Traditionally, windows have not been built to survive catastrophic circumstances. Conventional glass is not designed to resist earthquakes, windblown debris, explosions or terrorist attacks. Subject to such stresses, glass often brakes into lethal shards that are hurled from the window frame, endangering building occupants and passersby. Broken glass also causes property damage.

Security window film can help mitigate the impact of explosive force and windblown debris. The film’s primary function is to hold glass intact when it is broken. This prevents the glass from turning into deadly, flying projectiles. In some cases, the glass may even shatter but still remain intact.

Terrorist attacks
“The most significant damage in approximately 75 per cent of all bombings is the failure of architectural glass,” says Ron Massa, a security consultant quoted in magazine, Buildings.

This has become a major concern in recent years, especially in public buildings. The 1998 bombings of U.S. embassies in Kenya and Tanzania injured over 5,000 people. Many of them were hit by broken glass.

Broken glass in the 1996 terrorist bombing of the Khobar Towers military complex in Saudi Arabia resulted in over 330 injuries, 80 to 90 per cent of which were caused by broken glass.

According to an article in by Massa in another magazine, Public Works, “After buildings collapse, the most significant threat to people and property in bombings arises from the failure of conventional glass.”

The single most devastating terrorist attack to date was the destruction of the World Trade Center in New York in 2001. The event was of such a magnitude that, obviously, now window system would have been able to survive. However, the windows in adjacent buildings may not have broken had they been equipped with security film.

Natural disasters
Broken glass has also resulted from natural disasters, ranging from Hurricane Andrew, which struck the U.S. and the Bahamas in 1992, to devastating earthquakes, such as those that struck Northridge, Calif., in 1994 and Kobe, Japan, in 1995.

Hurricanes and tornadoes produce intense winds, which can result in damage and injury from flying glass.

Earthquakes twist or flex glass. The intensity of an earthquake will determine whether or not the glass breaks. In events of sufficient magnitude, thousands of panes of glass can be broken.

The insurance industry has adopted a new term, “mega-catastrophe,” for those events where insured losses exceed $1 billion. Before 1990, there were no mega-catastrophes. Since then, there have been seven.

Furthermore, of the 25 largest insured catastrophes in U.S. history, 21 have occurred in the last decade. Global warming indicates volatile weather may cause more large-scale property damage and glass-related fatalities and injuries in the future.

The dangers of broken glass
The most common glass-related injuries occur when someone walks into a windowpane. When it is broken, the glass falls and generally causes injuries to the knees and upper legs.

The next most likely areas of impact are the head, neck, and shoulders. Such injuries are not necessarily fatal, but can lead to severe damage of the tendons and the loss of limbs or limb function.

A hurricane can break glass by throwing an object into a window. In such a scenario, both the object and dagger-like shards of glass can strike the buildings occupants.

A bomb’s explosion creates a shock wave that’s shatters nearby glass. If the explosion is of sufficient magnitude, the glass can be atomized. This is a particular dangerous situation, because the explosive shock wave also causes victims in the area to gasp for air, so they can easily breathe in the atomized glass particles and die.

When small pieces of glass penetrate the body, they assume the colour of the surrounding organs. Also, glass particles cannot be detected by X-ray. For these reasons, emergency room physicians have great difficulty finding glass inside victims’ bodies.

Making windows safer
Windows are intended to transmit light and enable building occupants to see outside. On a building’s south exposure, they also generate passive solar energy.

Typical windows performance problems include unacceptable air infiltration, the transmission of noise and ultraviolet (UV) radiation, poor insulating capabilities, the inability to block solar heat and vulnerability to electronic eavesdropping. Security enhancements for glass become more economically feasible if they do not impede- but rather improve-window performance.

Existing glass can be replaced with laminated glass- i.e. two or more pieces of glass bonded by polyvinyl butyral plastic layer. Compared to conventional glass, laminated glass can provide increased resistance to windblown debris, seismic force, and explosive shock waves.

Security window film is the alternative to replacing existing windows with laminated glass. The clear, tinted, or reflective polyester film is applied to the interior surface of the glass. Typical film installations cover the visible portion of the surface of the glass, all the way to the edge of the frame, but do not extend to the glass edge within the frame.

Film can be applied to both single-pane and insulating glass. Proper application will not affect the integrity of an insulating glass sealant, nor will it generate thermal stress to the glass from the uneven heat absorption. Security window film is available with or without solar control- i.e. the ability to block unwanted solar heat.

Security window film can stretch without tearing, so it can absorb a significant degree of the shock wave of an explosion. As the explosive force moves toward the glass and pushes it inwards, the glass eventually cracks and breaks. However, the film applied to the rear of the glass continues to absorb the shock wave.

In some instances, the shock wave, while great enough to break the glass, is not sufficiently powerful to shear the film, which holds the broken glass intact. In these cases, not only are there no injuries, there is also no damage to the building.

In other cases, the shock wave is powerful enough to break the glass and shear the film behind it. The glass collapses, still attached to the security film, minimizing damage and injuries. In multi-story buildings, the film can prevent glass from falling to the street below.

Weighing the options
Both laminated glass and security window film can mitigate the impact of explosions, windblown debris and earthquakes. The performance of both options depends on the relationship of each to the existing window frames.
In the case of laminated glass, the window frame must support the weight and thickness of the glass for the total system to resist stress. Installing laminated glass in existing window frames that are not designed to support its weight runs the risk that the glass may separate from the frames when it is stressed.

The ability of security window film to resist force may increase if it is not only applied to the glass, but also attached to the frame. Many window film manufacturers market attachment mechanisms to secure their product to window frames.

Independent tests verify many security window films provide equivalent stress resistance to that of laminated glass. The optimum security window films also help reduce a building’s energy consumption by blocking solar heat.

Costs
The cost of laminated glass, including installation, is approximately $30 per square foot. The cost of conventional security window film applied to the interior surface of existing glass is approximately $7.50 to $9 per square foot. However, the most energy-efficient security films cost between $15 and $20 per square foot. A system that physically attaches the film to the window frames costs approximately $9 to $24 per square foot.

The cost of disruptions to building occupants when removing and replacing existing glass, compared to the process of applying security window film, also needs to be taken into account when considering laminated glass and security film.