12 percent between 1990 and 1993 (Center for the Study of Terrorism and Political Violence, n.d.). 5Assassination accounted for 3 percent of all terrorist attacks between 1968 and 1969, 9 percent in the 1970s, 13 percent in the 1980s, and 13 percent between 1990 and 1993 (Center for the Study of Terrorism and Political Violence, n.d.). 6Kidnapping accounted for just .01 percent of all terrorist attacks between 1968 and 1969, 9 percent in the 1970s, 10 percent in the 1980s, and 6 percent between 1990 and 1993. There were no barricade and hostage situations recorded between 1968 and 1969, though they accounted for 3 percent of all terrorist incidents during the 1970s, and just 1 percent in both the 1980s and between 1990 and 1993 (Center for the Study of Terrorism and Political Violence, n.d.). 7Followed by kidnapping (6 percent), arson (5 percent), robbery (3 percent), malicious destruction of property and hostile takeovers (2 percent each), acts of sabotage (1 percent), and hijacking (1 percent) (FBI, 1993). 8The data include bombings carried out by persons motivated by economic (profit) reasons, vengeance, or other personal grievances, rather than political causes and grievances. It is the political motivation that generally defines a terrorist from a nonterrorist act of violence.

OCR for page 7
Page 11 tion of sites and methods of attack. In choosing a target, terrorists assess buildings' physical layouts and locations, more specifically, the patterns of public vehicular and foot traffic into and out of a building, physical security measures and visitors' entrance procedures, and the existence of multiple entry and exit points. The lightly guarded public parking lot beneath the World Trade Center was very likely a factor that made this structure—in contrast to the considerably older Empire State or Chrysler buildings, which lack such facilities—operationally attractive to the terrorists who bombed it. Similarly, the unrestricted public access in front of the Murrah Building in Oklahoma City and its relatively short setback distance from the street may have influenced the choice of that target. TABLE 2-1 Analysis of Bombing Incidents in the United States by Target, 1989–1993 (Deaths, Injuries, and Number of Incidents)   Killed Target 1989 1990 1991 1992 1993 Total Residential 15 10 13 15 21 74 Commercial 2 1 1 0 9 13 Vehicles 8 1 8 5 6 28 Educational 0 0 0 0 0 0 Mail Boxes 0 0 0 0 0 0 Open Areas 2 5 3 0 5 15 Utilities 0 0 0 0 0 0 Law Enforcement 0 1 0 0 0 1 State and Local Governments 0 1 0 0 0 1 Federal Government 0 0 0 0 1 1 Banks 0 0 0 0 0 0 Military 0 0 0 0 0 0 Airports and Aircraft 0 0 0 0 0 0 Apartmentsa 0 0 0 4 4 8 Religious Facilitiesa 0 0 0 0 0 0 Energy Facilitiesa 0 0 0 0 0 0 Parksa 0 0 0 0 2 2 Medical Facilitiesa 0 0 0 0 0 0 Otherb 6 3 2 2 1 14   TOTAL 33 22 27 26 49 157 aNew target category added in 1992. bOther category does not include accidental/noncriminal explosives incidents. cStatistical information for 1992 and 1993 encompasses actual and attempted bombings. The years prior reflect only functioned bombs and incendiary devices. SOURCE: ATF (1993). (table continued on next page)

OCR for page 7
Page 12 (table continued from previous page)   Injured Target 1989 1990 1991 1992 1993 Total Residential 114 64 84 112 99 473 Commercial 52 69 34 60 1075 1,290 Vehicles 26 38 27 22 26 139 Educational 15 11 41 73 29 169 Mail Boxes 1 5 3 3 14 26 Open Areas 77 41 26 26 44 214 Utilities 0 1 1 0 0 2 Law Enforcement 4 2 3 4 8 21 State and Local Governments 1 2 1 2 0 0 Federal Government 0 0 1 0 2 5 Banks 0 1 0 3 0 4 Military 0 0 0 2 1 3 Airports and Aircraft 0 0 0 0 0 0 Apartmentsa 0 0 0 35 17 52 Religious Facilitiesa 0 0 0 14 16 30 Energy Facilitiesa 0 0 0 0 0 0 Parksa 0 0 0 1 3 4 Medical Facilitiesa 0 0 0 0 0 0 Otherb 14 17 25 6 2 64   TOTAL 305 251 246 349 1,323 2,474 (table continued on next page) A building's physical location may similarly be a critical factor in target selection. Terrorists may be unable to gain access to their primary target site and therefore must settle for detonating a bomb inside or outside an adjacent building. A block of apartments in London's exclusive Kensington Palace area was bombed in July 1994 apparently because of its location adjacent to the well-protected Israeli Embassy. Several facts suggest that the terrorist threat to buildings will remain significant. First, there is the relative ease with which access can be gained to commercial establishments. Such structures are necessarily open to the public, are subject to heavy daily pedestrian and vehicular traffic, and generally have few formidable access barriers. In contrast, other attractive terrorist targets—such as embassies, consulates, and military facilities—are often better protected and guarded, and sometimes physically and structurally reinforced. The latter are deliberately formidable targets, but an unintended effect of their "hardening" may be to displace the terrorist threat onto more vulnerable structures, such as commercial buildings. Another reason the terrorist threat to buildings will likely persist, as the Oklahoma City, the World Trade Center, and other numerous recent major bomb-

OCR for page 7
Page 13 (table continued from previous page)   Number of Incidents Target 1989 1990 1991 1992 1993c Total Residential 367 372 453 662 699 2,553 Commercial 205 262 297 369 335 1,468 Vehicles 284 294 286 426 408 1,698 Educational 76 86 93 151 167 573 Mail Boxes 204 352 495 789 872 2,712 Open Areas 81 124 91 126 146 568 Utilities 27 25 37 38 16 143 Law Enforcement 14 17 15 38 24 108 State and Local Governments 14 17 38 50 36 155 Federal Government 11 7 9 11 10 48 Banks 8 16 17 16 15 72 Military 4 2 8 5 8 27 Airports and Aircraft 2 1 3 2 2 10 Apartmentsa 0 0 0 146 98 244 Religious Facilitiesa 0 0 0 14 16 30 Energy Facilitiesa 0 0 0 4 7 11 Parksa 0 0 0 45 44 89 Medical Facilitiesa 0 0 0 12 14 26 Otherb 87 89 157 85 63 481   TOTAL 1,384 1,664 1,999 2,989 2,980 11,016 ings demonstrate, is that attacks on buildings (especially by bombing) are a proven means for terrorists to attract attention to themselves and their causes. It is interesting to note that simple homemade devices fabricated by amateurs have proven just as destructive and lethal as more sophisticated terrorist weapons. The explosive device used at the World Trade Center, for example, was made out of ordinary, commercially available materials—including lawn fertilizer (urea nitrate) and diesel fuel—and cost less than $400 to make.9 The device used in the Oklahoma City bombing was likely of similar construction. The 9The World Trade Center bomb was composed of some 1,200 lbs of common sulfuric and nitric acids used in dozens of household products and urea used to fertilize lawns. The detonating device was a more complex and extremely volatile mixture of nitroglycerine enhanced by tanks of compressed hydrogen gases that were designed to increase the force of the explosion (see Barnes. 1994: Bernstein. 1994a–d: and Morganthau. 1994). Similarly, in April 1988 a Japanese Red Army terrorist was arrested while en route to New York City on a bombing mission. Found in his possession were gunpowder, hollowed-out fire extinguishers in which to place the explosive materials, and roofing nails as crude anti-personnel weapons (see BRI. 1988: Hanley, 1988).

OCR for page 7
Page 14 sophistication of terrorist weapons, then, especially bombs and explosive devices, may well be in their simplicity. Unlike military ordnance, such as plastic explosives for example, the materials used in homemade bombs are readily commercially available; they are thus perfectly legal to possess until actually concocted or assembled into a bomb. These ordinary materials are also far more difficult for authorities to trace or for experts to obtain a "signature" from. For foreign governments seeking to commission terrorist attacks, such homemade bombs also help enable the state sponsor to avoid identification and any possible military retaliation or international sanction. Furthemore, terrorists can improve their methods readily. As the World Trade Center bombers included a chemical engineer in their operations, future terrorists targeting a building may, for example, include a structural engineer in their plans to help ensure the collapse of the building. Terrorists study the "lessons" of comrades and other terrorist groups, and are often more sophisticated in their operations and know more about security-force tactics and countermeasures than their predecessors. Finally, again, the increase in internal security at many buildings in the wake of recent bombings may not eliminate the terrorist threat to buildings, but rather displace it to more accessible buildings. Bomb Damage to Buildings and Occupants A bomb explosion within or immediately nearby a building can have catastrophic effects, destroying or severely damaging portions of the building's external and internal structural framework, collapsing walls, blowing out large expanses of windows, and shutting down critical fire- and life-safety systems, such as fire detection and suppression, ventilation, light, water, sewage, and power. Loss of life and injuries to occupants can result from many causes, including direct blast-effects, debris impact, fire, and smoke. The indirect effects can combine to inhibit or prevent timely evacuation, thereby contributing to additional casualties. The following sections describe examples of these damaging effects. Bomb damage to buildings depends, of course, on the type of building and the nature of the explosive device and its location relative to the building. Other factors determining damage relate to the specifics of the building's design and the immediate surroundings and the location and disposition of its occupants. The first section below describes the structural damage to a building from the direct effect of an explosion. Collateral damage to building subsystems is discussed, and the final section describes how injuries are sustained by people within the building. The nature of explosions and a more quantitative discussion of their induced physical behavior is presented in Chapter 3.

OCR for page 7
Page 15 Structural Damage Recent terrorist attacks against commercial buildings dramatically illustrate the influence of bomb placement and building design on the nature and extent of direct structural damage. The devastating car bomb attack against the Jewish Community Center in Buenos Aires (July 1994) illustrates the damage potential of an external explosion against a multistory building of masonry load-bearing construction. A similar attack against a multistory office building of more modern concrete column and slab construction at St. Mary Axe in London, (April 1992) inflicted a different type and level of damage. In marked contrast, a much larger car bomb detonated in an underground garage of the World Trade Center, a modern steel and concrete skyscraper (February 1993) did substantially less structural damage than was probably intended. The structural consequences of these terrorist attacks and bombings in Staples Corner, London, and Oklahoma City are briefly reviewed here. Jewish Community Center, Buenos Aires On July 18, 1994, a van loaded with about 275 kg of high explosive was detonated in front of the Jewish Community Center located in a densely constructed area of Buenos Aires. The explosive is thought to have been arranged to focus the blast on the building, 3 to 5 meters away. The exterior walls of this five-story building were of brick masonry construction, which supported the floor slabs. The air blast from the bomb totally destroyed the exposed load-bearing walls which, in turn, led to progressive failure of the floor slabs and virtually total collapse of the building. Figure 2-1 illustrates the resulting damage to the building. Such wall-bearing buildings are notable for their tendency to be brought down in this manner by localized damage. St. Mary Axe, London A car bomb containing an estimated 350 kg of TNT10 was detonated in the densely built-up St. Mary Axe section of London near midnight on April 11, 1992, causing extensive damage to a number of neighborhood buildings. Figure 2-2 is a photograph of the damaged European Bank for Reconstruction and Development, a 10-story tower block atop a three-story pedestal, located at an estimated 115–160 m from the car bomb. This building was of modern concrete column and slab construction with nonbearing masonry walls on the lower three 10This estimate was obtained from newspaper accounts and does not necessarily represent official estimates.

OCR for page 7
Page 16 Figure 2-1 The Jewish Community Center, Buenos Aires, showing vulnerability of brick masonry construction. Source: Embassy of Argentina, Washington, D.C.

OCR for page 7
Page 17 Figure 2-2 St. Mary Axe, London, showing general damage to multistory office build- ing of the European Bank of Reconstruction and Development. Photo reproduced courtesy of Safe Special Services Group.

OCR for page 7
Page 18 Figure 2-3 St. Mary Axe, London, showing potentially lethal glass shards. Photo reproduced courtesy of Safe Special Services Group. floors. It was directly shielded from the explosion by another building and did not suffer significant structural damage; nevertheless, there was extensive glass damage and attendant hazard potential to personnel.11 Window damage to the bank building illustrates the influence of glass size, strength, and orientation. Windows were completely broken on the two up-wind faces and survived almost completely on the down-wind faces.12 All large windows in the first-floor pedestal (1.5 m × 2.8 m, 10-mm annealed glass) were blown in, and glass shards were thrown into adjacent offices to a distance of about 3 m, as illustrated in Figure 2-3. This office either did not have venetian blinds or the blinds were not lowered at the time of the explosion. Where blinds were in place in other offices, they effectively reduced the fragment hazard by 11Because the explosion occurred just before midnight when the offices were unoccupied, it is estimated that injuries were less extensive than if the explosion had occurred during the day. 12Windows that blew out on the down-wind faces were designed as smoke vent windows and were actuated by the internal explosion pressure.

OCR for page 7
Page 19 Figure 2-4 Staples Corner, North London, showing damage to a single-story steel- framed warehouse. Photo reproduced courtesy of SAFE Special Services Group. capturing many of the glass shards and reducing the throw of others. Similar glass fragment behavior was observed on upper floors as well. It is also evident from this photograph that the blast effects of the explosion on the interior were of very low intensity, since only negligible disturbance of desk papers occurred. The only windows to survive on the up-wind face of the building were the second-floor podium windows (2.5 m × 2.5 m) that were made of toughened, 10-mm thick, double-glazed glass, and the picture windows on either side of the main entrance, which were laminated and survived the explosion but were crazed. Basement windows at street level were 33-mm-thick laminated glass and survived without crazing. Staples Corner, North London On April 11, 1992, an explosion occurred at Staples Corner. Figure 2-4 shows the resulting damage to a single-story steel-framed warehouse measuring 56 m × 57 m. The building's nearest corner was 17 m from the explosion and its farthest corner 95 m away. The building was clad with light-profiled steel sheeting on light purlins in the walls and roof and is a good example of frangible construction, that is, where the resistance of the cladding is much less than that of the supporting framework. Severe damage occurred to the cladding, sheeting, purlins, blockwork lining wall, and finishes and fittings in areas nearest the blast. However, the main steel frame sustained only minor damage. The stripped sheeting generally rebounded outward toward the bomb. World Trade Center, New York City The terrorist attack against the 110-story World Trade Center on February

OCR for page 7
Page 20 26, 1993, involved a much larger car bomb than the London or Buenos Aires attacks. While damage and injuries in the World Trade Center bombing were extensive, they differed substantially from those in the other cases because of differences in bomb placement and building structure. The bomb, estimated to be about 900 kg of high explosive, was detonated against the south wall of the north tower in an underground garage two levels below grade. The most severe structural damage occurred to the subgrade levels, with the bomb crater measuring 24 m to 36 m across on some of the levels. Two levels of reinforced concrete slabs, about 280 mm thick, were blown out and the debris completely covered refrigeration equipment on the fifth subgrade level, rendering it inoperable. A shock wave propagated throughout the subgrade structure, causing the parking-level slabs to fail at column capitals and other ''hard points.'' Steel columns that supported both the adjacent Vista Hotel and parts of the World Trade Center Plaza and concourse area, that before the explosion were braced by the closely spaced parking levels, stood naked as high as 21 m without definable lateral support. At the crater's edge, the slabs had sheared free of their supporting columns, settling several feet to form "ski jumps" into the crater. Elsewhere, multi-ton portions of concrete were literally dangling from reinforcing steel. Figure 2-5 shows the aftermath of the explosion at the level of the detonation. The photograph was taken at the south end of the parking garage; the explosion occurred at the north end. Segments of the masonry wall along the Figure 2-5 World Trade Center, New York City, showing the aftermath of the explo- sion within the parking garage. Source: ATF (1993).

OCR for page 7
Page 21 south wall of the north tower were blown into the building; masonry work surrounding elevator shafts was blown into those shafts, causing the air locks to be breached, particularly for the north tower. With the loss of the air locks, smoke and dust-laden air was drawn into and upward through the towers by stack action, a phenomenon discussed in Chapter 4. Breaching of the air locks accounted for the majority of personnel injuries, as discussed later in this chapter. While the intent of the terrorists may have been to bring down the tower, its structural integrity was never threatened. This achievement undoubtedly can be attributed to the ductility of the structural steel shell and the conservative requirements used in its design: to withstand a 240-kph wind storm, the loss of perimeter columns by sabotage, and the impact of a fully fueled 707 aircraft anywhere along the tower height. A more immediate reason the tower did not collapse is because the lateral (horizontal) blast pressure was not large enough to cause the column to fail in shear or combined axial load and bending. The adjacent column bracing and floor framing did fail due to the blast pressure. The adjacent Vista Hotel, less massively constructed, did sustain extensive damage, which threatened its structural integrity. Alfred P. Murrah Federal Building, Oklahoma City The bombing of the Federal Building in Oklahoma City on April 19, 1995, was the largest such terrorist attack in the United States to date (ENR, 1995). A car bomb, estimated to contain about 1,800 kg of high explosive and located 3–5 m from the north face of the building and about 12–15 m from the east end, caused 168 fatalities, numerous injuries, and an estimated $50 million in damage to about 75 buildings in the area. The Murrah Building was a nine-story tower of reinforced concrete slab and column construction measuring 61.5 m × 21.5 m. The frame had 10.7 m × 6.2 m typical bays, created by a column line along each face and one down the center. Four of the north-face columns, spaced at 12.3 m and unsupported for two stories, formed an atrium at street level. A 61.5-m-long third-floor spandrel beam transferred loads from the columns on floors above to the 8-m-tall exposed columns. The explosion destroyed three of the four front columns and a centerline column. With four columns shattered, the upper floors toppled northward as the 200-mm-thick slabs separated from centerline columns. As a consequence, 8 of the 10 bays along the northern half of the building collapsed into a heap. In the southern half of the building, two bays collapsed on either side of the failed center column. Inside the south entrance, slabs collapsed in adjacent bays on two floors. Photos are shown in Figures 2-6 and 2-7. Damage to Building Subsystems Certain building subsystems, if lost, render the building unable to protect the occupants or assist in their survival and otherwise make the building uninhabit-

OCR for page 7
Page 22 Figure 2-6 Alfred P. Murrah Federal Building, Oklahoma City, showing catastrophic effects on the building's north face. Figure 2-7 Alfred P. Murrah Federal Building, Oklahoma City, showing detail of the reinforced concrete column and slab con- struction.

OCR for page 7
Page 23 able or unusable. Typical of these subsystems are fire-detection and suppression systems; water and sewer service, including sanitation; means of egress, including corridors, stairs, lobbies, and exit doors; elevators; primary and emergency electrical systems; and rescue operation systems, including voice and data communications, ventilation, and smoke control. A bomb detonation inside a building's parking garage probably would cause the most serious damage to building subsystems simply because several critical subsystems originate there, along with much of the control and distribution equipment. A garage-level detonation has a significant potential for fire and smoke production because the parked vehicles contain large amounts of combustible materials. Also, the fire-suppression system would likely be made inoperable, since it is exposed and very fragile. The World Trade Center bombing, unfortunately, was a very good example of these observations: extensive damage occurred to communications, life-safety, electrical, and mechanical systems; the emergency generator plant shut down because of loss of cooling water; the elevator and stair shafts were breached; smoke from burning automobiles on the parking levels was forced up the shafts of both towers; and the underground tower's operations control center was put out of commission, leaving building occupants without important information. Street-level explosions can also cause serious damage to critical building subsystems. Most urban office buildings have extensive street-level fenestration consisting of glass panes, some as large as 2 m × 4 m set in narrow aluminum extrusions. This assembly typically has little or no blast resistance, and a blast wave can enter the street-level lobby area virtually unattenuated. Consequently, extensive damage can occur to the fire control room usually located near the lobby, to elevators, to egress stairways, and to the service risers that pass through the street-level floor. Service risers generally are concentrated in one of a few vertical shafts or "chases" that rise up through the core of the building, usually contiguous with the elevator shaft(s). Typically they contain the heating and cooling hydraulic lines, domestic and sanitary systems, electrical distribution and communications lines, fire standpipes, and supply/return ducting and condenser water lines. Building codes require that these chases be fire-rated, but they are generally constructed of relatively light materials. Their blast resistance typically is poor and at least some of the service risers would likely be severed. The lateral distribution of services on the upper floors is accomplished by equipment and conduits above the ceilings of occupied spaces—systems that can be damaged by a street-level depending on the location of the service risers relative to the point of detonation. The ceilings are commonly constructed of loose fiber tiles laid in a lightweight gridwork and offer little to no protection. Blast damage to the elevators from either a garage- or street-level detonation is usually extensive and most disruptive to occupancy in mid- to high-rise buildings. Elevator doors may collapse into the hoistway, and there will be damage to hoistway components directly from the explosion and from flying structural de-

OCR for page 7
Page 24 bris. Elevator cabs that are at rest on the floor where the explosion occurs will be damaged, perhaps beyond repair. In multi-elevator banks, damage to only one cab or hoistway may cause the electrical and mechanical safeties to shut down the entire system, thereby preventing the surviving cabs from being operated manually or on fire service. Stairways used to evacuate the building in the event of a fire or explosion are similar in construction and vulnerability to the elevator shafts and would experience similar damage from a street-level blast. Collapse of the stairway shaft walls may make the stairway impassable, impeding or even preventing evacuation, and the situation is made still more serious because the main lobby level is often one means of egress from the building. Hazards to People Personnel injuries and loss of life can result directly from the bomb's explosion; blast pressure, impact of high-speed glass fragments or other structural debris, collapse of structural members, fire and smoke inhalation, or a variety of other causes associated with the general confusion that may follow an explosion and a possibly prolonged evacuation period. If the explosion is sufficiently close to a wall or floor, there can be gross disintegration, with either spalled pieces on the back side or the wall materials themselves being propelled as missiles. These missiles can injure people and damage property, and if structural support is sufficiently disrupted, the building may collapse. Except when people are trapped in collapsed building spaces, most injuries occur from missile penetration or from smoke inhalation. Breaching of elevator and stairwell doors (more likely for street-level explosions ) allows smoke to migrate upwards into the building, carried by the building's stack effect during winter months. Elevators are likely to be occupied throughout the day, and persons may be trapped within them, either as a result of damage to the elevator shaft or hoists, or as a result of loss of power or controls. In the World Trade Center bombing, the north tower air locks were destroyed and smoke and dust-laden air was forced to the upper floors, accounting for most of the over 1,000 personnel injuries. References Adams, J. 1986. The Financing of Terror. London: New English Library. ATF (Alcohol, Tobacco, and Firearms). 1993. Explosives Incidents Report. Washington, D.C.: U.S. Department of the Treasury. Barnes, E., et al. 1994. The $400 Bomb. Time, March 22. Bernstein, R. 1994a. Expert Can't Be Certain of Bomb Contents at Trial. New York Times, January 21. Bernstein, R. 1994b. Nitroglycerin and Shoe at Center of Blast Trial Testimony. New York Times, January 27. Bernstein, R. 1994c. Witness Sums Up Bombing Evidence. New York Times, February 7.

OCR for page 7
Page 25 Bernstein. R. 1994d. Lingering Questions on Bombing: Powerful Device, Simple Design. New York Times, September 14. BRI (Business Risks International). 1988. Risk Assessment Weekly 5(2):July 22. Center for the Study of Terrorism and Political Violence. (n.d.). The RAND-St. Andrews University Chronology of International Terrorism. St. Andrews University, St. Andrews, Scotland. ENR (Engineering News-Record). 1995. OKLAHOMA CITY: The Blast, The Repercussions, and a Special Report on Defensible Space. The McGraw-Hill Construction Weekly. May 1. FBI (Federal Bureau of Investigation). 1993. Terrorism in the United States, 1982–1992. Terrorist Research and Analytical Center, Counterterrorism Section, Intelligence Division. Washington, D.C.: U.S. Department of Justice. Hanley, R. 1988. Suspected Japanese Terrorist Convicted in Bomb Case in New Jersey. New York Times, November 29. Hoffman, B. 1983. The bombing of the King David Hotel. Midstream (Aug./Sept.):61–62. Morganthau, T. 1994. A Terrorist Plot Without a Story. Newsweek, February 28. Wald, M.L. 1993. Figuring what it would take to take down a tower. New York Times, March 21.