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

by brittney_cutler | July 31, 2022 4:00 pm

[1]

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:

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

In imperial units, the equation is expressed as:

q_z = 0.00256 V², where q_z 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]).

[2]

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.

[3]

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.

[4]

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.

Basic factor considerations

The engineering design rules cover five basic mechanical factors:

1. Load distribution and magnitude

The uniform wind load acting on the product is assumed to be distributed in a manner depicted as a triangular (or in a combination triangular-trapezoidal) pattern across the framing members supporting glass or opaque panels. Diagrams within AAMA 2502 indicate the areas of glass to be considered as collecting and transmitting the wind load to individual members for a variety of window configurations. The total load acting on a member is calculated by multiplying the defined area of the glass or panels that it supports by the applicable design pressure for the building’s location.

2. Section properties

These include the area moment of inertia, the section modulus, and the bending moment. The behavior of a framing member of a given material with a specific cross-sectional configuration depends principally on two of these properties: a) the area moment of inertia, which together with its modulus of elasticity, governs deflection; and b) its section modulus—the ratio of moment of inertia to the greatest distance of the beam’s edge from its neutral axis—which governs stress. The bending moment (the resistance of a member to bending due to the application of external forces) is also a factor.

Section properties for many sizes of solid rectangles, square bars, round rods, square tubes, round tubes, pipes, standard structural shapes (e.g. channels, I-beams, wide flange beams, angles, and zees) as well as certain channels and tees are given in various, material-specific engineering texts. For example, one source of data on aluminum is the Aluminum Design Manual, published by the Aluminum Association.

3. Strength

This is defined by stress calculations for both tension and compression. The calculated stress cannot exceed guidelines in NAFS and/or other applicable industry standards for different framing materials. If not otherwise defined in the referenced documents, the material yield stress is subject to a minimum safety factor of 1.5 as the basis for allowable stress design.

4. Connections

[5]

To ensure proper design, connections must be analyzed for tensile, shear, and bearing strength. The total strength in tension and in shear provided by the fasteners must be the same or greater than the tensile and shear loads at that connection. For further reference, single-shear strengths for screws and bolts of various alloys are given in AAMA TIR-A9-14, Design Guide for Metal Cladding Fasteners. Also, the capacity of fasteners can be calculated per AAMA 2501-20, Voluntary Guideline for Engineering Analysis of Anchorage Systems for Fenestration Products Included in NAFS.

Where the framing members are rigidly and securely fastened together, the assembly will act as one integral member. The fastening devices or welds connecting the members together must be strong enough to withstand the loads applied to them.

5. Deflection

Formulas are provided for figuring deflection and stress limits of simply supported beams, including the calculation of modulus of elasticity (which varies by material), for both symmetrical and unsymmetrical profiles. Formulas are also given for calculating the deflection of composite framing members such as thermally broken framing. In the case of composite framing members with integral thermal breaks, it is recommended they are tested per AAMA 505-17, Dry Shrinkage and Composite Performance Thermal Cycling Test Procedure, and analyzed per TIR-A8-16, Structural Performance of Composite Thermal Barrier Framing Systems.

Portions of the uniform load considered to be applied to the overall product are assumed to act upon each member in a triangular or trapezoidal loading profile, or as a concentrated load. Formulas are given for calculating the bending moment for these loading patterns, and for both loads concentrated at the center of the member and for loads concentrated at any off-center point along it. Diagrams of 15 different example cases of load distributions upon window and door framing members, based on different framing configurations, are given—square, oblong, and oblique using two-lite, four-lites of equal size, four unequal lites, two lites with an integral transom, and two main lites below four-lite transom configurations. The proper treatment of load deflection is discussed for each different configuration.

Finally, formulas are provided for determining the maximum deflection of a framing member of a given length subjected to bending from a uniform load. This takes into account the previously calculated area moment of inertia and modulus of elasticity, as well as—in the case of a trapezoidal loading configuration—an appropriate moment coefficient.

The maximum permissible deflection of a framing member is typically expressed as a fraction of the length (L) of its unsupported span. Under NAFS, the total maximum deflection cannot exceed the limitation of L/175. Note, the calculated deflections cannot exceed those set forth in the referenced performance standard for products with framing spans longer than those of the tested version. Also, the span-to-deflection ratio of the calculated unit cannot be less than that of the tested specimen. This deflection is subject to the limitations imposed by AAMA TIR-A11, Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads, or ASTM E1300/CAN-CGSB 12.20, Standard Practice for Determining Load Resistance of Glass in Buildings. The glass edge is considered to be supported, as set forth in ASTM E1300 or CAN-CGSB 12.20.

To analyze profiles with complex multi-element cross-sections, the entire section should be divided into any number of simple subsections to calculate the section elements of each, tabulating the calculations for all subsections.

The analysis procedure for each subsection unfolds as follows:

• Record the width and depth of each subsection, which when multiplied together, yields the area of each subsection.
• Locate the center of gravity of each subsection and record its distance from its defined “0-0” axis.
• Calculate each subsection’s area moment of inertia about its own center of gravity.
• Total the results for all three subsections to provide the total area, first moment, and area moment of inertia of each subsection about its own “0-0” axis.
• Calculate the position of the neutral axis (“X-X”) of the entire profile and, in turn, the greatest distance from the neutral axis to the extreme fiber can be calculated.
• Calculate the area moment of inertia of the entire profile about the “X-X” axis.
• Calculate the section modulus based on the results indicated above.

Be aware: formulas are provided for all the referenced calculations in AAMA 2502.

An example five-subsection profile is given, showing the calculation of the area moment of inertia of each subsection about its own center of gravity and about the “0-0” axis for the complete profile. These are totaled from the tabulation to yield these moments for the entire profile. Formulas are given for calculating the moment of inertia of the entire profile about its defined neutral (“X-X”) axis and for calculating the overall section modulus.

The comparative analysis procedure in AAMA 2502 is especially suited for code jurisdictions where it is desirable to document the performance of each window and exterior door size to meet specific structural DP criteria. For window and door manufacturers, the procedure provides a uniform approach for dealing with different code jurisdictions and specific DP for each size of fenestration product openings.

Author

Aaron Blom joined the Fenestration and Glazing Industry Alliance (FGIA) in 2021, and is responsible for conducting and developing the association’s professional education programs. He began his career in 2007, as a technical representative for an aluminum framing systems manufacturer, and represented a major fenestration producer, where he focused on the specification and sale of fenestration systems for monumental commercial construction projects. Blom also has worked for a multidivisional specialty subcontractor. He can be reached at ablom@fgiaonline.org.

Source URL: https://www.constructionspecifier.com/verifying-fenestration-structural-strength-without-lab-testing/

---