…. That All Blast Resistant Film Installations Are Not Created Equal?
Over the years, federal governments, private corporations and international non-governmental organizations have been responding to terrorist attacks on their people and their properties by instituting physical security criteria and requirements.
One of the first measures that is often implemented is the installation of blast resistant (a.k.a. anti-shatter, fragment retention) film on the exterior windows of inhabited and occupied buildings.
There are many valid reasons that film installation is one of the early anti-terrorism measures adopted:
• Glass laceration injuries account for the majority of the personnel injuries (once outside of the close-in blast area) and the installation of blast resistant film can mitigate these injuries.
• The relative cost of blast resistant film installations can be significantly less than many of the other hazard mitigation measures (such as building hardening, protected perimeter installation, relocation of assets from vulnerable locations).
• A blast resistant film installation program can be rapidly implemented.
However, security and facility management personnel should remember that there are many flavors to film applications; that not all existing window systems are appropriate for all window film applications; and that the installation of blast resistant window films will only go so far in protecting the occupants of your buildings.
What is Blast Resistant Film?
Blast Resistant Film is generally a polyester film that is adhered to the interior (occupied side of window) surface of glass and is intended to reduce the number of glass shards propelled into an occupied space by keeping them adhered to the film itself. The film is thicker than traditional solar films and the mounting adhesive (the glue that keeps the glass shards attached t to the film) is more robust than those used for solar or thermal applications.
Blast resistant film does not make the glass itself stronger. Rather, it mitigates hazardous breakage by either holding the glass shards together (and the whole glass/film combination enters the room at a slower velocity than untreated glass) or by retaining the glass/film combination in the frame by attachments of the film to the framing system.
Installation Types
The two basic types of blast resistant film installations are Daylight applied and Attached installations.
• Daylight Application: a Daylight application of blast resistant film refers to installation of the film on the vision portion of the glass pane only, with no attachment to the existing framing system. This is the least expensive approach, but provides less protection than an attached installation.
• Attached Installation: when additional protection is required, the window film can be attached to the existing window framing system. Care must be taken when attaching the film as additional loads will be transferred to the frames and the walls supporting the frames during a blast event. If the existing window or supporting structure is weak (for instance, wood framed windows or unreinforced masonry walls) this can cause more widespread failure – and potentially more injuries – than breaking the glass alone. If the window framing and supporting structure are strong enough to support the loads transmitted then the final system can provide a significant increase in protection over both Daylight installations and non-filmed window systems.
Things to Consider
• Compatibility: Not all film is compatible with all existing panes of glass. Special care should be taken when the existing windows are wire glass, insulating glass units, laminated glass, tinted, have large surface areas, large glass thicknesses or have variations in shading over individual panes of glass. Most film manufacturers have what is known as a “Glass to Film” table that should be reviewed. Most of the restrictions are concerned with avoiding spontaneous glass breakage due to thermal issues. If film is installed on glass that is not within the manufacturers’ recommendations, the warranty may not be valid and you may end up with broken windows.
• Is it Really Blast Film: it is important that the blast film you select has been tested in a blast environment. There are a number of accepted test methodologies that have been developed over the years, these include:
o ASTM F1642 Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loadings;
o GSA-TS01-2003 US General Services Administration Standard Test Method for Glazing and Window Systems Subject to Dynamic Overpressure Loadings;
o GSA Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loadings;
o ISO 16933, Glass in building – Explosion-resistant security glazing –Test and classification for arena air-blast loading;
o ISO 16934, Glass in building – Explosion-resistant security glazing – Test and classification by shock-tube loading.
• Are Existing Window Frames and Walls Strong Enough: If you are looking for the additional protection provided by an Attached Installation, it is important to confirm that the existing window frames and walls are able to support the increased loads that will be created by a blast. Retain the services of a blast consultant to assess your existing conditions.
Will Blast Resistant Film Meet the Protection Requirements of your Facility?
Generally, blast resistant film can increase protection in lower pressure environments (i.e. around 4 or 5 psi (27.5 to 34.5 kPa) for Daylight applications and around 10 psi (69 kPa) for Attached applications) but will rapidly be overcome from higher pressures. Even small VBIED’s can result in pressures far in excess of these limits and the installation of blast film alone will not provide a substantial increase in protection.
When looking at protecting people, buildings and operations from blast effects, there is no single mitigation measure that will adequately meet all needs. Rather, implementation of a variety of measures is most likely required. These may include increasing standoff distances to potential locations of explosive devices, implementing security screening and access restrictions at your facility, mitigating the hazardous effects from window breakage, and hardening your buildings themselves.
In the last Volume of our Industry Briefing, we gave you a heads-up that Project Manager Arturo Montalva had an upcoming article in STRUCTURE Magazine – well here it is! Follow this link to read the article on the use of Open Source Software in structural engineering http://bit.ly/9ICysY.
An explosion is a rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave travels outward, in all directions, from the source of the explosion and is the primary source of building damage. The duration of the shockwave is fast, measured in milli-seconds rather than seconds (think of a blink of the eye) and the forces imposed on anything it its path (be it a building or a person) are enormous – many times greater than hurricanes.
Many factors contribute to how a building will respond to an explosive event, the most critical are.
• The size of the bomb
• The distance from the bomb to the building
• The type of building construction.
The size of the bomb (also termed Net Explosive Weight) and the distance from the bomb to the building (standoff distance) determine the magnitude of the pressure (force over area) and the duration that the pressure acts on the building element. Pressures decrease as the standoff distance increases and durations tend to increase with greater standoff. Shorter durations and smaller pressures cause less damage than higher pressures and longer durations.
The type of construction is also a significant factor in how much damage a building will experience from an explosion. It is important to remember that the vast majority of buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to a bomb does not mean that the building was poorly designed. Blast loading is an extremely abnormal event and stresses buildings in ways never anticipated during the original design and construction. Buildings are generally designed to hold up gravity (downward) loads and lateral wind loads. In earthquake regions, they are also designed to withstand forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitudes associated with explosions. Very lightweight buildings and buildings built of unreinforced masonry (brick or concrete block units) tend to respond the worst to explosions, while concrete and steel framed buildings built in high seismic zones tend to respond the best.
