|Fastener Specification Checklist|
To aid in the fastener selection process, American Architectural Manufacturers Association (AAMA) Technical Information Report (TIR)-A9-14, Design Guide for Metal Cladding Fasteners, provides the following suggested fastener specification checklist.
A safety factor (SF) is used in the allowable strength design (ASD) method. This method was used to determine the allowable values presented in TIR-A9, which have been determined after a study of several industry standards. There is also another design approach—the load and resistance factor design (LRFD) method—in which the combined use of a load factor ‘m’ (greater than 1) and a resistance factor ‘j’ (less than 1) is the equivalent of using a safety factor. That is, SF= m/j. Load factors are given in the governing building code. Resistance factors, also termed strength-reduction or capacity factors, are given in the specification for the structural material/components being connected.
For fasteners of 6.35 mm (0.25 in.) or less in diameter, a SF of 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 exceeding 6.35 mm, but are 25 mm (1 in.) or less, a SF of 2.5 is used. This is more conservative than the safety factors recommended by others, especially for the 12.7 to 25 mm (0.5 to 1 in.) range, by the cold-formed steel specification, Specification for Cold-formed Stainless Steel (ASCE-8), and the AISC Specification for Structural Steel Buildings ranging between 2 and 2.42.
Protection from corrosion, hydrogen embrittlement
It is essential fasteners have adequate protection against corrosion to prevent eventual failures due to moisture from rain and condensation. Additionally, a defense must also be provided against galvanic corrosion, which occurs when dissimilar metals are in contact in the presence of moisture, especially in harsh environments such as seacoast locations.
Stress corrosion is the effect of corrosion on a metal under stress. When metals are under constant or cycling stress (the latter of which causes metal fatigue), the effect of corrosion can be much more severe than when metals are not stressed. Stress corrosion failures can occur shortly after the load is applied, but may not occur for months or years later. Such failures occur without warning and can be dangerous.