By Valerie L. Block, CDT, LEED AP
The capabilities of laminated glass performance have expanded thanks to the introduction of stiff, structural polymer interlayers. This means reduced overall pane thickness and weight, improved durability of exposed laminate edges in exterior applications, and better post-glass breakage performance in minimally supported glazing systems.
Laminated glass is typically made with two plies of heat-treated glass bonded together with a polyvinyl butyral (PVB) or ionoplast interlayer. The glass can be annealed, heat-strengthened, or fully tempered, depending on the application. Annealed glass is the basic product, typically made by floating molten glass over a bath of molten tin, and then slowly cooling under controlled conditions to remove residual stresses in the material. The controlled cooling process is known as ‘annealing.’
There are two types of heat-treated glass: heat-strengthened and fully tempered. The former is twice as strong as annealed glass and the latter is four times as strong. The improved strength of these glass types is produced by running regular annealed glass through a furnace, heating it just above its annealing point (about 720 C [1328 F]),
and then rapidly cooling it to increase its surface compression.
Standards providing useful information on the acceptable tolerances and quality of architectural glass include:
- ASTM C1036, Standard Specification for Flat Glass;
- ASTM C1048, Standard Specification for Heat-treated Flat Glass, Kind HS, Kind FT Coated and Uncoated Glass; and
- ASTM C1172, Standard Specification for Laminated Architectural Flat Glass.
Glass thickness considerations
Laminated glass thickness varies based on load requirements and application. Load resistance and deflection of the glass depend on the support conditions, which can range from traditional four-sided continuous framing to minimally supported examples, such as a railings or canopies, and glass point-supported with bolted fittings.
Laminated glass provides safety from falling glass in overhead glazing by virtue of the interlayer’s ability to retain fragments after breakage. Flexible interlayers, such as PVB have been used for many years in skylight laminate applications. Stiffer interlayers are now available that not only provide glass retention after breakage, but also result in thinner, and overall lighter, laminated glass construction. For example, in 2005, ionoplast interlayers were used to reduce the thickness over PVB laminates in the design of an 18.3-m (60-ft), barrel-vaulted atrium of laminated glass at Toronto’s Yorkdale Shopping Centre.
Safer glass railings
Building code requirements for glass railings have typically led architects to tempered or laminated glass made with heat strengthening. While load requirements are typically found in building codes, two ASTM standards are useful in the specification and testing of glass in railing systems. ASTM E2358, Standard Specification for the Performance of Glass in Permanent Glass Railing Systems, Guards, and Balustrades, addresses the design and performance requirements for glass railings. ASTM E2353, Test Methods for Performance of Glass in Permanent Glass Railing Systems, Guards, and Balustrades, presents test protocols to assess the static strength, impact performance, and post-breakage retention characteristics of these systems.
Over the past few years, numerous occurrences of falling monolithic tempered glass from upper- storey balconies have resulted in increased awareness about the need for laminated glass. Specific incidences with Toronto, Seattle, and New York City high-rises have encouraged the replacement of traditional monolithic tempered glass with its laminated counterpart to prevent falling shards in the event of breakage.
In Toronto, the threat of falling glass prompted building officials to form an expert advisory panel to make recommendations around balconies. The panel suggested prescriptive requirements for glazing used in interior and exterior balcony guards be part of the building code. Additionally, the panel suggested glass located within 50 mm
(2 in.) of the edge of the floor of the balcony railing be heat-strengthened laminated glass. The Canadian Standards Association (CSA) has assembled experts to begin developing the requirements for glass balcony railings.
As the demand for laminated glass increases, so will systems designed to incorporate the material. An example already on the market is a dry-glazed railing assembly with laminated glass installed in an aluminum-extruded base shoe serving as a guard for fall protection in structures. This system, and others like it, can be installed in both interior and exterior weather-exposed applications.
Glass railings are often specified to enhance the view—this is especially true in sports stadiums. To afford the best line of sight possible onto the playing field, supports are kept to a minimum. In some cases, the top rail is omitted and the glass is only supported on its bottom edge. In these instances, it is important to consider post-breakage glass behaviour. Stiffer interlayers are more tear-resistant than flexible ones, and broken minimally supported laminates are more likely to remain upright or attached to bolts. This is also important for taller glass panels installed on rooftops.
It is significant to note excessive exposure to moisture and using incompatible sealants and grouts can lead to edge defects and, in extreme cases, delamination of laminated glass. Edge defects may consist of a few bubbles or whitish blush along the glass edge. In more serious cases, the defects can extend throughout the laminated panel, at which point the interlayer’s bonding strength may be compromised. This is called delamination and requires immediate glass replacement. The selection of sealants and grouts used with laminated glass products should be based on compatibility testing of these materials.
Designing glass floors
Glass floors and stair treads are growing in popularity. In some cases, laminated glass walkways enable a greater opportunity for daylighting. In other cases, the material’s transparency is intended to enhance the view, as in the glass walkway at the Grand Canyon or at the proposed Glacier Discovery Walkway in Alberta’s Jasper National Park, where a viewing platform will extend over the Sunwapta Valley.
However, glass floors and stair treads present a design challenge. Not only are load calculations part of the design process, but slip resistance of the glass must also be considered for human safety. After many years of development, an international standard is available to aid the designers of glass walkways. ASTM E2751-11, Standard Practice for Design and Performance of Supported Glass Walkways, contains both a calculation and testing approach, as well as requirements for the top surface of glass.