Case Study: The 1995 Oklahoma City Bombing
How the future of federal facility design was forever changed
The Alfred P. Murrah Building was a United States federal facility named after Federal Judge and Oklahoma native Alfred P. Murrah. The structure was designed as a 9-story low rise office building with a reinforced concrete ordinary moment frame, shear wall core, and curtain wall facade with concrete glass. The structure was designed to accommodate offices for the federal government. On the morning of April 19, 1995, a 4,800 lb. bomb was detonated at the north side of the building, causing a major collapse of ⅓ of the structure. The explosion was considered a domestic terrorist attack when coconspirators Timothy McVeigh and Terry Nichols were found guilty of the crime. The explosion tragically killed 168 people, 15 of them being young children in the daycare center. Additionally, the blast damaged hundreds of nearby buildings and injured over 600 people.
This incident paved the way for the United States Government to make a change in the way federal facilities are to be designed and built. Blast/seismic loads were never considered in the design process in regions such as Oklahoma. The engineering process as we know it today always considers the potential effects of both wind and seismic activity on structures before they are built, and this incident is one of the reasons that the engineering world has had to shape its processes with a more preventative approach. Keep reading as we discuss the background of the Alfred P. Murrah Building, the events that took place on April 19, 1995, the type of blast that occurred, the structure’s response to the blast, the alternatives that could have changed the outcome, and the future of U.S. security and building standards after the attacks.
Building Background
In the early 1970’s, the Alfred P. Murrah Building was designed using ACI 318–71. It was constructed between 1974–1976 and opened for use March 2, 1977. The federal building contained offices for the U.S. Department of Housing and Urban Development, the United States Secret Services, the Social Security Administration, the Drug Enforcement Administration (D.E.A), the Department of Veterans Affairs vocational rehabilitation counseling center, and the Bureau of Alcohol, Tobacco, and Firearms (ATF). In addition, America’s Kids daycare center was available for employees and the U.S. Army used some space for recruitment. Design standards in the early 1970’s did not require consideration of seismic loading in locations such as Oklahoma, so the structure was designed for wind loading only. The design was a reinforced concrete ordinary moment frame. Ordinary Moment Frames (OMF) are typical for buildings that expect limited inelastic deformations and have non/low seismic regions. A shear wall was placed at the core of the building to aid in resisting wind loads. The shear wall surrounded the elevator and stairs and the facade was a curtain wall with concrete glass. This design feature was a major reason the building sustained so much damage from the 1995 bombing. The pressure waves from the blast burst through the windows and allowed the wave to travel throughout the building. The uplift on the floor slabs was detrimental to the progressive collapse.
The Attack
The terrorist attacks on the Alfred P. Murrah Building took place at 9:02 AM on April 19, 1995. Two domestic terrorists, Timothy McVeigh and Terry Nichols, were found guilty of bombing the building. The motive behind the attacks was due the two mens shared hatred toward the Federal Bureau of Investigations (FBI). The men met at U.S. basic training for the army and voiced their outrage and anger at how the FBI handled the Waco Siege. For those of you who are unfamiliar, the Waco Siege was a 51-day standoff between the Branch Davidian cult and the FBI over a botched search warrant by the Bureau of Alcohol, Tobacco and Firearms (ATF). The standoff ended with a fire fight that resulted in 75 people burning or being shot to death including one of the main leaders, David Koresh. The terrorists decided that bombing the Oklahoma City federal building on the two-year anniversary of the Waco Siege would provide justice for the deaths of those people.
Casualties and Damages
The explosion resulted in the death of 168 people, 15 of them including young children from the daycare center. Over 680 people were injured from the blast either in the building or in the surrounding area. A large portion of the injuries were caused from glass shattering and shooting directly towards unsuspecting people. The explosion caused severe structural damage as well. 324 buildings were destroyed or damaged within a 16-block radius of the location of the blast. At least 258 buildings experienced shattered glass. 86 cars in the area were damaged or destroyed. The damages in total were estimated at 652 million dollars.
The Blast: Location and Effects
The blast was considered a surface burst. A surface burst is when an explosion on the ground creates an initial shock wave and a reflected shock wave from the ground surface. The initial incident wave and the reflected wave then collide forming an even more powerful wave that could cause massive damage to any nearby structures.
The truck contained 13 barrels of explosives, 9 containing a mix of ammonium nitrate and nitromethane and 4 containing a mix of fertilizer and 4 U.S. gallons of diesel fuel. The blast was equivalent to 4,000 lbs. of trinitrotoluene (TNT). TNT contains a high detonation velocity of 6,640 m/s and an energy content of 4.6 megajoules per kilogram. Detonation velocity is how fast a shock wave travels through a detonated explosive. Both the power of the explosive and the location where it detonates are crucial to its effects on the surrounding area. The bomb was approximately 15.6 feet away from column G20, a column on the first floor that supported a transfer girder on the third floor.
After the bombing, the Building Performance Assessment Team (BPAT), the American Society of Civil Engineers (ASCE) and Federal Government Engineers investigated the damages to determine what led to the buildings destructive collapse. A thorough analysis of the damages in the field and calculations of effects of the explosion concluded that column G20, located about 16 feet from the blast location, was destroyed by brisance, the shattering capability of a high explosive. The higher the detonation velocity, the faster the shock wave travels, and the tinier the particles of the nearest object become. Since column G20 was close to the initial blast location, it was destroyed immediately on impact. In addition, columns G16 and G24 reached their yield capacities, failing in shear (See below for column location).
These 3 columns carried the load down from a transfer girder on the third floor and with the their failure the transfer girder was quick to follow. The girder itself only had top bars continuous. When the building was designed, there was no code stating that the girder needed any bottom bars reinforcing it. From a designer’s point of view, if the weight of steel can be reduced without compromising a member, it will be reduced. The girders ultimate failure was due to the reduction of reinforcing bars and change in bending moment of the girder from the explosion. Once the transfer girder failed, every load it carried from the fourth to the ninth floor was left unsupported. The chain reaction of each failure led to the disproportionate/progressive collapse of the building.
Disproportionate/progressive collapse is when one single element failure can cause major collapse to other major parts of a structure. The column failure caused the girder failure which caused the floor slabs and columns in the above floors to fail. Think of it like a series of dominos!
A statistic outlining the damages to the building showed that 4% of the structure was destroyed by the blast and 42% of the building was destroyed by the blast and progressive collapse. The floor slabs in the structure were already weakened due to uplift pressures from the shock wave traveling through the building after shattering all the windows. Once that wave traveled through, the weight of the top floors of the building was too much for the failed members on the first and third floor to carry.
Alternative Blast Scenarios
To aid in the design of future blast resistant structures, several analyses were performed to reimagine the incident by changing the location of the bomb or the structural integrity of major elements. One alternative was increasing the reinforcement in the transfer girder. Since the failure of the girder led to the progressive collapse of the structure, it is important to look at what could have been different about that element and how it could help future designs of a transfer girder. The reinforcement bars for this analysis were assumed continuous with 7 square inches at the top and bottom of the girder. The reinforcement area in this scenario is double what the original design was. After modeling the structure, it was determined that the failure would only be localized in column G20 and the floor slabs would fail due to the pressure wave of the initial blast. The girder maintained its strength, stiffness and stability since the bars were able to resist the change in bending moment from the blast.
A second alternative worth researching was placing the blast at a distance 20 feet back from where it initially detonated. Since a surface blast causes most of its damage from the reflected incident wave, the distance alone could reduce the amount of damages. After modeling the structure with the blast 20 feet away, it resulted in high uplift pressures on the floor slabs alone. The column experienced no failure from brisance since the blast was at a significant distance. The initial incident wave and reflected wave were far enough away to cause no member failures, just floor slab failure. There was no progressive collapse in this scenario.
The Future of U.S. Standards and Security
As an engineer it is important to learn from the mistakes of the past. Without the 1995 bombing of the Oklahoma City Federal Building, there would have never been any change in the way structures for federal facilities are designed. As a result of this tragic event, President Clinton implemented Executive Order 12977 which directed an Interagency Security Committee (ISC) to be made to create standards for federal facilities. The ISC immediately included changes such as shatter resistant window film, standoff distances for facilities, structural design to prevent progressive collapse, and requiring X-rays and magnetometers at any entrance to ensure there were no weapons of any kind entering a facility. The shatter resistant film was an extremely important rule because the broken glass from the explosion alone accounted for major injuries as well as allowing the shock wave to travel through the building. If the glass was shatter resistant during the 1995 attacks, the building may have been able keep more civilians safe.
A standoff distance, as proven by the alternative analysis, would save a building from losing major structural elements on impact. According to the United Facilities Criteria (UFC) Section 3–2.1, the minimum standoff distance for new construction is 20 feet. Where there is no clear zone outside the perimeter of the building, the minimum standoff distance is 50 feet. It can be seen in the design of the new Oklahoma City federal building that a standoff distance of 50 feet was implemented to evade a future disaster. Bollards are typical for locations with a standoff distance as well to guarantee no vehicle can access that space in front of the structure.
Structural design measures to prevent progressive collapse include increasing reinforcement in beams, columns and slabs as well as using Special Moment Frames (SMF) versus Ordinary Moment Frames (OMF). A SMF can resist large inelastic deformations at its connections and are used for seismic resistant structures. It is essential for all existing and future federal facilities to consider a seismic load on the structure in case of another terrorist attack. Increasing the reinforcement will often increase the structural integrity of its members if it is within the ratio of concrete to steel. The final change by the ISC immediately after the attacks would be X-ray machines and magnetometers at every point of entry of a facility. Protecting the public from not only a blast but also a weapon is what the ISC intended to do. It is not enough to simply design for the building’s safety but also for those inside and vulnerable to an attack at any moment.
The 1995 bombing of the Oklahoma City Federal Building proved to be tragically detrimental but, in some ways, actually beneficial to the future of the United States. The lives lost on that fatal day will never be forgotten, and the families of those deceased will forever feel the pain and loss of their loved ones. With that being said, the engineering community gained a real-life perspective on the safety standards that need to be considered when designing public facilities that serve a large community. The United States government began to open its eyes to the increasing violence in the country as well as a need for change in order to protect its people.
Everyday engineers strive to serve the public and work towards a better tomorrow. It is our job to adapt to the ever evolving design and construction practices of our time especially if it means ensuring public safety. The 1995 OKC bombing was just a small subset of the iterative process that comes with finding new solutions to prevent future tragedies. In 2022, our engineering technology has advanced exponentially and will continue to do so indefinitely. With tools like civils.ai, we are joining the effort to ensure reliable and trustworthy engineering data is in the hands of society’s biggest problem solvers.
References
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