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FGIA: Strengthening the code

Published glass strength guidelines do not adequately address multi-cavity IGUs.

November 26, 2021  By Amy Roberts



Canadian window and glass-related design guidelines and codes are in a state of flux, and the glass and fenestration industry is stepping up to help define and unify performance factors and to offer more user-friendly solutions.

A work group consisting of representatives of the Fenestration and Glazing Industry Alliance, Quebec’s AVFQ, the Fenestration Association of BC, Fenestration Manitoba and Fenestration Canada tackled the issue of confusion in the Canadian marketplace as to which energy-efficiency indicator to use: U-factor or the Energy Rating index. The goal is to develop a white paper that will provide end-users with a clear understanding of the different approaches.

As a corollary to the energy rating issue, the latest and proposed energy efficiency codes and regulations have the effect of pushing the industry toward increased use of triple-glazed windows. However, published glass strength and other design guidelines do not adequately address multi-cavity insulating glass units. There are three compliance paths available for glass strength.

CAN/CGSB 12.20, Structural Design of Glass for Buildings does not address triple glazing. Therefore, the IGMA glass design software for the CAN/CGSB 12.20 does include the capability of triple glazing. ASTM E1300, Standard Practice for Determining Load Resistance of Glass in Buildings can also be used, per the National Building Code of Canada. It has been noted that current published requirements do not provide sufficient guidance to cover all variables.

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For example, with regard to glass strength calculations, the issue focuses on Table 4.1.7.3 in NBC Part 9 dealing with importance factors for determining wind loads. The importance factor is a multiplier that increases or decreases the calculated wind load at a particular location depending on the importance of the building’s use and occupancy. Importance factors are given in each category for ultimate limit state. If this is exceeded, the building is considered to be unsafe. Then, there is serviceability limit state, which indicates the maximum stress at which the building is still considered comfortable and safely usable for its intended purpose. Under the current table, the importance factors vary from 0.75 SLS for all building categories to a high of 1.15 for ULS in the high importance category. Note that the ULS-SLS spread varies from 0.05 for low importance buildings to 0.4 for high importance (0.5 for post-disaster). In essence, the industry notes that Table 4.1.7.3 does not address multiple-cavity IG units, tempered or laminated lites, or cavity width. In addition, it does not account for aspect ratio and is considered to be excessively conservative. Also, a strict interpretation would be that it applies to site-built rather than factory-manufactured fenestration products.

Thus, some of the same entities involved in energy efficiency have formed a new Joint Glass Strength Design work group to develop a code change request for NBC Part 9.6. This is based on a Fenestration Canada paper on glass strength design under NBC 2015 and on the use of CSA A440S1-19 to determine Part 9 glass design pressures for use with CAN/CGSB 12.20 or ASTM E1300. Work on this issue has led to a recognition of the need for code changes to focus on new tables for glass strength. The new tables would address design pressures, aspect ratios, multiple cavity IG units, tempered and laminated glass, varying cavity widths, support conditions, length and width and designation of glass thicknesses, which the current table 4.1.7.3 does not provide.

The proposed code change would unfold in two stages: CCR number one will focus on aspect ratios one to three for the 2025 code. CCR number two will develop full tables for the 2030 code cycle to address multiple-cavity IG units, use of laminated glass and other glass configurations.

For more information and to stay up to date on CCR development, feel free to contact me at aroberts@fgiaonline.org. •


Amy Roberts, FGIA director of  Canadian and technical glass operations


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