Through the window: Security glass magnified

by Matt Jacobsohn

Photo © Matt Jacobsohn
Photo © Matt Jacobsohn

Security glass has mainly been used by the military and other government entities, such as prisons and jails, police stations, and healthcare facilities. The scope has now expanded to markets and industries where security, particularly glass, used to be an afterthought. For instance, until recently, schools, retail outlets, pharmacies, corporate offices, religious institutions, hospitals, and data centers were rarely threatened by violent crime, active shooters, digital warfare, and natural hazards, and therefore, did not require security glazing. These market segments are employing security glazing products, but much of the money spent on them is being done with limited direction and research.

Security glazing is designed to protect against an array of situations and meet numerous criteria. It comes in various materials, weights, and thicknesses. These products undergo tests designed for specific threats or events, and often require surrounding material to carry an equal rating. Whether a door, frame, window, U-channel, or fitting, all the components of a system are designed to work in tandem to mitigate a threat. It is important that safety glass employed in security glazing systems comply with the American National Standards Institute (ANSI) Z97.1, For Safety Glazing Materials Used In Buildings – Safety Performance Specifications And Methods Of Test, and 16 Codes of Federal Regulations (CFR) 1201, Safety Standard For Architectural Glazing Materials, Category I and II ratings.

Test methods

The standards mentioned above differ in many ways, including who mandates its use, types and number of specimens tested, and the overall details of test setup, analysis, and reporting. Federal standard 16 CFR 1201 preempts all non-similar and state regulations, and ANSI Z97.1 is determined by building codes and glass/fenestration specifications. Only one specimen of each glass thickness requires testing in 16 CFR 1201, but ANSI Z97.1 needs four of each thickness. Unlike ANSI Z97.1, 16 CFR 1201 does not perform tests on plastics and bent glass. Both tests have multiple categories with the highest one requiring a pendulum impact from 1219 mm (48 in.) high, and the second categories need a pendulum impact from 457 mm (18 in.) high. ANSI Z97.1 has a third test category involving a pendulum impact from 305 mm (12 in.) high. However, this is only applicable for fire-rated glazing materials. In both standards, the criteria to pass means selecting the 10 largest broken pieces and weighing them to see if they are lighter than a 6452-mm2 (10-si) area of the original specimen.

Figure 1: Ratings of bullet-resistant materials as identified by Underwriters Laboratory (UL) 972, Standard for Burglary Resisting Glazing Material. Image courtesy Cellular Material International Inc.
Figure 1: Ratings of bullet-resistant materials as identified by Underwriters Laboratory (UL) 972, Standard for Burglary Resisting Glazing Material.
Image courtesy Cellular Material International Inc.

Many security glass test methods require complete system testing. However, some do not incorporate the system in which the glass is installed, thereby leaving its performance in field conditions unknown. The whole purpose of employing a strong piece of glass is lost if it falls out of its framing system post-installation.

Underwriters Laboratory (UL) 972, Standard for Burglary Resisting Glazing Material, measures the effectiveness of a 1.2 x 1.2-m (4 x 4-ft) glass specimen by laying it parallel to the ground and then dropping a 2-kg (5-lb) steel ball on it from four different heights. Failure of this test happens when the steel ball passes through the specimen. However, the test requirements do not cover the framing system used in the installation of burglary-resistant glazing material. In this author’s experience, the UL 972 test is usually specified because of its name and implication. People want glass that can stop a burglar. Therefore, designers looking for a test method are first directed to UL 972. To see the specifics of the test and its setup requires a test procedure document costing more than $500. This is where the research often stops, as people do not want to spend money on such documents. Hence, details of the test procedure and setup are missed and the test is taken.


It is important to note there is no such thing as ‘bulletproof glass.’ The term perpetuates the belief no bullet, not even one, will penetrate the glass. This provides a false sense of security and is widely misused. When glass passes a ballistic test, it is either referred to as ‘bullet-resistant’ or ‘bullet-rated’ to meet a specific level within a standard.

Bullet-resistant glass is tested according to UL 752, Protection Standards For Bullet Resistant Glass Products, or the National Institute of Justice (NIJ) 0108.01, Ballistic Resistant Protective Materials. The testing methodology employed depends on the client. NIJ is the U.S. State Department’s ballistics standard for the military and federal government applications.

UL 752 can be used for any other application where ballistic resistance is desired. The testing for ballistic resistance is set up with the end of the specified firearm barrel being 4.5 m (15 ft) from the surface of the glass, and a witness panel (aluminum foil or corrugated cardboard) placed 45 m (18 in.) behind the test specimen. If a bullet goes through the test specimen and the witness panel, the product fails. For instances where it is desirable to have exposed glass surfaces, a UL 752 tested product may receive an added layer of glass. If such a product is used, it will prevent bullet penetration as tested, however, it will be susceptible to spalling on the interior surface.

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