Volume 38, Issue 1, January 2003
Protective Glazing Guidelines
Basics for Protecting Buildings Against Terrorist Threats
by Julie Schimmelpenningh
“It’s not if, but when.”
These words, offered by Donald Rumsfeld, U.S. Secretary of Defense, regarding the potential of another security attack are etched in the minds of many Americans. The realization that we live in a threatening world has been brought to the forefront and remains ever looming. Although no single answer can be found to relieve our collective anxiety, the glass industry is making a difference. Security glazing made with laminated glass provides architects and building owners with the ability to design buildings that retain the feeling of openness, while providing a safe and secure environment for occupants.
Laminated Glass as a Deterrent
Laminated glass has been used to enhance the performance of glass for more than 60 years. It consists of a minimum of two lites of glass bonded together by an interlayer. Laminated glass has been used in the architectural market for well more than 30 of those years, but not to its fullest potential until recently. Laminated glass has always been an option for safety glazing, although most people equate the words tempered glass with safety glass. Laminated glass is capable of delivering the impact safety that is called out in the standards and required through the buildings codes, but it also offers the ability to be retained in the frame should breakage occur. Until now, this performance benefit has not garnered much attention. Now that we have fallen under attack, we realize how critical it is to maintain the building envelope during a catastrophic event to help minimize damage and protect lives.
In recent years, laminated glass has been recognized for its versatility and ability to stand up to Mother Nature. The adoption and propagation of windborne debris protection in hurricane zones, the inclusion of performance language in the National Earthquake Hazards Reduction Provisions (NEHRP) for seismic protection and the everyday use of glass for wind and snow load have proved to be challenges easily met by laminated glass products.
When designing for security, the real performance properties of laminated glass products have become crystal clear. Laminated glass has been used globally for forced-entry, ballistic and blast resistance for many years. The recent awakening to terrorist activities, however, has thrust the subject of security glazing to the forefront for buildings that were once considered low risk.
The most complex configuration of security glazing utilizing laminated glass is glass that is designed for ballistic resistance. There are several standards that outline test methods and ratings of ballistic-resistant products. The most common one referenced is the Underwriters Laboratory (UL) 752 test method. This method provides the guidance to test and rate glazing at various temperatures and with multiple rounds of shots from a prescribed weapon. The typical pass/fail criteria relies on the absence of glass spall toward the observer. The penetration of the bullet can also be considered, although the lack of penetration alone does not permit a rating under UL 752, significant life safety can be achieved with glazings capable of stopping the bullet.
Typical configurations of bullet-resistant glazing consist of multiple plies of thinner glass, glass less than 12 mm (0.5 inch), separated by thin layers (0.38 mm (0.015 inch)) of interlayer. The minimum thickness to resist the lowest level of ballistic attack outlined in UL 752 (9 mm handgun) is about 32 mm (1.25 inches). The interlayer in ballistic-resistant glazing is required to perform differently than when utilized in other safety and security glazing—the interlayer is simply in the composition to hold the mass of glass together so that the glass bulk is there to fend off the bullet.
In forced-entry resistant glazings, the interlayer is utilized in the configuration to provide retention performance and to absorb the energy of the impacting device. There are two traditional test methods commonly cited for forced entry ratings: the American Society of Testing and Materials (ASTM) F 1233 and UL 972. Both test methods involve the multiple impacts of glazing by an object or objects with the intention of not allowing an opening to form through which entry can be forced. Typical configurations for forced-entry resistance are fairly simple: they involve annealed, heatstrengthened or tempered laminated glass with a minimum of 1.52 mm (0.060 inch) interlayer.
Jewelry storefronts, museums, residences and government buildings are common areas where burglary-resistant glazing can be found. This glazing typically protects against the opportunist thief, but thicker and more complex structures can be utilized for detention and institutional facilities where the object is to keep people in rather than to keep them out.
The area of security glazing that truly has been thrust to the forefront is blast-resistant glazings. Since the early 1980s, data on the performance of laminated glass has been available for use in the design of buildings. The need for such a performance attribute has gone virtually unnoticed in the United States as there was a lack of perceived need and an “it won’t happen here” attitude. With the bombing of the Alfred Murrah building in 1995 and subsequent bombings around the world, people have begun to understand the potential damage that can be caused by glass during such an event. In Oklahoma City, glass damage was recorded for 23 blocks surrounding the targeted building. Most of the injuries not associated with structural collapse of the building can be attributed to flying glass.
Does this mean buildings should be designed as bunkers? That allowing natural light into a building is too risky a proposition? No. Glass, however, has received a bad rap. True, glass breaks, but with the proper selection of glazing for a potentially targeted building and for the surrounding buildings, many injuries and building serviceability issues can be avoided.
Although blast-resistant glazing seems like it would be a very complex issue, it actually is the simplest of all the security glazings discussed in this article. Standard two-ply laminated glass with a 0.76-mm (0.030-inch) interlayer is capable of servicing a very wide range of blast threats. It may be necessary to move to a 1.52-mm (0.060-inch) interlayer due to some design considerations or at the higher government load specifications, but single laminated or laminated insulating units can—and do—perform very well in this application. The standard test method that has been developed by national consensus is ASTM F 1642. It is currently under revision to take into account the ratings of glass spall toward the interior. There are many design techniques that can improve the physical security of a building. The steps to remember when designing for resistance are as follows:
1. Know your neighbors. Do a risk assessment on your building and surrounding buildings.
2. Allow as much distance between your building and the potential location of attack as possible (stand-off distance).
3. Put windows in your building. Allowing vision to the outside may help deter attacks.
4. Design the glass to withstand structural wind and snow loads appropriate to the building and location.
5. Determine if you are concerned with protecting only the occupants or pedestrians as well. For example, in Oklahoma City there were significant injuries to people on the nearby streets.
6. Incorporate laminated glass as a single unit or as one or both plies in insulating glass units. The laminate should always be toward the interior.
7. The glass should be anchored correctly into the window.
8. Design proper anchoring of the frame to the building.
9. Install the windows according to the manufacturer’s instructions.
10. Enjoy your facility. You now have improved the building’s ability to withstand an attack.
The formation of the Homeland Security Office is sure to bring about major changes in how we all deal with security issues. Our industry has the ability to make a tremendous difference in this world. With more awareness, education and participation in this market we will see more adoption, innovation and hopefully the removal of the no-glass bunker mentality that usually surfaces during troubled times like these.
|Test Methods—At a Glance|
|UL972: Impact resistant glazing
Summary: Glazing subjected to vertical impacts from 2.26 kg (5 lb.) steel ball.
Number of performance levels: Four.
Typical laminated glass that passes: 1.52mm (0.060 inches) thick interlayer
|UL752: Bullet Resistant Glazing
Summary: Glazing subjected to impact by various guns.
Number of performance levels: Four.
Typical laminate thickness to pass each level:
Level 1: 1 1/4 inches.
Level 2: 1 1/2 inches.
Level 3: 1 3/4 inches.
Level 4: 2 inches.
|ASTM F1233: Security Glazing Standard
Summary: Glazing subjected to impact one to five times by various guns.
Number of performance levels: 11.
Typical laminate construction to pass each performance level: 1 ¼ inch minimum—3 plus inches.
|Forced Entry Test:
Summary: Glazing system is subjected to a series of assaults with a range of weapons.
Number of performance levels: 5 (Class I through Class V).
Typical laminate construction to pass each level:
Class I: Laminate with 0.060 inches thick interlayer.
Class II: Multi-layer laminate more than ¾ inches thick.
Class III: Multi-layer laminate more than 2 inches thick.
Class IV: Specialty laminates.
Class V: Specialty laminates.
Julie Schimmelpenningh serves as architectural applications manager for Solutia Inc., in St. Louis.
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