Designing Project-Specific Fenestration Anchorage
by Marcy Marro | October 1, 2019 12:00 am
When a fenestration product conforming to the North American Fenestration Standard (NAFS) is specified by the type, class and performance grade, this indicates that the product meets a specific design pressure as tested in one specific anchorage system. In the field, however, fenestration products are often installed with various mullions, transoms, subframe systems or substrates that could require vastly different anchorage systems, or fastener location and spacing, than those tested in the laboratory.
Fortunately, no matter which anchor system is utilized, the principle engineering methodology does not change. All anchorage systems that are to be analyzed (such as nail fins, framing receptors, anchor clips and more) require similar dimensioning and performance information.
If a specific fenestration product installation is different than that which was tested, the newly published AAMA 2501-19, Voluntary Guideline for Engineering Analysis of Anchorage Systems for Fenestration Products Included in NAFS, provides a useful reference to verify that its anchorage system conforms to the project specifications.
While fenestration anchorage systems are too numerous and varied to cover every possible detail in this voluntary guideline, an industry-approved methodology is provided, outlining the minimum requirements for structural analysis by engineering calculation that can be referenced in project specifications and contract documents. Note that the analysis of curtainwall systems is more complex and is therefore not covered.
AAMA 2501-19 establishes minimum requirements applicable to anchorage of a NAFS-compliant fenestration system to provide a load resistance with a safety factor appropriate for the project-specific Design Pressure requirements. If the latter has not been established by the project architect, then the minimum required by local code requirements, the current ASCE/SEI 7, or the current Canadian supplement to NAFS (A440S1), whichever is more appropriate, should be used.
Information required to conduct the engineering analysis of the anchorage system is listed in AAMA 2501, along with the recommended content of the engineering analysis report.
As described in a companion document, AAMA TIR-14, Fenestration Anchorage Guidelines, the mode of failure for the material and fasteners along the load path of each anchor (the path that the load on the window must follow to reach the building envelope) can change. Each component in the load path should be analyzed to ensure that the material, material thickness and quantity of screws and/or fasteners is adequate for the load.
The physical anchorage of an aluminum window is critical to ensure that it performs and operates as designed and tested. Image by Oleg March, courtesy of Apogee Enterprises Inc.
Three-Step Analysis Process
TIR-A14 describes a three-step process for anchorage design:
- Step 1: Check the material being anchored.
For the material, the mode of failure could be loading along the weak axis of the framing member, bending or buckling. Specifically, if shear is present, then the bearing strength of the base needs to be checked. If tension is present, then pull-over needs to be considered. Analysis equations for both factors, as well as for pull-over of flat head fasteners, are provided for reference. - Step 2: Check the fastener.
For the fasteners, the mode of failure could be the same as that of the material being anchored (bearing or pull out/pull-over) or of the anchor material itself (shear, tension or combined loading). Besides using AAMA’s TIR-A9 for load values of fasteners, many manufactures publish their own load table specific to their fasteners, which gives allowable loads to which the calculated loads can be compared. Note that no structural strength is assumed for sealants or flashing materials. - Step 3: Check the material being fastened to.
For the base material, bearing or pull-out and/or pull-over govern. The shear and/or tension in the fastener is compared to the allowable bearing (shear) and fastener pull-out of the base material. For fastener pull-out in aluminum substrates, reference is made to the Aluminum Association’s “Aluminum Design Manual.”
TIR-A14 looks at the anchoring implications for four different building envelope substrates: metal, wood, concrete, and concrete masonry units (CMU) or red face brick.
Taking metal as an example, the design considerations when fastening into a metal structure depend on whether 1) the base material, typically steel, is thick enough to allow the full tensile capacity of the fastener to manifest, or 2) the base material is of a thin gauge that will result in pullout of the fastener before its tensile capacity is reached. When the base appears thick enough, so that the fastener material will fail in tension before it pulls out, the base material should still be verified that its bearing and pull-out capacity are greater than that of the fasteners. When fastening into a thin base material, bearing and pull-out failure modes need to be calculated once the internal stresses in the fastener have been checked.
Both AAMA 2501-19 and AAMA TIR-A14 may be obtained by visiting aamanet.org/store.
Steven Saffell serves as AAMA’s technical director, overseeing the standards, product certification, and codes and regulatory affairs aspects of the association. He can be reached at ssaffell@aamanet.org.
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