Spandrel Glass Design: Considerations for Determining Load Resistance

H., Scott, P.E.; Barry, Christopher, PE; M., Stephen, P.E.

Overview

Abstract

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.

Authors

Scott H. Norville, P.E., Ph.D.

Professor

Texas Tech University

scott.norville@ttu.edu

Christopher Barry, PE

Glass Consultant

chrisbarryglassconsultant@gmail.com

Stephen M. Morse, P.E., Ph.D. M. FTI, M. ASCE

Assistant Professor

Michigan Tech University

Keywords

curtainwall, component performance, glass performance, glass, laminated glass, design processes

Introduction

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.

Design Scenarios

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