Printer Friendly

Panes that pay.

WHEN COLLABORATING ON design, security professionals, architects, and engineers often disagree over window size, placement, and expense. Such disagreement is a major hurdle to designing blueprints that meet the needs of the specialists as well as the client.

Architecture/engineering design firms have aesthetic goals that haven't traditionally addressed security requirements. Unfortunately, when security concerns and fenestration design are at odds, the professionals from each discipline have had no comprehensive source of information with which to make informed choices - until now.

No existing building codes require glass to defeat or deter forced entry, to deflect ballistic projectiles, or to remain intact after explosions. Moreover, intact after explosions. Moreover, there has been no quantitative information on glazing products capable of deterring attacks, whether bombs, bullets, burglars, or electronic invasion.

Recently, however, a study of glass performance in security applications was completed. The study was the most extensive privately funded study on the subject since World War II. It offers guidance for building designers who demand that glazing be able to serve the dual purposes of preserving the integrity of a building envelope from attacks while providing visual communication with the outdoors.

It's often untenable for security specialists to find fault with designs that use glass, for in a very real sense the terrorists are winning if the threat they pose consigns potential targets to windowless monoliths. Plainly speaking, people want windows.

The study reflects a new view - that security and fenestration are practical and affordable when the primary design goal is the survival of people and assets behind the glass, rather than imperviousness of building facades to various attacks.

Security glazing refers to a wide range of glass products that share the common attribute of one or more interlayers of energy-absorbing, high-performance plastic. Standard laminated glass consists of two or more plies of glass bonded by a polyvinyl butyral (PVB) plastic interlayer. Laminated security glass products can be specified for virtually any application, regardless of the architect's requirements for heat transfer, acoustics, visibility, or aesthetics.

The study and test procedures were designed in part and overseen by security consultant Dr. Ron Massa of the Lorron Corporation of Burlington, MA. The tests focused on the most common types of threats; blast effects, whether accidental or the acts of terrorists; various caliber ballistic attacks against a variety of window lites (panes); forced entry through glazings; and electronic invasion.

"Specifying laminated glass has traditionally been considered an extraordinary measure," Massa states. "We tested laminated glass against what systematic study of terrorist and criminal arsenals reveals to be the highest-percentage attack scenarios. Life safety can be practical and affordable with laminated glass. Specifying laminated glass should be standard practice in the security community."

Test results indicate that while architectural monolithic glass meets some requirements for accidental damage, monolithic glass constructions in common architectural thicknesses and strengths offer little or no resistance to forced entry, ballistic attack, or electronic invasion. More significantly, monolithic glass in common architectural thicknesses becomes part of the weapon when shattering under blast overpressures.

Failure in monolithic glass under bomb attack is dramatic. After shattering, shards fall or become projectiles.

When speaking of laminated glass, however, failure should be redefined. Failure becomes a matter not of whether the glass survives an attack but of whether the building envelope is intact. The retentivity laminated glass exhibits can provide protection for people and assets after a blast. That factor puts laminated glass in a different class than monolithic glazing and should be considered in the purchasing process.

Laminated glass under main-force (forced-entry) attack, for instance, in 1/4-in. or thicker constructions, can prevent quiet cutting or single-blow breakage from rocks, small tools, and other hand-held objects.

In addition, tests have shown that laminated glass in 1 1/6-in.-thick constructions can provide life-safety protection from small handguns while protection from long guns requires laminated glass of 1 1/2-in. or thicker constructions.

The series of blast-effects studies revealed that laminated glass configurations can withstand blast overpressures of up to 60 lb. per square inch for durations of up to 3 milliseconds, pressures monolithic glass cannot withstand.

Note that the research was conducted on laminated glass fabricated with Saflex PVB interlayer. Extrapolation to other systems, whether PVB-based or not, should be avoided.

According to FBI Bomb Data Center statistics, 95 percent of the bombs that terrorists detonate weight under 10 lb. In 1989, 1,227 bomb attacks were reported in the United States, an increase of about 20 percent compared to the previous year. Of those attacks, only 5 percent involved bombs weighing more that 100 lb., while more than 70 percent involved bombs that weighed less than 30 lb.

When a bomb is detonated, the pressure shock wave propagates at many times the speed of sound. After this positive phase passes, a negative phase occurs, during which local ambient pressure drops before returning to normal. Although blast pressures may be scaled and predicted to a degree, computing blast waves in the air is only the first step in determining the effects of bomb blasts on structures that include glass.

Dr. Scott Norville, director of the Glass Research and Testing Laboratory at the Texas Tech University School of Civil Engineering in Lubbock, was chosen to conduct the bomb-testing portion of the studies. For the test program, more than 300 glass lites were subjected to blast loadings from detonations of 3.64 kg and 45.38 kg of C-4 plastic explosives - the equivalent of 10 lb. of TNT.

Fully tempered and heat-strengthened monolithic lites and laminated glass constructed of combinations of those two were tested. Annealed glass and insulated glass configurations were also tested at varying distances from ground zero.

In addition, some samples with a so-called "security" film of PET (polyethylene terephthalate) applied after framing were tested. Each test was conducted with the samples mounted in frame-neutral configurations and at similar ambient air temperatures and barometric pressures.

After each blast, the researchers examined the results for three factors: whether the lite had broken; if so, what percentage of the lite remained in the frame; and what the sizes and final locations of any glass fragments that left the frame were.

A comprehensive report of test results on all 300 samples tested would be too long to include here. But a synopsis of the test results as published by Norville is revealing:

At test frame S6, 15 ft. [4.5 meters]

from ground zero, all monolithic

glass lites fractured, with

glass shards flying as far as 40

meters from the frame. All monolithic

lites of nominal 6 mm thickness

coated with PET (polyvinyl

terephthalate) security films fractured

and were torn from the

frames. Shards from these lites

separated from the PET films and

flew up up 9.14 meters from the


One or more plies from the laminated

glass lites fractured, but the

laminated glass units remained in

the frame. No shards of measurable

dimensions left the laminated

glass units. The monolithic lites

in the insulating glass units always

fractured, showering large shards

up to 1.22 meters in front of the

frame. Laminated glass in the insulated

glass units usually fractured,

but the interlayer kept them

intact, with no glass shards whatsoever

flying behind the frame into

the protected area.

The most significant damage in

approximately 75 percent of all

bombings is the failure of architectural

glass. The glass acts as

part of the bomber's weaponry.

Unfortunately, that includes the

glass in untargeted structures near

the blast.

However, laminated security glazing supplied in common architectural thicknesses and weights can substantially reduce injury caused by flying glass and even reduce the cost of repairing a bombed facility near a bomb attack by reducing the extent of damage and the amount of debris to be cleared. "What we've learned," Massa notes, "is that laminated glass becomes part of the defense rather than part of the attack."

The Texas Tech study provides the security professional with several new avenues of approach when collaborating with architecture/engineering design firms on bomb-resistant fenestration. It is proven, for instance, that laminated glass units in common architectural thicknesses and sizes offer building inhabitants the greatest chance of surviving a bomb attack when compared to monolithic glazing of similar dimensions.

The implications of the security glazing study reach beyond the concerns of new construction and should be considered for security retrofits. "Bombs are indiscriminate," Massa explains. "Some day a potential target may move in across the street, and there you are, an unwitting victim." Retrofitting films of PET to monolithic lites as an after-thought, although probably better than nothing, does not strengthen the glass to which they are applied.

Massa believes the bomb tests may suggest to the security community different ways of looking at attacks. "Maintaining the building envelope during the short-duration blast loadings is what we are learning about," he says.

In the past, mass was thought of as the only defense against bombs. Most of the previous tests were performed in the governmental arena, and although the results weren't made public, the government security community prefers massive lites of clear polycarbonate as a deterrent to perceived bomb threats.

That response, however, is almost universally regarded in the private sector as prohibitively expensive. The government security community's reliance on seemingly indestructible masses of polycarbonate is due mainly to test protocols, in which blast-loading tests assumed a bomb size of 4,000 lb. Polycarbonates have the inherent problems of ultraviolet degradation (including becoming brittle and opaque) and scratching.

Massa thinks the merit of the tests for security professionals is that they have quantified the tendency of laminated glass to bend but not break. "If, during the life of a window, it's asked to withstand for 3 milliseconds blast overpressures that would send shards of monolithic glass across a room and embed them in a wall, then the laminated glass has provided life safety," Massa explains, "So life safety is achievable at replacement costs for a cracked but intact lite."

THE H.P. WHITE LABORATORY IN Street, MD, was contracted to perform ballistic and main-force tests on laminated glass in accordance with standards set by the American Society for Testing and Materials (ASTM). The ballistics testing lab has conducted stress and failure analysis on materials since it was founded in 1936. All told, the test program called for more than 50 ballistic and main-force tests on dozens of configurations of laminated, tempered, and heat-strengthened glass products.

In the past, there has been no science of glass performance under ballistic attack. One school of thought suggests that total mass is the only sure protection against ballistic attack. That theory would require glass lites 3 in. and thicker. And even then there are no guarantees.

"Detailed test protocols that call for a glass lite to remain completely intact after systematic ballistic attack, however, may be unrealistic," Massa suggests. "It is possible for a lite to fail according to ASTM test protocols while still offering life safety to those behind the glass."

Like the blast testing, the ballistics tests focused on attack scenarios and weapons most likely to be chosen by terrorists and criminals. During one sequence of the ballistics test program, a range technician for H. P. White fired an Uzi submachine gun from behind a protective barrier.

The laminated, 2-in.-thick, 30" X 30" high-security sample was initially subjected to a group of five 9 mm rounds in three seconds within an 8-in. circle. A failure, according to the ASTM test standard, is determined by any spalling (glass fragments ejected from the back of the sample).

After absorbing 20 Uzi rounds, the sample showed a slight bulge on the protected side. Finally, after being hit with 25 Uzi rounds, according to the ASTM standards, the sample failed. Several fingernail-sized chips had dropped from the back of the sample to the floor.

However, should a terrorist or criminal ever find the time and circumstances to slowly and deliberately fire dozens of Uzi rounds at a similar window, it seems improbable that a person might view the attack with his or her nose against the glass.

Of course, when developing a standard for materials meant to protect life, ASTM must assume a worst-case scenario. But as Massa points out, "A laminated lite failing the ASTM test can still offer an outstanding security value."

Finally, after two more 13-round clips were fired on full-automatic into an area the size of a palm print, a penetration occurred. It was possible to push a pencil through a hole in the glass. That hole was probably made by the 51st bullet to hit the sample.

"If life safety defines the value of glass for a bank teller," Massa states, "there are known constructions of laminated glass that offer protection from an array of common ballistic projectiles. If, after an attack, your bank teller is unharmed physically, even though there is some ejecta or spalling from the protected side of the lite, that defines a successful and practical security measure."

Although the tests showed that laminated glass lites offer significant life-safety advantages over monolithic glass of the same thicknesses, too many variables (shot obliquity, distance from discharge, ambient temperature, muzzle velocity, etc.) remain to guarantee life safety. Future research may prove that both the thickness and the cost of ballistic glass can be reduced.

The major discovery of the ballistics testing is that even glazings that fail in the ASTM sense can offer substantial security benefits. This emerging rationale for specifying laminated glass as a means of surviving terrorists' weapons of choice provides security specialists with a new model for anticipating and reacting to ballistic threats.

Perhaps the most interesting findings of the studies were on the subject of main-force attacks. Massa explains, "We all think of glass as fragile, but laminated glass in security scenarios is one of the toughest building materials available. Laminated glass gives a composite response to energy inputs that laypeople find hard to fathom." Indeed, an 8-in.-thick, concrete-block wall can be breached with less than 10 blows of a sledgehammer, while a 1/2-in. laminated glass lite can resist dozens of blows from an identical tool.

ASTM attack sequences, however, may not reflect the reality of most breaking attempts. "Aggressors seem to have boundless energy and creativity," Massa explains. "But they are cognizant of with a risk that increases exponentially with each passing second. They know, too, that as they work, other security systems are sensing and responding to their attempts."

A reasonable risk would be a one-shot attack with a brick or rock, a reach through the breached glass, and escape. "We've found that even the thinnest laminated glass offers better forced-entry protection than monolithic glass of the same thickness. It takes at least one minute of hard, loud effort to achieve a smash-and-grab result through a 1/4-in. laminated glass lite."

With targets of opportunity being the stock-in-trade of burglars, Massa feels the use of laminated glass should be standard practice in any architectural application where a reasonable possibility of a break-in exists.

The forced-entry tests were performed according to the test sequence for ASTM Standard F1233-89. Samples were subjected to both blunt and sharp impact weapons as well as thermal stress and chemical deterioration weapons.

The tests revealed that heat-strengthened laminated glass is superior to annealed or fully tempered laminated glass. Several plies of thinner glass are superior to fewer plies of thicker glass, and thicker interlayers are more resistant to main-force attacks than thinner interlayers.

Security professionals must also prevent the losses that occur from theft or destruction of information. Such losses can occur as a result of electronic eavesdropping and electromagnetic interference induced by sources as disparate as solar flares and construction equipment. Laminated security glazing with metallized screen or fabric protected between two interlayers of PVB can provide protection from such losses.

When the laminated lite is manufactured, the metallized fabric is extended about an inch beyond the glass edges. when installed in a four-sided metal frame, the protruding mesh is mechanically and electrically connected to an effective ground, thus isolating the protected facility from electromagnetic radiation and ensuring that electromagnetic signals cannot be intercepted through the glass.

Laminated glazings selected for an architectural or security application can be fabricated for electronic security as well. The mesh slightly but uniformly tints the glass but is practically invisible.

Window frames are critical to more than just electronic security measures. Laminated security glazing alone does not obviate the need to design frames that can withstand the dynamic loads that these lites must withstand.

Laminated glass actually strengthens the frame because it distributes the load over the entire frame. As laminated glass lites bend, they absorb energy and spare the frame from receiving the entire load. Thus it is unlikely that all the framing anchors in a window would fail simultaneously.

The cost of lost time and business after an attack are measurable, as are comparative glazing costs. As a rule of thumb, laminated glass costs 15 to 20 percent more than tempered glass of the same thickness.

Massa points out, "Plain-vanilla glass is rarely specified by architects. With the many types of specialized glazings available today, fenestration systems are regarded as serving multiple purposes beyond simply providing visual communication with the outside.

"Laminated glass can be made in configurations that accommodate all the architectural niceties. Laminated glass should be standard practice in curtain walls and any other applications where security professionals are called in because threats are perceived. If the nominal cost increase seems out of line, compare it to the cost of inadequate security."

PHOTO : How does security glazing measure up against blasts, ballistic attacks, and electronic invasion? Read here and see.

Tom Harpole is a free-lance writer in Helena, MT, and a member of the International Society of Explosive Engineers.
COPYRIGHT 1991 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Special Seminar Issue; security glazing
Author:Harpole, Tom
Publication:Security Management
Date:Sep 1, 1991
Previous Article:When employees beat the system.
Next Article:Sobering up for success.

Related Articles
Setting the standard for security glazing.
Window fix not open and shut.
Windows 2000.
Glazing Vision. (Specifier's Information).
Pilkington. (Specifier's Information).
Thermal resistance of a window with an enclosed venetian blind: guarded heater plate measurements.
Solar heat gain coefficient measurements for glazings with indoor window attachment products.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters