2018 Codes Clarify Deflection Limits for Glass Wall Systems

The 2018 International Building Codes clarify the deflection criteria to be used in the design of glass wall systems, addressing a lack of specificity about limits for glass wall systems in the 2015 version. The update in the 2018 code comes in the form of a footnote in Table 1604.3, which establishes serviceability deflection limits for structural systems and members of structural systems. The table specifically addresses roof members, floor members, exterior walls, interior partitions, farm buildings and greenhouses. 

Deflection limits

Serviceability deflection limits address the amount of movement that could occur within an assembly under design load, while still maintaining its ability to perform the function it is designed to perform. For example, the serviceability deflection limits for exterior walls are concerned with the amount of movement that could occur in an exterior wall when subjected to design wind pressure, while still maintaining its function as a weather-resistive barrier. 

Glass wall systems

In the 2015 IBC, the body of Table 1604.3 limits the deflection of exterior walls supporting plaster or stucco to L/360, with “L” being the span of the wall. Although it does not specifically name glazed components or assemblies, or framing supporting glazing, it does establish a deflection limit of L/240 for exterior walls supporting “brittle finishes other than plaster or stucco.” Historically, this is the deflection limit that was applied to glass wall systems such as curtain wall or floor-to-ceiling storefront.

Empirical evidence, however, has demonstrated that glass wall systems that deflect as much as L/175 maintain their function adequately, when established methods, such as pressure equalization, have been applied within the design of the wall system itself and the wall span is not excessively high. 

The American Architectural Manufacturers Association, aamanet.org, addresses this in its standard, AAMA TIR-11 Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads. AAMA TIR-11 establishes a deflection limit of L/175 for spans less than 13 feet 6 inches and L/240 + ¼ inch for spans greater than that. The additional ¼ inch provides for a smooth transition between the L/175 criteria below 13 feet 6 inches, and the L/240 + ¼ inch criteria above it. Specifically:

L = 13 feet 6 inches = 162 inches

L/175 = 162/175 = 0.925 inch

L/240 + ¼ = (162/240) + ¼ = 0.925 inch 

Although there are footnotes to Table 1604.3 of the 2015 IBC that establish the wind pressure that is to be used in the application of this table, the deflection limit to be used in the structural design of glass and for framing supporting glass in sunrooms and patio covers, none of them specifically address the serviceability deflection limit for glass wall systems.

2018 IBC

New provisions in the 2018 IBC contain a section adding the criteria from AAMA TIR-1 for framing supporting glass. One of the existing footnotes will also be revised to specifically refer to this new section. This will help to clarify the criterion that is to be applied in the design of a glass wall system.

Industry impact 

The new section and revised footnote will help to clarify the appropriate deflection criteria to be used in the design of glass wall systems. It is hoped this will help ensure these systems are being designed appropriately. 

It is not anticipated that this change will impact windows and doors that have been tested and labeled in accordance with AAMA/WDMA/CSA 101/I.S.2/A440. In that instance, the serviceability of the system and structural design of the glass are addressed by the test protocol given in the standard. The specifics of this protocol override the more general requirements of Section 1604.3.