Defining “Standard Practice” in Engineering Evaluations
We have often spoken of the “101” series of international window and door performance standards. The current version of this is AAMA/WDMA/CSA 101/I.S. 2/A440-08, which allows window and door products of different framing materials and operator types to be objectively and fairly assessed for structural performance in response to wind loading, air leakage and water penetration resistance. These standards are referenced by the International Residential Code and the International Building Codes and thereby adopted (with or without additional requirements) in many states and local code jurisdictions.
The simplest and most direct means of verifying compliance with the applicable standard is a certification label based on laboratory tests. This compliance is typically authorized by an ANSI-accredited certification entity, such as AAMA. However, there are special cases where it can be prohibitively expensive and time-consuming to test and certify special project-specific or one-of-a-kind designs. In many cases, therefore, local codes and project architects provide the option for window suppliers to obtain an engineering evaluation of the product instead of requiring the array of test results and certification labels. The manufacturer’s installation instructions may also be needed to confirm the integrity of recommended fasteners, their locations, and anchoring techniques.
The Florida Building Code pioneered this approach on a large scale when the state’s product approval system took effect in October 2003. Under that system, one of the paths for product approval was defined as “an evaluation report from a Florida registered engineer or architect.” Such evaluations, which in some jurisdictions must be stamped by a registered professional engineer, are defined with reference to “accepted engineering practice.” However, there can be divergence of opinion on what constitutes “accepted” practice.
To eliminate confusion and multiple interpretations, AAMA has developed a standardized process for engineering evaluation of windows and doors, in the form of AAMA 2502-07, Comparative Analysis Procedure for Window and Door Products, released last November (available for purchase at www.aamanet.org).
This document has replaced the engineering design rules in AAMA Procedural Guide 103-07, and will also replace that portion of the AAMA 203-03 Procedural Guide: Window Inspection and Notification System. This publication provides a means for licensed manufacturers to list window parameters (installation instructions, comparative analysis load figures, etc.) beyond standard, permanent-label information, on an AAMA-validated, temporary label recognized by the Florida Building Code.
In essence, AAMA 2502 gives evaluators a solid foundation for evaluation with an industry-consensus standard for the engineering process. The methods outlined in AAMA 2502 are worthy of reference in the codes and acceptance by codes and specifiers as the touchstone for “standard engineering practice.”
Essentially, the methodology provides a way to mathematically interpolate the successful results of a single series of performance tests (per AAMA/WDMA/CSA 101/I.S. 2/A440-08 or one of its predecessors) for a given size of window and door product as a means to qualify smaller sized units of the same design at a higher performance grade (but within the same performance class). This allows manufacturers to establish performance ratings for a range of sizes at higher pressures without having to actually test all the units, so long as there is no variation in any of the components from those used in the tested unit. This shortcut offers a significant savings in time and costs. Windows that are so qualified may be identified by a validated temporary label.
AAMA 2502 provides a procedure to allow higher positive and negative design pressure ratings of a tested window or door design by decreasing the size of the unit compared to the size of the test specimen. All possible failure modes are to be analyzed, and the allowable and actual loads applied to installation fasteners must be determined considering varying substrates, installation tolerances and frame stresses. For determining the capacity of fasteners used in field installations, AAMA 2501, Voluntary Guideline for Engineering Analysis of Window and Sliding Glass Door Anchorage Systems, is referenced.
Once the largest production unit of a given design (equal to, or larger than, the gateway size for the applicable product type and performance class) is tested per the applicable “101” standard, and the production unit of that design with the highest design pressure (typically the smallest size) is tested, computations can be used to qualify all sizes in between. The two units at the extremes must have a design pressure greater than or equal to that of the largest tested unit, and fastener spacing must not be greater than the tested spacing.
AAMA 2502 goes on to set forth engineering design rules (formerly listed in the AAMA 103 certification procedural guide), by which products that differ from the size of the test unit can be qualified for code compliance for structural performance without further testing. These rules can also assist manufacturers in the product design effort.
This comparative analysis procedure 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 design pressure criteria. For window and door manufacturers, the procedure provides a uniform approach for dealing with different code jurisdictions and specific design pressure for each size of fenestration product opening.
Rich Walker is president and CEO of the American Architectural Manufacturers Association, 847/303-5664, firstname.lastname@example.org.