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Glazing Hazard Mitigation

by Joseph L. Smith, PSP and Nancy A. Renfroe, PSP
Applied Research Associates, Inc.

Last updated: 07-23-2010

Introduction

While windows and glazing are important architectural and functional components of a building (e.g., for daylighting) glass fragments caused by accidents, natural disasters, or intentional events such as terrorist attacks can lead to serious injuries to building occupants. In order to mitigate glass fragment hazards, designers must consider a multitude of factors, including a building's occupancy, functional requirements, and anticipated threats and risks to people and mission. As a result, protective glazing material design and selection may not be simply a matter of following building code requirements. It may be a governing component of building envelope, site, or interior design, around which other building systems must be designed and integrated.

Photo of Las Vegas Federal Courthouse

Fig. 1. Las Vegas Federal Courthouse used one-inch thick insulating glass panels fabricated with two panes of laminated glass. At critical locations the lobby curtain wall is anchored to the steel framework.
(Courtesy of Peter Aaron/Esto)

This Resource Page describes the fundamentals and basics of glazing hazard mitigation practices for both new and existing buildings. See WBDG Retrofitting Existing Buildings to Resist Explosive Threats for a more detailed discussion on protective design of window systems in alterations projects.

Description

Protective glazing is used to counter many threats to buildings and occupants including bomb (blast) attacks, ballistic attack, burglary or robbery incidents, forced entry, detention containment, and natural disasters such as seismic occurrences, hurricanes and tornados. A selection of the most appropriate protective glazing systems must be made for each project to address the specific threat.

A. Types of Protective Glazing Products and Systems

There are a wide variety of protective glazing materials and systems available to designers that will satisfy a projects' unique set of design conditions and needs. Selection of the most appropriate glazing hazard mitigation system will involve considerations of threat, vulnerability, and risk, the envelope design concept, (fenestration style (see also WBDG Style), cost, daylighting needs, and other design objectives such as the need to balance security/safety and sustainability objectives. The type of protective glazing material selected may also vary within a building depending on the window's or building's location (proximity to threats), size of the glazed opening, occupant load, and criticality of functions and missions housed within the facility, as well as other considerations such as whether windows will be fixed or operable.

Glass construction is generally divided into four main categories:

  • Monolithic
  • Laminated
  • Insulating
  • Miscellaneous (includes a variety of products, such as wired, textured and patterned glass, that modify the basic flat monolithic glass).

Other products commonly employed to reduce glazing hazards include, but are not limited to, glass clad polycarbonate systems, applied protective window films, protective window curtains and shields and cable reinforcement systems. See Figure 2.

Drawings of selected types of glazing products and hazard protection: Monolithic glass, Laminated glass, Insulating Glass Unit, Glass-clad Polycarbonate, Applied Window Film, Curtain/Shield System, and Cable System.

Fig. 2. Selected types of glazing products and hazard protection

Monolithic Glass refers to a single, flat piece of glass of constant thickness. The basic annealed glass product, also called "Float Glass", will typically break into very hazardous, dagger-like shards. Float glass can be made safer by several processes including heat treating, heat strengthening, chemically strengthening, and thermal tempering. Tempered float glass is available in thickness from 1/8" to 3/4". All of these processes, and resulting performance characteristics, are described in ASTM and ANSI standards.

Laminated Glass is composed of two or more layers of monolithic glass bonded with a thin but tough sheet interlayer material, most commonly polyvinyl butyral (PVB). The interlayer material is available in thickness from 0.015 to 0.090", depending on performance requirements of the project, with 0.030" being the most commonly used with annealed glass and 0.060" being the most commonly used with fully tempered glass. The layers of monolithic glass can be combinations of clear, tinted, or solar-reflective glasses. Laminated glass is widely used in the automotive industry for safety because it holds glass fragments together after breakage of the unit. See WBDG Retrofitting Existing Buildings to Resist Explosive Threats—Laminated Glass for additional information.

Insulating Glass is a sealed unit fabricated by combining panes of monolithic or laminated glass to create internal air spaces that provide improved energy performance. See WBDG Windows and Glazing for more information on insulating glass thermal properties. Insulating glass units may be configured with a variety of glass pane materials and may often use laminated glass on only the interior pane. If glazing is selected with the laminated glass on the interior surface only, then it is critically important to ensure that the windows are installed in the desired configuration. Accidentally installing the monolithic lite on the interior and the laminated lite on the exterior may create significant hazards to occupants.

Glass-Clad Polycarbonate is made by combining multiple plies of monolithic glass exterior layers with sheet polycarbonate interior layers (such as Lexan) and is typically used where there is risk of high-powered ballistic attack. Sometimes called bullet-resistant glass, the product may also employ air gap layers in heavier assemblies that may reach 2-1/2" thickness or thicker.

Applied Window Films (Security Films) are generally used in renovation or retrofit situations where entire window unit replacement with protective window systems is not practical or cost effective. These tough, transparent films consist of multiple, micro-thin polyester layers that work to hold shattered glazing together. The films are applied with adhesive to the interior-glazing surface and are available within a wide variety of strengths and thermal benefit values. Clear security films can be used on projects where historic preservation of existing windows and frames is a major design objective. See WBDG Retrofitting Existing Buildings to Resist Explosive Threats—Anti-Shatter Film for additional information.

Curtain and Shield Systems, such as safety drapes, may be added on the window interiors to serve as a "catch-system" to further prevent the danger of flying glass fragments. These devices can range from heavy clear sheet plastic film hung from the window frame head in utilitarian buildings to mechanical security window shades that have the appearance of commercial decorative shades in federal office buildings. See WBDG Retrofitting Existing Buildings to Resist Explosive Threats—Blast Curtains for additional information.

Glazing Catch Cable/Bar Systems are used to enhance protective glazing performance in buildings at risk of window "blow-out" (where the deformed pane leaves the frame) in a blast attack. Blast attack air pressures on glazing surfaces tend to cause the pane to flex at the center and pull away from the window framing system. Cable system supports are rigid cables or rods of steel installed on the interior frame of windows that serve to "back up" the primary window framing system and dissipate air pressures more evenly over an entire windowpane. See WBDG Retrofitting Existing Buildings to Resist Explosive Threats—Glazing Catch Cable/Bar Systems for additional information.

B. Types of Protective Glazing Assemblies

Protective glazing materials are only effective when designed as a complete and unified assembly including glazing, frames, connection to other building envelope materials, and the structural frame. Of key concern is the connection between the glazing and the frame and the "bite", which is how far the glazing is imbedded into the frame. Note that protective glazing systems, that are designed to flex (deform under pressure), and rigid systems require different types of detailing and anchoring to the window framing system. Protective glazing anchoring systems are generally classified as either mechanical or "wet" systems.

Mechanical Anchoring Systems: For new construction, these systems will capture the glazing product either by the edges or through the pane with metal frames or connectors and are usually made of aluminum or steel. Gaskets are detailed to absorb design pressures to hold the glazed pane in place. For retrofit projects, depending on the existing framing condition, protective security films may be applied and secured by an applied mechanical anchoring trim. Many types of extruded aluminum anchors are available in various shapes and colors and are sometimes customized for special applications.

Wet Anchoring Systems: Depending on the protective design requirements of a given building envelope area and opening, retrofit projects will sometimes utilize a Structural Silicone Adhesive, or wet system, to connect the edge of security film that is applied to the interior surface of a window frame. Under blast conditions, the film retains glass fragments in a single sheet that is suspended in the frame by the adhesive bond. If installed properly, wet anchoring systems can both meet performance requirements and be a cost-effective application. The depth of the silicone penetration into the frame bite is critical for proper performance. Generally, at least ½ in. penetration is required to achieve protection.

C. Design Levels for Glazing Hazard Protection

Threat/vulnerability assessments and risk analyses conducted for a project set the protective level requirements for glazing hazard mitigation. It should be noted that protective glazing measures would also be appropriate for buildings that are located near high risk targets, even though the building themselves are not considered a target. Collateral damage to buildings surrounding the A.P. Murrah Building in Oklahoma City and the World Trade Center site illustrates this concern.

The GSA blast protection criteria has been adopted by the multi-agency Interagency Security Committee (ISC) and is the most widely recognized classification of design levels for glazing hazard protection. See Table 1.

Table 1. ISC Security Design Criteria Blast Protection Levels for Windows

Performance ConditionProtection LevelHazard LevelDescription of Window Glazing Response
1SafeNoneGlazing does not break. No visible damage to glazing or frame.
2Very HighNoneGlazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.
3aHighVery LowGlazing cracks. Fragments enter space and land on floor no further than 3.3 ft. from the window.
3bHighLowGlazing cracks. Fragments enter space and land on floor no further than 10 ft. from the window.
4MediumMediumGlazing 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.
5LowHighGlazing cracks and window system fails catastrophically. 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 greater than 2 ft. above the floor.
A graphs showing the side view of the test structure illustrating the Blast Level Table performance conditions. The test window should be in the design position or centerned on the wall. The graph shows 10 feet at the bottom and the left side is represented by 1 and 2. The window is divided into 5 regions labeled 3a ( measures less than or equal to 3.3 feet), 3b, 4 (reaches the right side and makes up 2 feet of the right side), and 5.

Fig. 3. ISC Security Design Criteria Performance Conditions for Windows

D. Related Glazing Design and Integration Issues

After establishing a project's initial security performance level and protective glazing requirements for each area at risk, protective systems are selected in concert with consideration of other design requirements. It is important to ensure appropriate products and systems integration in the total glazing design solution and to achieve the optimum of multiple design objectives. Related design concerns include:

  • Integration of building and site design to achieve "standoff" distances (the distance between the building and vehicular or pedestrian traffic), thereby minimizing the need for hardening of building envelopes. Natural devices such as berms, hedges, and ponds can be used very effectively to achieve stand off distances.
  • Consider the attributes of rigid versus flexible glazing protection concepts. Many structural consultants hold that highly deformable energy dissipating protective window systems that restrain the debris and control glass failure are superior solutions to rigid systems in many respects including reduced material use.
  • Provide adequate access to fire safety personnel. Blast-resistant windows can present an obstacle to fire fighters who may need to "vent" buildings for smoke or gain emergency access through windows. The conflicting objectives of fire safety and blast resistance require a balanced design that provides the weakest acceptable laminated glass within the most flexible frame. Increased strength is not always the best way to provide a multi-hazard glazing protective design.
  • Account for additional weight of heavier glazing systems and resultant additional structural frame and envelope attachment requirements.
  • Utilize available glazing hazard design tools and software such as WINGARD PE or WINGARD LE available from the GSA to authorized users.
  • Evaluate existing tests of products, systems, and technologies to determine appropriateness for intended design applications.
  • Evaluate existing tests of products, systems, and technologies to determine appropriateness for intended design applications.
  • Consider daylighting and workplace performance objectives of the project and select protective glazing products and systems that achieve an optimum of multiple design objectives. If selected properly, protective glazing products can also act as sun control and shading devices. Some protective films may block out too much daylight and produce excessive exterior reflectivity. Consider natural ventilation benefits and balance the design of protective glazing with building occupants' indoor environmental needs.
  • Evaluate the benefits and conflicts of protective glazing with sustainability goals and balance security/safety and sustainability objectives in as optimal and resource-efficient design solution as possible. Consider the thermal benefits of all glazing materials and systems and use economic analysis to evaluate facility investment decisions.
  • Note that the useful life of many protective films is approximately 7-10 years and, depending on an operating facilities' energy efficiency, costs of protective film installation or window replacement may require an extended return on investment period. Most manufacturers and installers now provide 10 year product warranties.

E. Designing for Blast Effects1

Among the threats, bomb blasts pose some of the most demanding glazing design challenges and can be a governing influence in the overall building envelope design. When a bomb detonates, shock waves are created that travel out from the detonation source. The blast pressures from an explosion decrease rapidly in magnitude with increasing distance from the explosion. In most cases, especially for design purposes, simplified methods may be used to estimate the blast loading. For design, the blast pulse is generally simplified and assumed to consist of a triangular shape. The overpressure is assumed to rise instantaneously to its peak value and decay linearly to zero overpressure (i.e., back to ambient pressure) in a time known as duration time. In this case, the area under the pressure-time waveform, or impulse, is simply the area of a triangle, or:

I=P*td/2

Where I = Impulse, psi-msec
P = Peak pressure, psi
td = Duration time, msec

Both the pressure and the impulse (or duration time) are required to define the blast loading. In order to maintain these values, a number of available tools may be used such as the program AT-BLAST. This program (by Applied Research Associates, Inc.) implements the standard Kingery-Bulmash air blast equations that are used in most Department of Defense technical manuals. Alternatively, tables of pre-determined shock parameters may be used to estimate blast pressure and impulse. Example tables for several bomb sizes as a function of standoff distance are provided in the following:

Airblast Parameters for 50 lb. TNT Detonation

50 lb. TNT20 ft.50 ft.75 ft.100 ft.200 ft.300 ft.
Peak reflected pressure (psi)122.812.66.34.11.60.9
Reflected impulse (psi-msec)136.547.530.622.510.97.1
Peak incident pressure (psi)34.755.52.91.90.80.5
Incident impulse (psi-msec)51.4622.015.611.96.14.0

Airblast Parameters for 100 lb. TNT Detonation

100 lb. TNT20 ft.50 ft.75 ft.100 ft.200 ft.300 ft.
Peak reflected pressure (psi)246.720.39.25.82.21.3
Reflected impulse (psi-msec)228.577.449.436.317.511.4
Peak incident pressure (psi)59.18.34.12.71.10.6
Incident impulse (psi-msec)80.935.224.518.79.66.4

Airblast Parameters for 500 lb. TNT Detonation

500 lb. TNT20 ft.50 ft.75 ft.100 ft.200 ft.300 ft.
Peak reflected pressure (psi)1,182.679.527.414.64.62.6
Reflected impulse (psi-msec)783.0246.0153.5111.152.534.3
Peak incident pressure (psi)197.024.910.76.32.11.3
Incident impulse (psi-msec)209.196.667.552.427.418.5

Airblast Parameters for 1,000 lb. TNT Detonation

1,000 lb. TNT20 ft.50 ft.75 ft.100 ft.200 ft.300 ft.
Peak reflected pressure (psi)2,133.7157.149.024.06.53.6
Reflected impulse (psi-msec)1,354.0409.6252.4181.384.655.0
Peak incident pressure (psi)314.742.017.19.63.01.7
Incident impulse (psi-msec)222.8150.0104.181.043.129.2

In any bombing attack, there are three basic types of effects that the occupants may experience:

  • Primary Effects: Primary effects include the human body's response to direct blast loadings. These can be the result of exterior or interior detonations, which produce reflected, incident, and possibly gas pressure loadings. The blast forces produced directly interact with the occupants causing injury or possibly death.
  • Secondary Effects: Secondary effects include fragment and debris impacts. Heavy and/or high velocity fragments and debris interact with the occupants of the facility causing injury or possibly death.
  • Tertiary Effects: Tertiary effects include loss of balance and subsequent impact of the person into his/her surroundings due to the passing blast wave or violent movement of a supporting structure.

The debris generated, or the collapse of structures produced, during an explosive (blast) attack causes the majority of injuries and death in a bombing event. As an example, over 5,000 people were injured by flying glass and debris in the bombings of two American embassies in Africa in 1998. The types of injuries that occurred included deep lacerations, eye injuries, etc. Approximately 90 people were blinded in the attack on the U.S. embassy in Kenya.1

When designing window systems to resist blast forces it is important that the glazing, framing, and anchorage all be designed to withstand the required forces. Generally, the glazing should be the weak link (i.e., it is not desirable for the window system to prematurely fail and blow into occupied spaces due to failure of the frame or anchorage). This approach is commonly referred to as balanced design.

Application

Building codes require protective measures for glass hazards in high traffic circulation areas that would be prone to easy breakage in order to ensure occupant safety and health. Typically, these codes require that door lights, side lights, and interior glass walls and balcony rails be made of tempered or other strengthened glass. Even the threat of fire in buildings dictates the need of protecting exiting occupants from the danger of flying glass that could be caused by heat breakage. In addition to providing protection, emergency ingress and egress from buildings must by considered in the design of protective glazing and window systems. Recent testing has shown that most systems can be readily breached by emergency personnel.

Beyond building codes, however, additional protective glazing measures have traditionally been employed in defense installations, industrial facilities, financial institutions, and detention facilities. Today, primarily because of the increased threat of terrorism, all federal buildings require enhanced levels of protection from the hazards of glass. Approximately 75 percent of all damage and injury from bomb blasts can be attributed to flying and falling glass following the explosion. Even high profile private buildings such as tall buildings and places of public assembly are now routinely evaluated for enhanced glazing protection measures. Protective glazing is a key component of "hardened" buildings and the ultimate goal of hardened buildings is to provide security for building occupants and assets. See also WBDG Federal Courthouse, Office Buildings, and Research Facilities.

Relevant Codes and Standards

Except when mandated in building codes for fire safety and protection from accidents in buildings, there are no minimum requirements for protective glazing other than for governmental buildings. Federal standards and criteria are therefore widely recognized as the leading force in new materials and systems development and application, particularly with regard to blast related glass hazard mitigation.

Mandates

Federal Guidelines

Private Sector Guidelines

  • Blast Effects on Buildings: Design of Buildings to Optimize Resistance to Blast Loading by G.C. Mays and P.D. Smith. London: Thomas Telford Publications, 1995.

Public Testing Institutions

Private Testing Laboratories

  • Many private laboratories with expertise in protective glazing systems testing are also available. Contact the Protective Glazing Council for additional information and referral.

Additional Resources

WBDG

Building / Space Types

Applicable to most building types and space types, especially high occupancy buildings such as Educational Facilities, Federal Courthouse, Health Care Facilities, Office Building.

Design Objectives

Aesthetics—Engage the Integrated Design Process, Cost-Effective, Functional / Operational, Historic Preservation, Productive, Secure / Safe, Sustainable

Products and System

Section 07 92 00: Joint Sealants, Building Envelope Design Guide: Fenestration Systems, Glazing, Windows, Curtain Walls, Sloped Glazing, Atria Systems
Federal Green Construction Guide for Specifiers:

Project Management

Project Planning, Delivery and Controls, Building Commissioning

Tools

LEED®-DoD Antiterrorism Standards Tool

Security Criteria Centers

Organizations and Associations

Publications