Verifying fenestration’s structural strength when lab testing does not apply

Photo courtesy Keymark

By Aaron Blom

Laboratory testing is the typical way to verify if fenestration products meet specified structural design pressure criteria. These requirements are often based on the code-mandated AAMA/WDMA/CSA 101/I.S.2/A440, North American Fenestration Standard (NAFS)/Specification for windows, doors, and skylights, or local variations of this standard.

Under NAFS, there are four performance grades for windows and doors based on increasing thresholds of Design Pressure (DP) which are based on the pressures exerted by the maximum wind speeds expected at the building site. The National Building Code of Canada (NBC) provides data on expected maximum wind speeds in various areas depending on recurrence interval. It should be noted, the go-to U.S. wind speed reference from the American Society of Civil Engineers and the Structural Engineering Institute, ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, is rarely used in Canada. The two NAFS performance classes with the highest “gateway” structural strength are CW and AW. In addition to other tests required to demonstrate compliance with NAFS, successful laboratory testing at a structural test pressure (STP) equal to 1.5 times the DP is required when determining performance grade of a product. It is important to remember while testing is performed at higher pressures, products should be specified based on the DP, not the STP.

The load on the window exerted by a wind of given velocity at a given height (z) above ground is determined by the simplified equation:

qz = Velocity Pressure (N/m2) = 0.0613 V2 (m/s)

In imperial units, the equation is expressed as:

qz = 0.00256 V2, where qz is in psf and V is in mph

Alternatively, the pressure in pounds per square foot (psf) found in the imperial unit’s version can be converted to N/m2 by multiplying by the factor 47.89 (often rounded to 48). That is, Pa = 48 x psf [e.g. 240 Pa = 5 psf x 48]).

All fenestration systems must be designed to limit deflection. This is especially important in mulled units, like what is seen here. Photo courtesy Innotech

To attain the NAFS CW performance class, the minimum (gateway) DP is 1440 Pa (30.08 psf), so the minimum STP is 2160 Pa (45.11 psf). A product can qualify at higher DPs in increments of 240 Pa (5.01 psf) up to a maximum of 4800 Pa (100.25 psf). For the AW performance class, the minimum DP is 1920 Pa (40.10 psf), indicating a minimum STP of 2880 Pa (60.15 psf). AW products, unlike CW products, can be qualified at higher DPs with no upper limit.

NAFS also defines the maximum acceptable deflection of framing members subjected to such loading. Under NAFS, the frame deflection is determined by subjecting a specimen window to the laboratory test method ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference, Procedure A. This test method imposes a uniform load of 1.5 times the target DP, applied from the exterior (positive load) and then the interior (negative load) for 10 seconds each. The resulting amount of maximum permanent deflection of framing members is measured and must not exceed that defined for the performance class (i.e. no more than 0.3 percent of span length for CW, or 0.2 percent for AW). Also, there can be no damage and normal operation must be retained, with no disengagement of sash, frame, or glazing.

This testing is a proven way to verify code compliance of a specific fenestration design and its framing profile configuration. However, a given project can call for fenestration configurations different from those of the tested baseline assembly that has qualified for a given rating under NAFS—such as larger or smaller sizes, or the ability to withstand higher wind pressure levels. Conducting additional tests of all such variations to achieve code compliance is usually prohibitively expensive and time-consuming. A viable alternative is to employ engineering analysis according to “accepted engineering practice” by which such products can be qualified for structural performance by extrapolating test results obtained for the same basic configuration.

The engineering design rules specified in AAMA 2502 are intended to analyze the structural performance of products at non-tested sizes, in which the product is subjected to bending under a uniform load. Photo by Arina P. Habich–Shutterstock

Still, there are differing opinions about what constitutes “accepted” engineering practice. To provide a uniform approach and reduce engineering judgement, the former American Architectural Manufacturers Association (AAMA), now the Fenestration and Glazing Industry Alliance (FGIA), developed a consensus-based process for engineering evaluation of structural integrity: AAMA 2502, Comparative Analysis Procedure for Window and Door Products—which is now approved as a reference standard in the International Building Code (IBC). AAMA 2502 was last updated in 2019. The IBC cites it as a method for conducting engineering analysis of alternate sizes based on physical testing of a baseline assembly, rather than testing each alternative size.

The Engineering Design Rules specified in AAMA 2502 are intended to analyze the structural performance of products, at non-tested sizes, in which the product is subjected to bending under a uniform load acting against the face of the product perpendicular to the plane of the wall, such as that induced by wind.

Essentially, using the basic mechanical properties of moment of inertia, bending moment, modulus of elasticity, etc., determined by calculations based on the material and specific cross-sectional geometry of the framing, one can calculate the deflection of framing members in a unit with the same framing profile material and cross-sectional configuration as the tested assembly. The structural load could be equal to or greater than the load applied to the test unit. The results of these calculations ultimately enable the determination of frame deflection under load—the key indicator of the product’s ability to resist wind loads. This allows specifiers to verify structural wind load performance by extrapolating the test results for a larger or smaller product subjected to the same or greater design pressure. This enables the unit in question to be qualified for code compliance of structural performance without further testing and reduces ad hoc engineering judgement to a minimum. This approach also could be useful in the design and development of new products.

The higher the opening, the higher the windload requirements become. Therefore, the fenestration system must withstand a stronger load.

The calculations produce load distribution and magnitude, section properties (moments of inertia and bending), strength (in both tension and compression), strength of fasteners or anchors, and ultimately the maximum frame deflection. The maximum concentrated load imposed on any framing member, hardware, or fastener of the frame must not exceed the maximum equivalent concentrated load of the test unit. A resulting analysis report, signed and sealed by a registered professional engineer (PE), can be used to obtain code compliance.

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