Before Oklahoma City and 9/11, security window film was considered a practical way to protect building occupants against injury from flying shards of glass. The simple idea was to install the film on the inside of the windows, forming an invisible shield to capture blast-shattered glass and minimize its shrapnel effect.
This early method of applying window film was an important first step toward reducing injuries. However, the grim reality of terrorism has forced experts to understand blast phenomena and how buildings and their windows react under the stress of explosive forces.
Tests have underscored the limitations of film alone, demonstrating that a blast can blow filmed glass from its frame and turn it into a lethal projectile. As a result, new methods have been developed that secure filmed glass to window frames and “catch” filmed glass. These attachment and restraint systems prevent filmed windows from becoming dangerous projectiles while preserving the envelope of the building and speeding up the restoration of building operations.
Different attachment and restraint systems are designed to meet varied threat levels, window construction, and budgets. When deciding which system to use, it is advisable to seek the advice of structural engineers experienced in blast phenomena. Since buildings and windows vary structurally, they can evaluate the systems for their intended use and “load” requirements they are expected to meet. The following describes the anchoring and restraint systems commonly used:
FrameGARD™ Anchoring System. This mechanical anchoring system battens the security window film to the frames of a fixed window. Anchored with buttress-thread screws to two or four sides of a fixed frame, this system is designed to hold blast-shattered filmed glass in its frame. The mechanical components may be reused if the glass is replaced.
GullWing Attachment System. This system uses a flexible plastic strap to attach all four perimeter edges of the filmed glass to the frame, allowing significant deflection of the filmed glass while retaining the shattered pane in its frame. Using double-sided adhesive tape instead of screws, this system is suited where the window frames must remain in their original condition.
Wet Glaze Attachment. This system is built around carefully crafting a triangular joint of structural silicone - up to 0.5 inch at the perimeter edges of the window - connecting the filmed glass to the supporting framing.
Dry Glaze Attachment. This system employs an extruded rubber batten, creating a “factory-made” triangular joint around all four perimeter sides of a window, attaching the filmed glass to the frame.
Arpal Restraint. This structural cable restraint system is suited to high-load conditions for oversized windows, or to large explosions. It is used with 12- to 15-millimeter high-strength window film.
Emergency preparedness has forced us to take a close look at the windows around us. Considered a liability in many locations, windows that are properly secured can once again become an attractive and valuable feature in our buildings.
Daniel Venet is executive vice president at New York City-based CHB Industries Inc. (www.chbwindowfilm.com), a provider of security window film solutions since 1990.