On the night of June 25, 1996, a sewage tanker filled with explosives exploded in front of the Khobar Towers, a US Air Force residential complex in Dhahran, Saudi Arabia. The over 5,000 pounds of plastic explosives caused the complete failure of the northern facade and partial failure of the concrete floor slabs in that area, as shown in Figure 1. This massive explosion took the lives of 19 Americans and wounded 372 others (FBI, 2001 p. 1). Although the structure largely remained intact through the attack, the majority of the casualties were due to flying glass and debris. This attack was largely responsible for the creation of building codes specifically designed to prevent terrorist attacks and mitigate the damage caused to government buildings (Stevens, 2011 p. 4).
The Khobar Towers bombing, along with the 1983 bombing of a US Marine barracks in Beirut and the 1995 Oklahoma City Bombing, caused the United States Government to reconsider its building design codes and terrorism prevention measures. Draft Security Criteria, a 1997 General Services Administration (GSA) publication, was the first published building criteria in the US that prescribed progressive collapse and blast concerns to be addressed by designers. The Unified Facilities Criteria, published in 2005 by the Department of Defense, was created as a direct result of the Khobar bombings (Stevens, 2011 p. 4). These two government agencies used the lessons learned from this horrific event to write codes that will help to prevent future catastrophes.
Figure 1: Image showing the extent of the damage. From Department of Defense "News Photos"
Keywords
Terrorist Attack; Explosion; Blast; Facade Failure; Progressive Collapse; Department of Defense; General Services Administration
BUILDING DESIGN AND ATTACK
Building 131, which was attacked, was one building of a larger military complex named the Khobar Towers. These towers primarily served as residential units for the United States Airforce, but other structures in the compound housed non-military personnel, both American and Saudi. The entire complex stretched across 12 city blocks (Spence, 1996 p. 8). The attacked tower, along with the majority of the compound, was constructed from precast wall and floor systems. This redundant system later proved to save countless lives. In 1979 the Saudis constructed the Khobar Towers complex near the city of Dhahran in the Eastern Province of Saudi Arabia. The majority of the buildings remained unoccupied until the 1990 Gulf War when military forces from the United States, France and the United Kingdom, along with Saudi civilians moved in (GlobalSecurity.org, 2002). As can be seen in Figure 2, the US military occupied the Northern part of the complex and the entire compound was fenced off from unauthorized access.
Figure 2: Site plan of the Khobar Towers complex. Credited to GlobalSecruity.org
The 1984 bombing of a US Marine Corp facility in Beirut, Lebanon taught the military an important lesson: standoff distance. Soon after the US military began to occupy the Khobar Towers, a perimeter fence was erected around the compound (Byfield, 2006 p. 4). This fence consisted of a 7-8 foot high chain link fence, topped with barbed wire. Jersey barriers, illustrated in Figure 3, were placed in front of the chain link fence to prevent vehicular access. At Building 131, this fence was located approximately 30 meters or 100 feet from the Northern façade. Jersey barriers are common to highway construction and primarily rely on their 5,700 pounds of weight per 12 foot module to stop vehicles (FEMA, 2008 p. 4-31).
Building 131, along with most of the other buildings on the site, used precast load bearing walls as its primary structural system. The façade was also concrete precast panels, which were supported by the façade panel below. The modular architectural plan for this barrack allowed for the interior partitions to also be made from precast panels. The British reinforced concrete design code, BS8110, was used to design the structural system. At the time the British were the designers to address progressive collapse prevention because of the 1968 Ronan Point disaster. Using this code resulted in relatively strong connections between walls and slabs (Byfield, 2006 p. 4).
Figure 3: Illustration of a Jersey barrier. Credited to FEMA 430
At about 10:00 p.m. on June 25, 1996 rooftop sentries hurriedly awoke the US airmen in Building 131. These futile evacuation attempts did not succeed in removing people from the building because only 3 to 4 minutes after it was spotted, the suspicious sewage tanker detonated. The massive explosion destroyed the lower levels of the façade, which left the upper floors unsupported and led to their eventual collapse.
The investigation later reviled that three members of Saudi Hizballah terrorist organization drove a sewage tanker and a getaway car into a parking lot along the northern fence of the compound. The tanker was backed up to the fence and the attackers fled the scene. Rooftop sentries immediately became suspicious and began evacuation (Grant, 2006 p. 1).
Experts estimate that the truck was filled with at least 5,000 pounds of home-made plastic explosive made from fertilizers. This equates to 20,000 pounds of TNT and ranks as the most powerful detonated terrorist attack, even stronger than the explosion created by Timothy McVeigh in Oklahoma. (FBI, 2001 p.1) A 24.4 meter wide by 9.1 meter (or 80 feet by 30 feet) crater was created and can be seen in Figure 4. The blast sent debris and pressure waves crashing into Building 131’s side. The 5,700 pound concrete Jersey barriers were found in the first four floors of the building (Byfield, 2006 p. 2).
Figure 4: The 80’ diameter by 30’ deep crater left by the explosion. Credited to the Department of Defense
AFTERMATH
The majority of the structure withstood the attack relatively unscathed. Despite this, 19 American airmen lost their lives, 200 others were wounded and hundreds of Saudi soldiers and civilians were harmed (Spence, 1996 p. 5). Injuries can primarily be attributed to the northern facade failure and associated debris. Unlike several other terrorist attacks, the attack on the Khobar Towers did not result in a progressive collapse of the structure. The concrete partitions, shown in Figure 5, offered several redundant load paths for the forces to be redistributed One of the primary sources for explosion damage research comes from World War II. Francis Walley and Lord Baker surveyed bomb damaged structures in England and documented the failure mechanisms. They concluded that connection failures were the leading cause of complete building collapse and loadbearing wall structures tended to be more robust to explosions (Byfield, 2006, p. 3). By designing to the British codes, the connections were strengthened and withstood the blast.
Figure 5: A close up of the precast concrete wall system. Credited to Spence and the House National Security Committee
The most noticeable damage to Building 131 is the missing Northern façade. The explosion pressures, primary, and secondary debris (including the Jersey barriers) caused the lower levels of the façade to buckle into the building. The top three floors were no longer supported from below and proceeded to shear off the weak connections that tied them back to the floor slabs. The East and West facades were able to hold onto the building, but were displaced up to four feet from their original position (Grant, 2006 p. 2).
The façade failure created a much larger problem. The secondary debris from both the barriers and façade debris was the leading cause of injury and death. After the Oklahoma City Bombing a study was conducted, which determined flying glass and other projectiles was the leading cause of injury (Mallonee, 1996 p. 1). Medics on the site in Saudi Arabia encountered much of the same injuries.
In 2006 the FBI had enough evidence to bring 13 members of the pro-Iran Saudi Hizballah to justice. Among the charged are the three terrorists who parked the bomb next to the tower and Al-Mughassil, a high ranking leader in the organization (FBI 2001, p. 1). Immediately following the attack, many believed that Iran, itself, was behind the attack. Many feared that a US attack on the country would follow. The investigation did not reveal enough evidence to verify these allegations, but strong suspicions still remain.
CHANGES IN DESIGN CODES
This was not the first terrorist attack against Americans, but this attack was a key factor in the US government establishing design criteria that addressed it. In a three year span, from the first World Trade Center Attack in 1993 to the Khobar Bombing in 1996, three major attacks struck United States citizens. This shocked government officials pushed them into action. In October 1995, only months before the Khobar Bombings, President Clinton issued Executive Order 12977. This document ordered all federal facilities to address security and develop long-term construction standards, but gave no specifics or direction. Following this attack in Saudi Arabia, Congress directed the Department of Defense to develop codes explicitly for antiterrorist measures (Stevens, 2011 p. 2).
These proclamations had little effect on how federal and military buildings were designed until in 2000 the General Services Administration issued a design criteria for all federal buildings that addressed antiterrorism measures. The Department of Defense soon followed with their interim antiterrorism requirements. The 2005 release of the DoD’s Unified Facilities Criteria marked the point where design professionals had concrete guidelines for all Department of Defense buildings (Stevens, 2011 p. 3).
Both agencies’ criteria involve the three main areas of design described below. How each area effected or could have affected the Khobar Towers are also described.
Site Design
Figure 6: Examples of site barriers. Credited to FEMA
Unlike other types of antiterrorist measures, site design involves no direct analysis procedures. The main focus of site design is to prevent potential attackers from being able to reach their target. Explosive damage decreases at an exponential rate with respect to distance, so the farther away an attacker must detonate the explosive from the building, the less damage the attack will cause. A risk analysis is used to determine the likelihood of an attack and the type of attack. Two types of threats are most commonly considered: vehicular and pedestrian.
The officials at the Khobar Towers utilized this antiterrorism measure by installing Jersey barriers around the site. This is just one type of barrier used to bar vehicular access to a building. The Federal Emergency Management Agency (FEMA) released a 2007 manual that describes several types of antiterrorism site design that are diagramed in Figure 6. These elements are designed to limit larger vehicular threats from getting too close to the building, which is what saved the Khobar Towers from further destruction.
Some of these elements can double as pedestrian barriers. Pedestrians carrying backpack or satchel bombs are considered for several threat levels. These types of explosions are usually smaller in scale, but can often get closer or even inside the building before detonating. As with the Khobar Towers, fences are usually installed and a limited number of guarded access points are used to limit unauthorized access (FEMA, 2007 p. 4-1).
Blast Design
Blast analysis is the area of antiterrorist design that most directly sprang from the Khobar Bombings. As stated previously, a large number of the injuries in the Khobar attacks resulted from façade being turned into projectiles. The goal of a blast design is the prevent debris from the explosion or building from entering the structure's interior. Engineers and scientists primarily evaluate the building facade for split-second pressures. This kind of structural analysis relies on complicated finite element models and nonlinear material response.
Figure 7: Diagram showing how typical blast pressures are applied to buildings. Credited to Ryan Solnosky
The blast pressures are broken into two phases. The positive phase occurs immediately after the explosion and applies outward pressures from the blast onto the building, which can be seen in step 1 of Figure 7. The negative phase occurs after this and usually involves pressures much less than the positive phase. The negative phase pressures occur in the direction towards the explosion because the air is trying to rush back into the area voided by the blast. This is illustrated in step 3 in Figure 7. Designers typically encounter lateral pressures on the building that are orders of magnitude greater than the typical wind or seismic loads. Luckily these blast pressures are applied over a matter of milliseconds and therefore impulsive response the building must be considered, which offers more resistance than a non-impulsive response (Solnosky, 2011).
If the explosion is located close to the building or within the building uplift may occur in the floor elements. If the explosion is too far from the target the pressure waves act more as direct lateral pressures. Uplift can be very problematic for a structure as it can cause failure in several connections and leave slabs unsupported. This was determined to a major contributing factor in the 1968 Ronan Point Collapse. The British design field learned from this and implemented minimum connection strengths between walls and floors. The 1978 design used these British standards and therefore the wall to slab connections were able to withstand the imposed uplift.
Blast analysis has become a very important tool for designing the building's skin. As seen in this attack, facade failure and projectiles cause a majority of the injuries. Blast-resistant glazing has been developed to eliminate this hazard. Manufactures use lamination and tempering to strengthen the glazing to withstand lateral pressures which are 50-100 times higher then the typical wind pressures experienced by normal glazing. Other systems have been developed for larger glazing areas, like atriums and enterances. Cable net systems allow for large deflections and energy dissipation without shattering.
Progressive Collapse
The underlying principles involved with progressive collapse are redundancy, continuity and ductility. Tie-Force method was the first progressive collapse measure to be developed. Following the Khobar bombings Alternative Path and Enhanced Lateral Resistance analysis methods were developed to compliment Tie-Force method and add extra layers of safety into the building design. Refer to the Progressive Collapse article on this site for more information. Most design approaches use a prescribed level of protection for each building type, which is based on occupancy, mission, and other factors (Stevens, 2011 p. 9).
In 2005 the Department of Defense released the long awaited Unified Facilities Criteria that fulfilled the Congressional mandate made following the Khobar attacks. This manual included several sections on antiterrorism measures, of which section 4-023 addressed progressive collapse in an entirely new way. “UFC 4-023-03 can be seen as a starting point for the development of comprehensive guidelines for progressive collapse mitigations in the United States” (Stevens, 2011 p. 2). The design measures used in this code are summarized below.
Progressive collapse can be occur from both targeted attacks and accidental actions, such as volatile manufacturing materials. When designing for terrorist attacks the designer must provide alternative paths, sufficient strength for large displacements, and develop a continuous and ductile system.
Figure 8: Alternative Path Method computer analysis. Credit to Department of Defense, UFC 4-023-03
Alternate load paths are designed in two different methods. The Tie-Force Method was developed in Great Britain and relies on floor reinforcing or beams to develop enough tensile forces to safely support the floor through catenary action. If a high level of protection is required for a building type, a second, more rigorous method may be required. The Alternative Path Method involves removing exterior columns throughout the floor plan and elevation and ensuring the remaining elements have an acceptable amount of strength the hold the floor in place (DoD, 2005, p. 33). Figure 8 shows a computer deflection diagram from such an analysis.
A major reason why the Khobar Towers had only limited structural failure is the amount of redundancy offered by the precast partition walls. Figure 5 shows how the walls were able to redistribute the load from any failed members into other parts of the system.
If an even higher level of protection is required the Enhanced Local Resistance method may be implemented. The goal of this analysis is to strengthen all columns that may be attacked to the point where they dissipate large amounts of energy and shouldn’t fail. In the event the columns are overstressed and do fail, they are designed to fail in a ductile manor, usually flexure, which allows large amounts of energy to be absorbed by plastic deformations (DoD, 2005 p. 60).
LESSONS LEARNED
Although this horrific attack took 19 lives, some good did come from it. Following the attack on the Khobar Towers the Department of Defense was directed to develope antiterrorism design criteria. This Unified Facilities Criteria, along with the GSA's code, have made both US military and civilian workers safier.The Khobar Towers bombing reviled the importance of addressing the standoff distance to each building and the significance of blast resistance façade design.
Terrorist attacks are rare, as compared with other structural loading conditions. Since this attack few major terrorist bombings have caused structural failures. This is largely due to site planning and security measures. Since the 2001 start of the war in Afghanistan and the 2003 start of the Iraq war, several terrorist attacks take military and civilian lives. Unlike the Khobar Tower attack, these explosives largely target crowds of people in the open or roadway checkpoints. Standoff distances proved to drastically reduce the damage to the Khobar Towers as compared to the Murrah Building in Oklahoma City. That is largely why today’s military facilities have become much more difficult to strike.
Arguably the most important outcome of the attacks in Dhahran was that it shocked the government officials enough to force them to standardize antiterrorism measures. Multiple terrorist attacks struck US citizens in a short period, from the 1993 World Trade Center attack to the 1995 Oklahoma City bombing. The Khobar Towers turned out to be the straw that broke the camel’s back. Having written antiterrorism guidance allowed designers to create safer sites and buildings, which is a strong reason why few large scale building attacks have been successful since.
The Department of Defence's Unified Facilities Criteria design guide proscribes three levels of progressive collapse analysis. Buildings deemed to have a higher level of risk, such at military barracks, are required to pass more stringent criteria. The Khobar Towers successifully resisted a disportionate amount of collaspe because of the redundancy of the precast wall system. The government antiterrorism design guides also strongly address the site layout and facade design. Stronger and more ductile glazing systems are used in today's blast resistant buildings to mitigate injuries to occupants.
SOURCES
Annotated Bibliography
Byfield, Mike P. (March 31, 2006). “Behavior and Design of Commercial Multistory Buildings Subjected to Blast.” Journal of Structural Engineering, 20(4), 6 pages. The kinds and effects of explosives are described in this article. Findings from past explosive attacks are presented and conclusions are made.
Dept. of Defense (1996). "News Photos." United States Department of Defense, Arlington, VA. Online database of photographs taken by military personnel and other professionals associated with the department. Photos of the attack's aftermath will be used.
Fed. Bureau of Investigation (June 21, 2001). “Press Release.” United States Federal Bureau of Investigation, Washington, D.C. 2 pages. This FBI press release outlines the charges brought against the terrorist conspirators. It also describes the plot and attack.
FEMA (August 2008). “FEMA 430: Site and Urban Design for Security.”, United States Federal Emergency Management Agency, Washington, D.C., 4 pages. This design guide details how to lay out a secure site and incorporate indirect anti-terrorism design into a project. A chapter in this document describes some of the barrier techniques used in the Khobar Tower.
GlobalSecurity.org (2002). "Khobar Towers.", http://www.globalsecurity.org/military/facility/khobar.htm The author of this article describes the history and design of the Khobar Towers complex. The article also goes into the antiterrorist measures in-place before the attack.
Grant, Rebecca (June, 2006). “Death in the Desert.” Airforce-Magazine, 89(6), Arlington, VA. 5 pages. In this military magazine article, the author gives a detailed description of the attack and immediate aftermath.
Mallonee, S.; Shariat S.; Stennies, G.; Waxweiler, R.; Hogan, D.; and Jordan, F. (August 7, 1996). “Physical Injuries and Fatalities Resulting From the Oklahoma City Bombing.” National Institutes of Health, 276(5), Bethesda, MD. 3 pages. This epidemiologic study was conducted to provide description of the physical injuries and fatalities resulting from the Murray building bombing. The study concludes, as with the Khobar Tower Bombing, that the majority of the injuries resulted from flying glass and debris.
Solnosky, Ryan (November, 2011). "Blast and Progressive Collapse Lecture." Penn State University, University Park, PA. During a lecture to the structural students in AE 481W, Mr. Solnosky, a teaching assistant, detailed the steps involved with both blast and progressive collapse design. Material for the presentation was compiled from several text books, codes and courses.
Spence, Floyd D. (August 14, 1996). "Press Release." House National Security Committee, Washington, D.C. 25 pages. In this press release the chairman of the Congressional committee summarizes the events that led to and followed the attack. It also outlines the steps the government took to begin the developement of new design guides.
Stevens, David; Crowder, Brian; Sunshine, Doug; Marchand, Kirk; Smilowitz, Robert; Williamson, Eric; and Waggoner, Mark (August 15, 2011). “DoD Research and Criteria for the Design of Buildings to Resist Progressive Collapse.” Journal of Structural Engineering, 137(9). 11 pages. This article gives a brief history of the major terrorist attacks, including the Khobar Towers Bombing, that lead to the creation of the Department of Defense’s Unified Facilities Criteria code. The article then focuses on the development and thinking behind the analysis methods prescribed in the UFC code.
Table of Contents
INTRODUCTION
On the night of June 25, 1996, a sewage tanker filled with explosives exploded in front of the Khobar Towers, a US Air Force residential complex in Dhahran, Saudi Arabia. The over 5,000 pounds of plastic explosives caused the complete failure of the northern facade and partial failure of the concrete floor slabs in that area, as shown in Figure 1. This massive explosion took the lives of 19 Americans and wounded 372 others (FBI, 2001 p. 1). Although the structure largely remained intact through the attack, the majority of the casualties were due to flying glass and debris. This attack was largely responsible for the creation of building codes specifically designed to prevent terrorist attacks and mitigate the damage caused to government buildings (Stevens, 2011 p. 4).
The Khobar Towers bombing, along with the 1983 bombing of a US Marine barracks in Beirut and the 1995 Oklahoma City Bombing, caused the United States Government to reconsider its building design codes and terrorism prevention measures. Draft Security Criteria, a 1997 General Services Administration (GSA) publication, was the first published building criteria in the US that prescribed progressive collapse and blast concerns to be addressed by designers. The Unified Facilities Criteria, published in 2005 by the Department of Defense, was created as a direct result of the Khobar bombings (Stevens, 2011 p. 4). These two government agencies used the lessons learned from this horrific event to write codes that will help to prevent future catastrophes.
Keywords
Terrorist Attack; Explosion; Blast; Facade Failure; Progressive Collapse; Department of Defense; General Services AdministrationBUILDING DESIGN AND ATTACK
Building 131, which was attacked, was one building of a larger military complex named the Khobar Towers. These towers primarily served as residential units for the United States Airforce, but other structures in the compound housed non-military personnel, both American and Saudi. The entire complex stretched across 12 city blocks (Spence, 1996 p. 8). The attacked tower, along with the majority of the compound, was constructed from precast wall and floor systems. This redundant system later proved to save countless lives.
In 1979 the Saudis constructed the Khobar Towers complex near the city of Dhahran in the Eastern Province of Saudi Arabia. The majority of the buildings remained unoccupied until the 1990 Gulf War when military forces from the United States, France and the United Kingdom, along with Saudi civilians moved in (GlobalSecurity.org, 2002). As can be seen in Figure 2, the US military occupied the Northern part of the complex and the entire compound was fenced off from unauthorized access.
The 1984 bombing of a US Marine Corp facility in Beirut, Lebanon taught the military an important lesson: standoff distance. Soon after the US military began to occupy the Khobar Towers, a perimeter fence was erected around the compound (Byfield, 2006 p. 4). This fence consisted of a 7-8 foot high chain link fence, topped with barbed wire. Jersey barriers, illustrated in Figure 3, were placed in front of the chain link fence to prevent vehicular access. At Building 131, this fence was located approximately 30 meters or 100 feet from the Northern façade. Jersey barriers are common to highway construction and primarily rely on their 5,700 pounds of weight per 12 foot module to stop vehicles (FEMA, 2008 p. 4-31).
Building 131, along with most of the other buildings on the site, used precast load bearing walls as its primary structural system. The façade was also concrete precast panels, which were supported by the façade panel below. The modular architectural plan for this barrack allowed for the interior partitions to also be made from precast panels. The British reinforced concrete design code, BS8110, was used to design the structural system. At the time the British were the designers to address progressive collapse prevention because of the 1968 Ronan Point disaster. Using this code resulted in relatively strong connections between walls and slabs (Byfield, 2006 p. 4).
At about 10:00 p.m. on June 25, 1996 rooftop sentries hurriedly awoke the US airmen in Building 131. These futile evacuation attempts did not succeed in removing people from the building because only 3 to 4 minutes after it was spotted, the suspicious sewage tanker detonated. The massive explosion destroyed the lower levels of the façade, which left the upper floors unsupported and led to their eventual collapse.
The investigation later reviled that three members of Saudi Hizballah terrorist organization drove a sewage tanker and a getaway car into a parking lot along the northern fence of the compound. The tanker was backed up to the fence and the attackers fled the scene. Rooftop sentries immediately became suspicious and began evacuation (Grant, 2006 p. 1).
Experts estimate that the truck was filled with at least 5,000 pounds of home-made plastic explosive made from fertilizers. This equates to 20,000 pounds of TNT and ranks as the most powerful detonated terrorist attack, even stronger than the explosion created by Timothy McVeigh in Oklahoma. (FBI, 2001 p.1) A 24.4 meter wide by 9.1 meter (or 80 feet by 30 feet) crater was created and can be seen in Figure 4. The blast sent debris and pressure waves crashing into Building 131’s side. The 5,700 pound concrete Jersey barriers were found in the first four floors of the building (Byfield, 2006 p. 2).
AFTERMATH
The majority of the structure withstood the attack relatively unscathed. Despite this, 19 American airmen lost their lives, 200 others were wounded and hundreds of Saudi soldiers and civilians were harmed (Spence, 1996 p. 5). Injuries can primarily be attributed to the northern facade failure and associated debris.
Unlike several other terrorist attacks, the attack on the Khobar Towers did not result in a progressive collapse of the structure. The concrete partitions, shown in Figure 5, offered several redundant load paths for the forces to be redistributed One of the primary sources for explosion damage research comes from World War II. Francis Walley and Lord Baker surveyed bomb damaged structures in England and documented the failure mechanisms. They concluded that connection failures were the leading cause of complete building collapse and loadbearing wall structures tended to be more robust to explosions (Byfield, 2006, p. 3). By designing to the British codes, the connections were strengthened and withstood the blast.
The most noticeable damage to Building 131 is the missing Northern façade. The explosion pressures, primary, and secondary debris (including the Jersey barriers) caused the lower levels of the façade to buckle into the building. The top three floors were no longer supported from below and proceeded to shear off the weak connections that tied them back to the floor slabs. The East and West facades were able to hold onto the building, but were displaced up to four feet from their original position (Grant, 2006 p. 2).
The façade failure created a much larger problem. The secondary debris from both the barriers and façade debris was the leading cause of injury and death. After the Oklahoma City Bombing a study was conducted, which determined flying glass and other projectiles was the leading cause of injury (Mallonee, 1996 p. 1). Medics on the site in Saudi Arabia encountered much of the same injuries.
In 2006 the FBI had enough evidence to bring 13 members of the pro-Iran Saudi Hizballah to justice. Among the charged are the three terrorists who parked the bomb next to the tower and Al-Mughassil, a high ranking leader in the organization (FBI 2001, p. 1). Immediately following the attack, many believed that Iran, itself, was behind the attack. Many feared that a US attack on the country would follow. The investigation did not reveal enough evidence to verify these allegations, but strong suspicions still remain.
CHANGES IN DESIGN CODES
This was not the first terrorist attack against Americans, but this attack was a key factor in the US government establishing design criteria that addressed it. In a three year span, from the first World Trade Center Attack in 1993 to the Khobar Bombing in 1996, three major attacks struck United States citizens. This shocked government officials pushed them into action. In October 1995, only months before the Khobar Bombings, President Clinton issued Executive Order 12977. This document ordered all federal facilities to address security and develop long-term construction standards, but gave no specifics or direction. Following this attack in Saudi Arabia, Congress directed the Department of Defense to develop codes explicitly for antiterrorist measures (Stevens, 2011 p. 2).
These proclamations had little effect on how federal and military buildings were designed until in 2000 the General Services Administration issued a design criteria for all federal buildings that addressed antiterrorism measures. The Department of Defense soon followed with their interim antiterrorism requirements. The 2005 release of the DoD’s Unified Facilities Criteria marked the point where design professionals had concrete guidelines for all Department of Defense buildings (Stevens, 2011 p. 3).
Both agencies’ criteria involve the three main areas of design described below. How each area effected or could have affected the Khobar Towers are also described.
Site Design
Unlike other types of antiterrorist measures, site design involves no direct analysis procedures. The main focus of site design is to prevent potential attackers from being able to reach their target. Explosive damage decreases at an exponential rate with respect to distance, so the farther away an attacker must detonate the explosive from the building, the less damage the attack will cause. A risk analysis is used to determine the likelihood of an attack and the type of attack. Two types of threats are most commonly considered: vehicular and pedestrian.
The officials at the Khobar Towers utilized this antiterrorism measure by installing Jersey barriers around the site. This is just one type of barrier used to bar vehicular access to a building. The Federal Emergency Management Agency (FEMA) released a 2007 manual that describes several types of antiterrorism site design that are diagramed in Figure 6. These elements are designed to limit larger vehicular threats from getting too close to the building, which is what saved the Khobar Towers from further destruction.
Some of these elements can double as pedestrian barriers. Pedestrians carrying backpack or satchel bombs are considered for several threat levels. These types of explosions are usually smaller in scale, but can often get closer or even inside the building before detonating. As with the Khobar Towers, fences are usually installed and a limited number of guarded access points are used to limit unauthorized access (FEMA, 2007 p. 4-1).
Blast Design
Blast analysis is the area of antiterrorist design that most directly sprang from the Khobar Bombings. As stated previously, a large number of the injuries in the Khobar attacks resulted from façade being turned into projectiles. The goal of a blast design is the prevent debris from the explosion or building from entering the structure's interior. Engineers and scientists primarily evaluate the building facade for split-second pressures. This kind of structural analysis relies on complicated finite element models and nonlinear material response.
The blast pressures are broken into two phases. The positive phase occurs immediately after the explosion and applies outward pressures from the blast onto the building, which can be seen in step 1 of Figure 7. The negative phase occurs after this and usually involves pressures much less than the positive phase. The negative phase pressures occur in the direction towards the explosion because the air is trying to rush back into the area voided by the blast. This is illustrated in step 3 in Figure 7. Designers typically encounter lateral pressures on the building that are orders of magnitude greater than the typical wind or seismic loads. Luckily these blast pressures are applied over a matter of milliseconds and therefore impulsive response the building must be considered, which offers more resistance than a non-impulsive response (Solnosky, 2011).
If the explosion is located close to the building or within the building uplift may occur in the floor elements. If the explosion is too far from the target the pressure waves act more as direct lateral pressures. Uplift can be very problematic for a structure as it can cause failure in several connections and leave slabs unsupported. This was determined to a major contributing factor in the 1968 Ronan Point Collapse. The British design field learned from this and implemented minimum connection strengths between walls and floors. The 1978 design used these British standards and therefore the wall to slab connections were able to withstand the imposed uplift.
Blast analysis has become a very important tool for designing the building's skin. As seen in this attack, facade failure and projectiles cause a majority of the injuries. Blast-resistant glazing has been developed to eliminate this hazard. Manufactures use lamination and tempering to strengthen the glazing to withstand lateral pressures which are 50-100 times higher then the typical wind pressures experienced by normal glazing. Other systems have been developed for larger glazing areas, like atriums and enterances. Cable net systems allow for large deflections and energy dissipation without shattering.
Progressive Collapse
The underlying principles involved with progressive collapse are redundancy, continuity and ductility. Tie-Force method was the first progressive collapse measure to be developed. Following the Khobar bombings Alternative Path and Enhanced Lateral Resistance analysis methods were developed to compliment Tie-Force method and add extra layers of safety into the building design. Refer to the Progressive Collapse article on this site for more information. Most design approaches use a prescribed level of protection for each building type, which is based on occupancy, mission, and other factors (Stevens, 2011 p. 9).In 2005 the Department of Defense released the long awaited Unified Facilities Criteria that fulfilled the Congressional mandate made following the Khobar attacks. This manual included several sections on antiterrorism measures, of which section 4-023 addressed progressive collapse in an entirely new way. “UFC 4-023-03 can be seen as a starting point for the development of comprehensive guidelines for progressive collapse mitigations in the United States” (Stevens, 2011 p. 2). The design measures used in this code are summarized below.
Progressive collapse can be occur from both targeted attacks and accidental actions, such as volatile manufacturing materials. When designing for terrorist attacks the designer must provide alternative paths, sufficient strength for large displacements, and develop a continuous and ductile system.
Alternate load paths are designed in two different methods. The Tie-Force Method was developed in Great Britain and relies on floor reinforcing or beams to develop enough tensile forces to safely support the floor through catenary action. If a high level of protection is required for a building type, a second, more rigorous method may be required. The Alternative Path Method involves removing exterior columns throughout the floor plan and elevation and ensuring the remaining elements have an acceptable amount of strength the hold the floor in place (DoD, 2005, p. 33). Figure 8 shows a computer deflection diagram from such an analysis.
A major reason why the Khobar Towers had only limited structural failure is the amount of redundancy offered by the precast partition walls. Figure 5 shows how the walls were able to redistribute the load from any failed members into other parts of the system.
If an even higher level of protection is required the Enhanced Local Resistance method may be implemented. The goal of this analysis is to strengthen all columns that may be attacked to the point where they dissipate large amounts of energy and shouldn’t fail. In the event the columns are overstressed and do fail, they are designed to fail in a ductile manor, usually flexure, which allows large amounts of energy to be absorbed by plastic deformations (DoD, 2005 p. 60).
LESSONS LEARNED
Although this horrific attack took 19 lives, some good did come from it. Following the attack on the Khobar Towers the Department of Defense was directed to develope antiterrorism design criteria. This Unified Facilities Criteria, along with the GSA's code, have made both US military and civilian workers safier.The Khobar Towers bombing reviled the importance of addressing the standoff distance to each building and the significance of blast resistance façade design.Terrorist attacks are rare, as compared with other structural loading conditions. Since this attack few major terrorist bombings have caused structural failures. This is largely due to site planning and security measures. Since the 2001 start of the war in Afghanistan and the 2003 start of the Iraq war, several terrorist attacks take military and civilian lives. Unlike the Khobar Tower attack, these explosives largely target crowds of people in the open or roadway checkpoints. Standoff distances proved to drastically reduce the damage to the Khobar Towers as compared to the Murrah Building in Oklahoma City. That is largely why today’s military facilities have become much more difficult to strike.
Arguably the most important outcome of the attacks in Dhahran was that it shocked the government officials enough to force them to standardize antiterrorism measures. Multiple terrorist attacks struck US citizens in a short period, from the 1993 World Trade Center attack to the 1995 Oklahoma City bombing. The Khobar Towers turned out to be the straw that broke the camel’s back. Having written antiterrorism guidance allowed designers to create safer sites and buildings, which is a strong reason why few large scale building attacks have been successful since.
The Department of Defence's Unified Facilities Criteria design guide proscribes three levels of progressive collapse analysis. Buildings deemed to have a higher level of risk, such at military barracks, are required to pass more stringent criteria. The Khobar Towers successifully resisted a disportionate amount of collaspe because of the redundancy of the precast wall system. The government antiterrorism design guides also strongly address the site layout and facade design. Stronger and more ductile glazing systems are used in today's blast resistant buildings to mitigate injuries to occupants.
SOURCES
Annotated Bibliography
Byfield, Mike P. (March 31, 2006). “Behavior and Design of Commercial Multistory Buildings Subjected to Blast.” Journal of Structural Engineering, 20(4), 6 pages.
The kinds and effects of explosives are described in this article. Findings from past explosive attacks are presented and conclusions are made.
Dept. of Defense (1996). "News Photos." United States Department of Defense, Arlington, VA.
Online database of photographs taken by military personnel and other professionals associated with the department. Photos of the attack's aftermath will be used.
Dept. of Defense (2005). "Unified Facilities Criteria (UFC) DoD Minimum Antiterrorism Standards for Buildings." United States Department of Defense, Arlington, VA.
This is the antiterrorism design code that came about in part because of the Khobar Bombing. The UFC covers site design, balst design, and progressive collapse.
Fed. Bureau of Investigation (June 21, 2001). “Press Release.” United States Federal Bureau of Investigation, Washington, D.C. 2 pages.
This FBI press release outlines the charges brought against the terrorist conspirators. It also describes the plot and attack.
FEMA (August 2008). “FEMA 430: Site and Urban Design for Security.”, United States Federal Emergency Management Agency, Washington, D.C., 4 pages.
This design guide details how to lay out a secure site and incorporate indirect anti-terrorism design into a project. A chapter in this document describes some of the barrier techniques used in the Khobar Tower.
GlobalSecurity.org (2002). "Khobar Towers.", http://www.globalsecurity.org/military/facility/khobar.htm
The author of this article describes the history and design of the Khobar Towers complex. The article also goes into the antiterrorist measures in-place before the attack.
Grant, Rebecca (June, 2006). “Death in the Desert.” Airforce-Magazine, 89(6), Arlington, VA. 5 pages.
In this military magazine article, the author gives a detailed description of the attack and immediate aftermath.
Mallonee, S.; Shariat S.; Stennies, G.; Waxweiler, R.; Hogan, D.; and Jordan, F. (August 7, 1996). “Physical Injuries and Fatalities Resulting From the Oklahoma City Bombing.” National Institutes of Health, 276(5), Bethesda, MD. 3 pages.
This epidemiologic study was conducted to provide description of the physical injuries and fatalities resulting from the Murray building bombing. The study concludes, as with the Khobar Tower Bombing, that the majority of the injuries resulted from flying glass and debris.
Solnosky, Ryan (November, 2011). "Blast and Progressive Collapse Lecture." Penn State University, University Park, PA.
During a lecture to the structural students in AE 481W, Mr. Solnosky, a teaching assistant, detailed the steps involved with both blast and progressive collapse design. Material for the presentation was compiled from several text books, codes and courses.
Spence, Floyd D. (August 14, 1996). "Press Release." House National Security Committee, Washington, D.C. 25 pages.
In this press release the chairman of the Congressional committee summarizes the events that led to and followed the attack. It also outlines the steps the government took to begin the developement of new design guides.
Stevens, David; Crowder, Brian; Sunshine, Doug; Marchand, Kirk; Smilowitz, Robert; Williamson, Eric; and Waggoner, Mark (August 15, 2011). “DoD Research and Criteria for the Design of Buildings to Resist Progressive Collapse.” Journal of Structural Engineering, 137(9). 11 pages.
This article gives a brief history of the major terrorist attacks, including the Khobar Towers Bombing, that lead to the creation of the Department of Defense’s Unified Facilities Criteria code. The article then focuses on the development and thinking behind the analysis methods prescribed in the UFC code.
Additional Resources
Hinman, Eve (November 2009). “Blast Safety of the Building Envelope.” National Institute of Building Sciences, Washington, D.C.
Krauthammer, Ted; Hall, Robert L.; Woodson, Stanley C.; Baylot, James T.; Hayes, John R.; and Sohn, Young (May, 2002). “Development of Progressive Collapse Analysis Procedure and Condition Assessment for Structures.” National Institute of Building Sciences, Washington, D.C. 13 pages.
Sevin, Eugene and Little, Richard G. (Fall 1998). “Mitigating Terrorist Hazards.” National Academy of Engineering, 28(3), Washington, D.C. 4 pages.