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JWF, Sec. Fim Facts, Bomb Blast Protection
Background
Bombings have increased at an alarming rate over the last 25 years leading to a substantial amount of property damage, injuries, and loss of life. Unfortunately, the resulting glass breakage from these blasts is a major cause of injury and death at the point of attack, and surrounding areas as well. The use of bombs probably first entered the world consciousness back in the 70’s by anti-government IRA forces who used homemade bombs to champion their cause against British rule. Radical groups and terrorists have used this strategy again and again, highlighted by a number of notable U.S. attacks:
• World Trade Center, NYC – 1993
• Alfred P. Murrah Federal Building, Oklahoma City – 1995
• U.S. Air Force Facility, Saudi Arabia – 1996
• American Embassy, Kenya – 1998
• Trade Center (NYC) & Pentagon (Washington, D.C.) – 2001
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Oklahoma City Bombing - 1995
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After the Oklahoma City bombing, President Clinton issued an Executive Order to the Department of Justice to assess the vulnerability of federal buildings to acts of terrorism. A special committee was established to review existing security and to establish minimum standards for all government buildings. At the time, security film had been established around the world as a way to mitigate hazardous glass caused by bomb blasts, but only a few U.S. government facilities were treated with Security Film. With bomb blasts now becoming a more critical issue the General Services Agency (GSA), the government arm responsible for evaluating and purchasing materials for government facilities, established certain criteria for any protective material including glazing systems and security film. Working with the U.S. Army Corps of Engineers, a number of tests were developed to determine the effectiveness of security film and laminated glass when faced with the impact of a bomb blast.
Bomb Blast Tests
There are basically 3-bomb blast tests used to determine the effectiveness of glazing systems/security film:
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U.S. Embassy
Nairobi, Kenya - 1998
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1. Open Frame Test – The most basic of all bomb blast tests, the Open Frame method takes two glass framed panels and places them side by side. Security film is applied to one pane while the other piece of glass is untreated. An explosion is created nearby causing the untreated glass to be blown away in a thousand pieces while the glass pane with security film is shattered, but the pieces remain intact (sometimes the blast causes the entire piece to be blown out of the frame). The reason for the test is to demonstrate that security film offers protection against dangerous flying glass shards. In terms of credibility, independent laboratories do not conduct Open Frames tests, so they are not widely embraced by professional engineers and government agencies.
2. Shock Tube Tests – This test is conducted in a specially designed shock tube. An untreated glazing system is first placed inside the tube with a panel (called a “witness panel”) placed directly behind it. A pressurized blast occurs within the tube and the panel absorbs the flying glass shards. The test is repeated using a glazing system treated with security film. The results indicate that glass treated with security film reduces the amount of flying shards striking the panel. One of the drawbacks of a Shock Tube test is that only positive pressure (i.e. forward moving force) is utilized. In reality, a bomb blast exudes both a forward moving force as well a negative “sucking” effect that causes outward pressure to the frame. This test method also doesn’t allow for a decrease of energy over a period of time.
3. Open Air Test – The most recognized bomb blast test is called the Open Air test. This test is widely used and accepted by professional engineers and government agencies, because it provides a more accurate account of what occurs during an actual bomb blast. Various glazing & framing systems are placed in a confined, outdoor environment and explosive devices are then detonated. The entire bomb blast sequence is filmed at various speed levels, and the subsequent responses are reviewed and analyzed. The diagram to the right and Exhibit 1 illustrates the different glazing system performances when subjected to a blast.
Exhibit 1:
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Performance
Code
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Protection
Level
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Hazard
Level
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Description of Window
Glazing Response
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1
2
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Safe
Very High
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None
None
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Glazing does not break. No visible damage to glazing or frame.
Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable. |
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3a
3b
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High
High
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Very Low
Low
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Glazing cracks. Fragments enter space and land on floor
no further than 3.3 ft. from the window.
Glazing cracks. Fragments enter space and land on floor
no further than 10 ft. from the window. |
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4
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Medium
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Medium
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Glazing cracks. Fragments enter space and land on floor and impact a vertical witness panel at a distance of no more than
10 ft. from the window at a height no greater than 2 ft. above the floor. |
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5
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Low
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High
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Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel
at a distance of no more than 10 ft. from the window
at a height greater than 2 ft. above the floor. |
A number of conclusions for security film & glazing systems can be drawn from the different bomb blast test methodologies:
• When properly installed on windows, security film provides significant glass hazard mitigation
• Increasing the security film thickness will improve performance
• Security film performs better on tempered or heat treated glass
• Laminated glass and glass polycarbonates can be designed to withstand significant blast loads
• Improper design can lead to the entire glass sheet being blown out of the framing system.
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