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Window Film Attachment Systems Strengthen Buildings' Most Vulnerable Feature

Panoramic windows are a beautiful design element of modern high-rise office towers. But they are not without their risks. Given the threat of terrorism in today's uncertain environment, windows can be a building's most vulnerable feature.

Security experts, property managers, architects and engineers are now doing something about it. Emergency preparedness has become a high priority.

Before 9/11, security window film was considered a practical way to protect building occupants against injury from flying shards of broken window glass. The simple idea was to install the film on the inside surface of the windows, forming an invisible shield to capture blast-shattered glass and minimize the shrapnel affect.

This early method of "daylight" film application, in which the film is installed only up to the perimeter edges of the window glass, was an important first step in reducing injuries. However, as increased attention has been paid to understanding blast phenomena and how buildings and their windows react under the stress of blast load, the effectiveness of the daylight method is being reconsidered.

Advanced testing of filmed windows has underscored its limitations. With nothing to hold filmed glass to its frame, tests have demonstrated that filmed glass can become a heavy, lethal projectile, exploding from its frame with devastating results.

As a result, structural engineers are recommending more sophisticated methods of applying window films that secure the filmed glass to the window frame. New attachment systems have been developed that can save lives and assure the speedy recovery of building operations after a blast.

Different anchoring and restraint systems may be used according to the types of windows and the client's budget. Structural engineers play a critical role in this process, based on their knowledge of the available technology and the behavior of building structures under load. Their analysis is the first step in determining the best system for the building.

The following is a summary of the types of anchoring and restraint systems in use today. They are listed in order of effectiveness and cost and must be evaluated by an engineer for their intended use and the load requirements they must meet:

LifeLine™
This comparatively inexpensive system employs a flexible shock-absorbing cord that is easily attached to the left and right vertical frames, spanning the inside of the window to capture filmed window panes released in a blast. In a blast, the sheet of filmed fragmented glass will wrap around the flexible cord and collapse. This patented system prevents overhead glass from falling on people below and dramatically reduces the shock stress load transferred to the window frame and building structure.

This system is comprised of a die-cast aluminum cleat in black or metal gray; specially woven shock-cord; screws; and a cover cap. Depending upon the size of the window, any number of cords can be used per pane of glass, though standard windows usually need two.

Wet Glaze Attachment
This system employs Dow Corning*995 Silicone Structural Adhesive to attach the edges of the filmed glass to the window frame.

Best suited for aluminum window frames, this system is highly dependent on the installer's technique and skill. The system is built around the idea of carefully crafting a triangular joint of up to an inch at the perimeter edges of the window, connecting the filmed glass to the supporting framing. Dow Corning's Techtalk Bulletin provides key installation guidelines to ensure proper sealant application.

A key concern with this system is properly applying the silicone over rubber or neoprene window gaskets which create an unsecured gap between the filmed glass and the first secure surface with sealant adhesion. Bridging that gap created by a gasket may prove problematic, requiring either cutting back the gasket (which may destabilize the window system) or increasing the quantity of structural adhesive to ensure adequate adhesion surface area.

Great care should also be paid to the way the triangular joint is applied since Dow Corning advises that concave joint surfaces can reduce performance. Additionally, since rubber window gaskets are not designed to carry any load, care should be taken to properly attach the silicone to the window frame.

GullWing Attachment System
This system uses an extruded flexible plastic band which attaches to the four perimeter edges of the filmed window glass, securing them to all four sides of the window frame. Using a blast tested double-sided adhesive tape instead of screws to attach the film and frame, this system is ideally suited to short-term tenant occupancy where the windows must be easily restored to their original condition. The flexibility of this system allows for significant deflection of the filmed window while retaining the shattered pane within its frame. Importantly, since GullWing is an extruded product, there is no room for variation or failure in the installation process.

Dry-Glaze Attachment
Used with a daylight application of window film, this new system from Madico employs an extruded rubber batten which creates a factory-made triangular joint around all four perimeter sides of a window, attaching the filmed glass to the frame.

The Dry Glaze is attached with a combination of double-faced high strength adhesive and Dow Corning 995 Structural Silicone, and provides a high level of protection against blast forces.

FrameGARD™ Anchoring System
This mechanical anchoring system is designed to overlay and batten down security window film to the frames of a fixed window system. Anchored with buttress-thread screws, and subjected to years of government and private testing, this system is installed in government, corporate and transportation facilities worldwide. This system incorporates a specific movement zone which allows the film to absorb energy and stretch under applied loads without placing excessive force into fixings and frames.

Energy Absorbing Cable-Catch System
Arpal Defender's highly refined structural cable restraint system has particular application in high-load situations where protection of large windows against severe explosions must be considered. It is used with a daylight application of 12 to 15 mil. high-strength window film. In a blast, the sheet of filmed fragmented glass wrap around the flexible cable and collapses.

There are two important features that differentiate it from the LifeLine system described above. First, a substantial piece of channeled metal is used to catch the glass. Second, the catch bar system is very rigid and utilizes the structure of the window frame and the building to maintain its strength.

Emergency preparedness compels us to take a closer look at the windows that surround us. Considered a liability in many locations, windows that are properly secured can once again become an attractive and valuable feature in our buildings.

By: Daniel Venet.
The author wishes to thank Darrell D. Barker, P.E. (ABS Consulting) and Carl Kernander (Madico Inc.) for their contributions to this article.

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