Earthquake-Safe Window Glass
The following article appeared in the Toctra Newsletter, April 15, 1985. This publication features new technology developed by Federal laboratories, and is published by the School of Business and Public Administration, California State University, Sacramento, CA 95819. Call (916) 454-6640 or 929-8454 for more information, requesting Case W03-19.
The effect of earthquake-induced stress on window glass in buildings has been studied at the Naval Civil Engineering Laboratory, Port Huenema, CA. The results of this effort indicate that window glass in both the high-rise and low profile buildings is a potential hazard to building occupants and personnel in close external proximity to buildings during and after an earthquake. Extensive glass breakage is expected during an earthquake if measures to prevent or minimize its occurrence are not taken.
Fragments of broken glass falling from earthquake-damaged high-rise structures, either from window glass or glass outer walls, reach relatively high free-fall velocities and present a considerable missile hazard to personnel and structures immediately adjacent to a building. Glass, when subjected to earthquake-induced shear strain during building sway, develops an explosive fragmentation potential. Earthquake building sway generates energy that is transmitted and stored in brittle glass elements of a building. When the glass system fragments due to the energy build-up level exceeding the strength characteristic of the glass, the glass explodes propelling fragments at very high velocities. Fragment velocities high enough to cause severe gouging of wood and tile can be experienced when this occurs. Explosive glass fragmentation will occur before actual building structural damage occurs. Heat-treated, annealed, and tempered and semi-tempered glasses are subject to explosive fragmentation during earthquake-induced stress.
The hazard potential from earthquake-induced glass breakage is nearly eliminated when a film of 2-4 mil. thick polyester is bonded to the inner surface of window glass. The polyester film's adhesive holds the fragments of the broken glass together, preventing explosive propulsion of dangerous glass fragments into an interior space. The holding property of the film / adhesive combination prevents glass fragments from becoming hazardous free-falling missiles.
The fragment-retaining film also functions to maintain the weather integrity of a building after an earthquake caused window breakage. Interior spaces can be used until replacement of the broken glass is completed. The film remains in place as a temporary substitute for the broken window glass, allowing interior temperature control and keeping wind and moisture out.
It has been demonstrated that when the polyester film is fixed to an installed piece of window glass, the film need not cover the non-exposed areas of the glass where it is covered by the window frame, glazing compound, or retaining lip. This feature allows retrofit of existing windows without the requirement for removal of the glass from the frame or housing, a considerable cost savings is the result. However, for new installations, it is recommended that the polyester film cover the entire glass surface.
There are several spin-off benefits resulting from installation of the shatter-retaining film to building windows. When clear film covers the interior surface of a building's windows, it acts as an effective thermal barrier which contributes to improved interior environmental control. When tinted, the film, with its fragment retention characteristics, provides an additional measure of entry security and reduced vandalism opportunity.
A program for installation of fragment retention film on building windows should consider interior space use, window proximity to occupants of working spaces, window area, glass thickness, and whether windows are fixed or can be opened. Large area glass windows exhibit a higher breakage risk than smaller windows and when explosive breakage occurs, the glass fragments are propelled a greater distance with greater velocity. Windows that can be opened are less subject to shear strain build-up and explosive breaking because of their greater degree of isolation from the building structure than fixed windows. Consideration of these and similar conditions allow for installation or retrofit of fragment retention film throughout a building.
As new building structures are more earthquake-survivable due to the increased structural flexibility, the opportunity for both fixed and non-fixed windows and glass exterior walls to be subject to higher levels of earthquake-induced shear strain and explosive breakage is greater. Installation or retrofit of polyester fragment retention film can provide a significant decrease in the hazards of flying or falling glass from earthquake-induced window breakage.
