A curtainwall cladding system-composed of framing members, glazing panels, operable sash, opaque spandrel panels and sealants-must be designed and installed to accommodate a wide variety of forces, both static and dynamic. To manage these forces, the selection and use of proper fasteners to transfer the load to the building framework at designated anchor locations is critically important for safe and reliable system performance. Being key elements in the load path, all fasteners should be analyzed to ensure that the material strength, thickness and quantity are adequate for the load likely to be experienced at the building site.
Fasteners must have the hardness, yield and tensile strength to appropriately withstand these loads without compromising the integrity and weather-tightness of the wall. Different anchorage designs are also required for the different substrates typically encountered in commercial- or institutional-type structures-concrete, masonry or steel frame.
Fastener Guidelines
A particularly useful reference for fastener specification is the American Architectural Manufacturers Association‘s (AAMA) newly updated Technical Information Report, AAMA TIR-A9-14, Design Guide for Metal Cladding Fasteners.
The mode of failure for the material and fasteners along the load path should be considered. This includes the fastener itself as well as the material being fastened to the substrate, which can fail by bending, buckling or pull-out (the amount of force required to pull the fastener out of the base material) and/or pullover (the amount of force required to pull the material over the head of the fastener). TIR-A9 tables provide allowable pull-out values for different thicknesses of aluminum of different alloy designations (3003-H14, 5005-H34, 6061-T6, 6063-T5, 6063-T6 and 6005A-T61) for UNC and spaced threads, for different screw diameters.
Load bearing analysis equations presented in TIR-A9 are the basis for determination of the values used in the document’s Load Tables, which list allowable tension, shear and bearing loads for a range of different fastener sizes, for carbon steel and stainless steel alloys. A total of 21 fastener sizes are covered, ranging from #6-32 through 1-8. Every value in the tables links back to the several interacting equations via a source spreadsheet.
Metals used in fasteners, on which the data in the report is based, include various carbon steel and stainless steel alloys. Note that the use of aluminum fasteners is not recommended for curtainwall anchoring systems.
TIR-A9-14 also provides:
- Expanded information in Allowable Bearing at Bolt Holes to include screws and screw tilting in various thicknesses and substrates.
- Updated and expanded information in Pull-Out Strength Section to include equations for thick, thin, and transition regions and includes thread stripping of internal and external threads.
- Tables showing hole and drill size for spaced threads tapping screws for Type A, C, AB, B, BP, D, F, T, BF and BT steel in various substrates.
- Minimum spacing and edge distance guidelines for bolts and screws, with separate guidelines noted for those intended for use with aluminum substrates.
Safety Factors
The safety factors used in the current edition have been determined after a study of several industry standards. A working definition of safety factor (SF) is the ratio of an installed fastener’s nominal strength to its allowable value for a given failure mode.
For fasteners of 1/4 inch or less in diameter, SF equal to 3.0 is used to generate allowable values. This value is used in both the North American Specification for Cold-formed Steel Structures (2007 and 2001) and the 2010 Specification for Aluminum Structures for this size range of tapping screws. For fastener diameters that exceed 1/4 inch, but are less than or equal to 1 inch, a SF equal to 2.5 is used.
The AISC steel specification (Specification for Structural Steel Buildings; 2010) uses a nominal safety factor, designated as Ω, equal to 2.0.
Protection from Corrosion and Hydrogen Embrittlement:
It is essential that fasteners have adequate protection against corrosion to prevent eventual failures due to moisture from rain and condensation, especially in harsh environments such as seacoast locations. Carbon steel fasteners should be plated or coated with zinc, cadmium, nickel or chromium in accordance with the specifications offered in TIR-A9.
Hydrogen embrittlement-a condition of low ductility in metals resulting from the absorption of hydrogen, typically during the manufacturing process-can cause unpredictable and potentially disastrous failure, especially of a fastener under tensile load. Tests described in TIR-A9 can be applied to assess whether hydrogen embrittlement is present.
A complete fastener specification must include fastener size and type, as well as material, calculated minimum mechanical properties such as bending, shear, bearing and pull out loads, thickness and type of protective coating required. Quality assurance and corrosion resistance should also be addressed.
AAMA TIR-A9-14 can make the fastener specifier’s job easier as well as more precise. Copies may be obtained via the Publications Store page at the AAMA website, www.aamanet.org.
—
Dean Lewis is the educational and technical information manager for the American Architectural Manufacturers Association (AAMA). To learn more, visit www.aamanet.org.
*Top and bottom photos: University of Colorado Hospital, courtesy of Wausau Window and Wall Systems