Oversized Glass Considerations

Standard Glass Sizes

Contemporary building projects that use insulating glass units (IGUs) in their facades typically adhere to obtaining raw float glass with a standard size of 96" x 130" (approx. 244 cm x 330 cm). A larger size, referred to as full size, of 100" x 144" (approx. 254 cm x 366 cm) is also available from most architectural glass manufacturers.

Today, some of the world’s most dramatic and visually breathtaking buildings involve oversized expanses of glass. Using large, dramatic panels of insulated glass is one of today’s most popular design trends. To satisfy the growing demand for oversized glass lites, leading manufacturers can now offer oversized glass ranging from 130" x 204" (approx. 330 cm x 518 cm) to an industry-leading size of 130" x 240" (approx. 330 cm x 610 cm), even at heavy thicknesses of 8mm and 10mm.

Specifying Oversized Glass

Improvements in manufacturing and enhanced energy efficiency from high performance, low-emissivity coatings and spectrally selective tinted glass have made specifying large glass units easier.

However, while it’s gotten easier to specify large and oversized glass units, there are several factors that need to be considered to ensure a successful project:

• Wind Load: Heat-treating helps with wind load, but it doesn’t help with the center of glass deflection. Whether the glass is annealed, heat-strengthened or fully tempered, the deflection due to uniform wind loading is all the same.  Deflection occurs when the glass physically bows into or out of the building because of positive and negative uniform wind loads. Excessive deflection may cause discomfort and/or concern for the occupants of the building and could potentially result in loss of edge support of the IGU. Vitro Architectural Glass (formerly PPG glass) recommends that if a design deflection is not specified, units should not exceed ¾” (19mm) of center-of-glass deflection.
• Thermal Stress:A thermally induced stress break is a possibility with glass and that’s especially true with large panels. Breakage that is a result of thermally induced stress occurs when there is a large temperature differential between the center of the glass and the edge of the glass. This temperature differential causes the center of the glass to expand faster than the edges resulting in stress at the edges that exceeds the strength of the edge. Because of the long perimeter of the edge, large IGUs have an even greater risk for thermal stress breaks. The most likely time of day for a thermal break to occur is in the early morning hours. This is because at night, the glass panel becomes cool, then when the sun comes up and hits the glass, the center of the glass warms up quickly, which puts stress on the cooler edges. To ensure a large glass unit will perform under the anticipated thermal loads, Vitro glass recommends working closely with a qualified fabricator and getting a thermal stress analysis completed at the beginning of the design phase. Vitro glass has developed a thermal stress analysis tool that anyone can use.
• Heat Treating:With units of this size, chances are the glass will need heat treatment. However, while heat-treating makes glass more resistant to wind loads and thermal stress, it also increases the chance of distortion.  If visual aesthetics is a primary concern, consider increasing the thickness of the outer lite of the IGU.  Thicker glass typically has less visual distortion when heat treated.
• Fabrication: Large glass units can be tough to handle during fabrication. Large pieces of glass are simply more difficult to manage through the fabrication process than smaller ones. The larger size of the glass makes them more susceptible to handling damage within the fabrication process.
• Weight: Large glass units are HEAVY, and that weight increases the chance of damage during fabrication, handling and glazing. One way to help prevent damage includes making sure both the glazing contractor and the glass fabricator have the proper capabilities, equipment (such as lift assist equipment/cranes/etc.) and experience in handling large IGUs.
• Field Issues: Large glass can be extremely unwieldy, and that means the people working with it must be extra careful to prevent it from breaking or damaging the edges. It’s essential that the glazing contractor working on the project put safety first, including having enough people and the right equipment on hand to prevent injuries and experience working with large IGUs.
• Glazing: Large glass units require a glazing system that is specifically designed for these types of units because the system needs to be able to support the heavy weight of the glass. This is also where cost and budget come into play. Different glazings have different costs, and you will have to consider the cost of a particular glazing versus the benefit it provides.

For complete technical information about large IGUs, read Vitro Architectural Glass Technical Document TD-140. For any other glass questions, please contact Vitro glass or call 1-855-VTRO-GLS (1-855-887-6457).