Volume 36, Issue 11, November 2001

sacrificing
                                                                        Nothing 
Sacrificial Ply Laminated Glass Meets Debris Impact 
Requirements While Offering Cost-Effective Designs

by Joseph E. Minor, P.E.

Windborne debris impact requirements in new building codes invoke images of high-tech glazing products and shutters that are both sophisticated in design and expensive to install. Indeed, many window and shutter products currently on the market for use in high-velocity wind zones uphold this picture. Glazing designed according to the sacrificial ply concept uses a conventional glass product in conventional glazing systems to meet windborne debris impact requirements. Hence, the sacrificial ply concept meets these requirements in a cost-effective manner.

Sacrificial Ply Principle
The principle is simple. Sacrifice the outer glass ply in laminated architectural glass (LAG) to the incoming debris while ensuring the inner glass ply does not break. Then make sure that the inner glass ply can carry the design wind load when acting alone. If these conditions are met, the LAG unit will function in a conventional sense in a conventional glazing system. Broken LAG units will have to be replaced following a storm, but the building envelope will have remained integral, thus protecting people and property.

Conventional LAG consists of two plies of glass joined by an interlayer of polyvinyl butyral (PVB) (see below). LAG suppliers can laminate any combination of glass thickness and glass type (annealed, heat-strengthened, fully tempered) using any PVB thickness. Hence, the unit can be manufactured with glass that is strong enough and the interlayer is thick enough to ensure that the inner glass ply does not break under specified debris impacts on the outer glass ply.
  sacrificial ply
The sacrificial ply concept uses conventional laminated architectural glass in conventional glazing systems.  The outer glass ply may be broken by impacting debris; the inner glass ply does not break and carries the design wind load.

Innovative glass product suppliers have established that the sacrificial ply concept will allow conventional glazing systems to pass the small-missile impact test specified in the South Florida Building Code. By trial and error they have found combinations of glass type, glass thickness and interlayer thickness that allow laminated products to survive 30 impacts of a 2 gram steel ball traveling at 130 feet per second on the outer glass ply (the small-missile impact requirement) while not breaking the inner glass ply. If designed appropriately, the unbroken inner glass ply then carries the design wind pressure. Since the inner glass ply does not break, there is no need for special silicone anchor beads or modified window frames to hold the unit in place following debris impact.

Basis for Design
Acting with industry support under a U.S. National Science Foundation grant, researchers at Pennsylvania State University and the University of Missouri-Rolla have worked out the science that underlies the sacrificial ply concept. Using theory and experiments, they have established a basis for design of LAG that will assure the performance of a LAG product under specified debris impacts. First, they defined stresses in the two glass plies and PVB interlayer that occur when a LAG unit is impacted by an elastic spherical ball. Stresses predicted by a dynamic finite-element analysis were verified through laboratory experiments that measured transient strains using strain gages. Second, the predicted stress information was combined with the same failure prediction model employed to develop the ASTM E1300 glass design charts to produce a failure prediction model for debris impacts on laminated glass. This model enables researchers to predict failure of the inner glass ply once its strength is defined. Finally, glass surface strength characteristics were determined for the inner glass ply through extensive impact testing on a wide range of laminated constructions. Symmetrical and asymmetrical constructions with different combinations of glass types and interlayer thickness were tested. Results of the entire analysis will be presented in the form of charts or tables that will allow the designer to select combinations of glass type, glass thickness and interlayer thickness that will survive specified debris impacts without breaking the inner glass ply.

The final form of the sacrificial ply laminated design aid (i.e., curves and tables) is under development. Preliminary design aids list combinations of glass ply thickness and interlayer thickness that will satisfy specific debris impact requirements. Initial efforts have been directed toward single impacts, although design charts and tables for multiple impacts from specific types of debris also will be addressed.

Summary
If a laminated unit is designed so the inner glass ply will not break from windborne debris impacts on the outer glass ply, and if the inner glass ply can resist wind pressures for the remainder of the storm, then there is no need for specially designed connections to the window frame. This protection from windborne debris can be achieved at an acceptable cost. Hence, the sacrificial ply design concept is a more viable alternative than laminated glass in which both plies break (requiring specially designed connections to the frame), other sophisticated glazing products (employing high-tech materials) or specially designed and tested shutter systems. 

References:
    Behr, R.A., Kremer, P.A., Dharani, L.R., Ji, F.S., and Kaiser, N.D., “Dynamic Strains in Architectural Laminated Glass Subjected to Low Velocity Impacts from Small Projectiles,” J. Mat. Sci., 34 (1999) pp. 5749-5756.

    Kaiser, N.D., Behr, R.A., Minor, J.E., Dharani, L.R., Ji. F.S. and Kremer, P.A., “Impact Resistance of Laminated Glass Using ‘Sacrificial Ply Concept’,” J. Arch. Engrg., ASCE, 6(1) (2000) pp. 24-34. 

    South Florida Building Code, Metropolitan Dade County Edition, Metro-Dade County, Miami, FL (1993).

    “Standard Practice for Determining Load Resistance of Glass in Buildings,” ASTM Standard E1300-00, American Society for Testing and Materials, West Conshohocken, PA (2000).

Acknowledgments
The U.S. National Science Foundation has provided funding for this project with partial matching funds from the Missouri Department of Economic Development, E.I. duPont de Nemours and Co. and Solutia Inc. 

 

Joseph E. Minor is a consulting engineer with a practice in Rockport, Texas, and a research professor in the Graduate Center for Materials Research at the University of Missouri-Rolla. He has extensive experience with the behavior of window glass and glazing systems in windstorms. His e-mail is joeminor@dbstech.com.


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