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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.
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.
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:
- 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.
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 Condition||Protection Level||Hazard Level||Description of Window Glazing Response|
|1||Safe||None||Glazing does not break. No visible damage to glazing or frame.|
|2||Very High||None||Glazing cracks but is retained by the frame. Dusting or very small fragments near sill or on floor acceptable.|
|3a||High||Very Low||Glazing cracks. Fragments enter space and land on floor no further than 3.3 ft. from the window.|
|3b||High||Low||Glazing cracks. Fragments enter space and land on floor no further than 10 ft. from the window.|
|4||Medium||Medium||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.|
|5||Low||High||Glazing 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.|
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 Effects
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:
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. TNT||20 ft.||50 ft.||75 ft.||100 ft.||200 ft.||300 ft.|
|Peak reflected pressure (psi)||122.8||12.6||6.3||4.1||1.6||0.9|
|Reflected impulse (psi-msec)||136.5||47.5||30.6||22.5||10.9||7.1|
|Peak incident pressure (psi)||34.75||5.5||2.9||1.9||0.8||0.5|
|Incident impulse (psi-msec)||51.46||22.0||15.6||11.9||6.1||4.0|
Airblast Parameters for 100 lb. TNT Detonation
|100 lb. TNT||20 ft.||50 ft.||75 ft.||100 ft.||200 ft.||300 ft.|
|Peak reflected pressure (psi)||246.7||20.3||9.2||5.8||2.2||1.3|
|Reflected impulse (psi-msec)||228.5||77.4||49.4||36.3||17.5||11.4|
|Peak incident pressure (psi)||59.1||8.3||4.1||2.7||1.1||0.6|
|Incident impulse (psi-msec)||80.9||35.2||24.5||18.7||9.6||6.4|
Airblast Parameters for 500 lb. TNT Detonation
|500 lb. TNT||20 ft.||50 ft.||75 ft.||100 ft.||200 ft.||300 ft.|
|Peak reflected pressure (psi)||1,182.6||79.5||27.4||14.6||4.6||2.6|
|Reflected impulse (psi-msec)||783.0||246.0||153.5||111.1||52.5||34.3|
|Peak incident pressure (psi)||197.0||24.9||10.7||6.3||2.1||1.3|
|Incident impulse (psi-msec)||209.1||96.6||67.5||52.4||27.4||18.5|
Airblast Parameters for 1,000 lb. TNT Detonation
|1,000 lb. TNT||20 ft.||50 ft.||75 ft.||100 ft.||200 ft.||300 ft.|
|Peak reflected pressure (psi)||2,133.7||157.1||49.0||24.0||6.5||3.6|
|Reflected impulse (psi-msec)||1,354.0||409.6||252.4||181.3||84.6||55.0|
|Peak incident pressure (psi)||314.7||42.0||17.1||9.6||3.0||1.7|
|Incident impulse (psi-msec)||222.8||150.0||104.1||81.0||43.1||29.2|
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.
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.
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.
- Energy Policy Act of 2005 (EPACT)
- Executive Order 12977, "Interagency Security Committee"
- Interagency Security Committee (ISC) Security Design Criteria—Unites all federal protective design requirements (official use only)
- Department of Defense
- DOD Anti-Terrorism Construction Standards (official use only)
- FM 3-19.30 Physical Security—Sets forth guidance for all personnel responsible for physical security
- UFC 4-010-01 DoD Minimum Anti-Terrorism Standards for Buildings—Establishes prescriptive procedures for Threat, Vulnerability, and Risk assessments and security design criteria for DoD facilities.
- UFC 1-200-01 General Building Requirements
- UFC 4-020-01 DoD Security Engineering Facilities Planning Manual
- UFC 4-020-02FA Security Engineering: Concept Design (FOUO)
- UFC 4-020-03FA Security Engineering: Final Design (FOUO)
- UFC 4-021-02 Electronic Security Systems
- General Services Administration (GSA)
- PBS-P100 Facilities Standards for the Public Buildings Service—Chapter 8
- Other "official use only" documents may be obtained from the Office of Design and Construction.
- The Risk Management Process for Federal Facilities: An Interagency Security Committee Standard
- U.S. General Services Administration Standard Test Method for Glazing and Window Systems Subject to Dynamic Overpressure Loadings
- Department of State
- Architectural Engineering Design Guideline (5 Volumes) (limited official use only)
- Physical Security Standards Handbook, 07 January 1998. (limited official use only)
- Structural Engineering Guidelines for New Embassy Office Buildings, August 1995. (limited official use only)
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
Security Criteria Centers
- Defense Threat Reduction Agency—Department of Defense (DOD) Anti-terrorism body—Pentagon's J34
- Department of Homeland Security Federal Protective Service (FPS)
- Naval Facilities Engineering Service Center (NFESC), Security Engineering Center of Expertise ESC66. E-mail: email@example.com
- U.S. Army Corps of Engineers, Protective Design Center
- U.S. Department of Defense
Organizations and Associations
- American Architectural Manufacturers Association (AAMA)
- Blast Mitigation Action Group [Government use only]
- Federal Facilities Council (FFC) Standing Committee on Physical Security & Hazard Mitigation
- Glass Association of North America (GANA)
- International Window Film Association (IWFA)
- National Institute of Building Sciences (NIBS)—Multi-hazard Mitigation Council (MMC)
- Protective Glazing Council
- Safety Glazing Certification Council (SGCC)
- Anti-Terrorism: Criteria, Tools & Technology by Joseph L. Smith, Applied Research Associates, Inc.
- Architectural Design for Security and Security and Technology Design by Donald M. Rochon. June 1998.
- Designing for Crime and Terrorism, Security and Technology Design by Randall I. Atlas. June 1998.
- Safety/Security Window Film by the International Window Film Association
- Security Glazing Specification by The Protective Glazing Council