Spandrel Glass Design
Considerations for Determining Load Resistance
Presented on October 13, 2022 at Facade Tectonics 2022 World Congress
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Currently, most spandrel glass in commercial curtain walls comprises insulating glass fabricated using an interior heat strengthened glass lite that has a full flood coating of opaque ceramic enamel frit. Recently, numerous thermally induced fractures have been documented on lites with ceramic enamel frit. In addition, investigation has indicated fracture origins were located at the heat strengthened glass surface (not at the cut edge) to which the flood coat had been applied. This indicates the ceramic enamel frit significantly reduces the magnitude of stress at which glass fracture initiates, i.e., full flood ceramic enamel frit weakens the heat strengthened glass. This paper investigates approaches to the design of spandrel glass through determination of load resistance. Specifically, the cases investigated include (a) use of full flood ceramic enamel frit with a reduction factor applied to the load resistance of the lite that has the full flood ceramic enamel frit application, (b) use of laminated interior lites fabricated with high performance opaque interlayers, and (c) use of an opaque silicone coating of appropriate color on the interior lite. The results indicate that any one of these approaches will produce adequate designs to resist wind loads although two of the methods result in larger values of load resistance. The final design selection will be dictated by economics which this discussion does not address.
Spandrel glass, unlike vision glass, is opaque. Its purpose is to glaze areas between floors while preventing views of the structure behind the glass. Traditionally, spandrel glass achieves opacity through coatings such as ceramic enamel frit or opaque silicone coatings applied to the glass surface. Other methods to achieve opacity include using laminated glass with opaque interlayers or applying an opaque silicone coating to the glass.
Ceramic enamel frit consists of ground glass mixed with inorganic compounds to produce a desired color. The frit is then applied to the glass surface through various methods, the most common being a full flood coating. After application of the frit, the glass and frit are heated to high temperatures to melt the glass particles and fuse them to the glass surface. Consequently, glass with frit must be either heat strengthened or fully tempered. Proponents of ceramic enamel frit promote it as being resistant to scratching, peeling, fading, chipping, and chemical attacks (Oldcastle Manufacturer’s Literature). Ceramic enamel frit has been shown to reduce glass load resistance (Barry and Norville, 2015; Bergers, et al., 2016; Natividad, et al, 2017). European standards (EN 1863-2,2004; prEN 13474, 2012) currently reduce the strength of glass lites with ceramic enamel frits.
Laminated glass comprises two or more glass plies bonded together by elastomeric interlayers. The most common elastomeric interlayer is polyvinyl butyral (PVB) although manufacturers have developed several high-performance interlayers, such as ionomer, epoxy, and polycarbonate. Laminated glass has found widespread use in safety glazing, blast resistant, security, and impact resistant applications, among others. The glass plies that encapsulate the elastomeric interlayers protect them from scratching. Most chemical reactions that might impact the interlayers must get at them through exposure at the glass edge. Adverse or unsightly effects of chemical reactions in interlayers are usually contained within fractions of an inch from the edge. Polyvinyl butyral is not recommended for spandrel applications due to the high temperatures developed in spandrel glass. Some other high-performance interlayers can withstand the high temperatures in spandrels and for later discussions we will assume the use of one of these.
Silicon coatings may be applied to any glass type: annealed, heat strengthened, or fully tempered. As with ceramic enamel frit, silicone coatings provide a plethora of colors and, of course, opacity. They may, unfortunately, be vulnerable to scratching and mechanical damage. But once glass with a silicone coating is installed in a spandrel environment, interactions that might cause scratching or chemical threats are minimized.
This paper will present procedures that illustrate design of insulating glass units based on ASTM E 1300 (ASTM, 2016). The design examples will consider current knowledge concerning the load resistance of laminated glass and the impact on glass load resistance resulting from applications of ceramic enamel frit or silicone.
For all design scenarios, we will consider an insulating glass unit having rectangular dimensions of 1520 mm x 1830 mm ( 60 in. x 72 in.) subjected to a 3-second
Spandrel Design 1: Silicone Coating
Blanchet et al. (2020) demonstrated that silicon coatings on heat strengthened glass increased load resistance marginally. To err on the conservative side, though, take the strength of silicon coated glass
Spandrel Design 2: Using a Laminated Glass Lite with an Opaque High-Performance Interlayer
Denote the outer monolithic lite as Lite 1 and the inner laminated lite as Lite 2. Although the laminated glass lite can use a strong, high-performance interlayer, which performs well
Spandrel Design 3: Monolithic Interior Lite with a Full Flood Ceramic Enamel Frit
The impact which an application of ceramic enamel frit has on the LR of a heat treated glass lite varies with several factors including application method, frit color, and frit
The authors have presented three design scenarios for spandrel glass. The first scenario, a silicon coating to achieve opacity, leads to no reduction in LR whatsoever. The second scenario, laminated
The authors have pointed out alternative design possibilities that do not weaken heat strengthened glass as a full flood ceramic enamel frit application generally does. In addition, they pointed out
Rights and Permissions
ASTM. (2016). “Standard practice for determining load resistance of glass in buildings.” ASTM E1300-16, ASTM International, West Conshohocken, PA.
Barry, C. J. and Norville, H.S. (2015): “Unexpected breakage in ceramic enameled heat strengthened insulating glass spandrels” Proceedings IGMA Winter Conference, Feb. 3-7.
Bergers, M., Natividad, K., Morse, S.M., and Norville, H.S. (2016). “Full scale tests of heat strengthened glass with ceramic frit,” Glass Structures and Engineering, Springer, 1(1): 261-276. http://link.springer.com/article/10.1007/s40940-016-0030-5.
Blanchet, S., Morse, S.M., and Norville, H. S. (2020). “Effects of silicon coatings on heat strengthened and fully tempered glass,” Journal of Architectural Engineering, ASCE, https://doi.org/10.1061/(ASCE)AE.1943-5568.0000403
EN 1863-2 (2004). “Glass in Building – Heat Strengthened Soda Lime Silicate Glass – Part 2: Evaluation of Conformity/Product Standard” BSI, Brussels, BE
Krohn, M., Hellmann, J., Shelleman, D., Pantano, C., and Sakoske, G., (2002): “Effect of Enameling on the Strength and Dynamic Fatigue of Soda-Lime-Silica Float Glass," Journal of the American Ceramic Society, 85(10): 2507-2514.
Morse, S., Natividad, K., Norville, H, S., Cavalcanti Fonseca, J., Erickson, B. (2016). Bending Tests of Heat Treated Glass with Ceramic Frit. Boston, MA: GlassCon Global.
Oldcastle Building Envelope Manufacturer’s Literature. https://obe.com/products/ceram... 13474 (2012). “Glass in Building – Determination of the strength of glass panes by calculation and testing” BSI, Brussels, BE.
prEN 13474 (2012). “Glass in Building – Determination of the strength of glass panes by calculation and testing” BSI, Brussels, BE.