Metal Fasteners UNC vs Spaced Thread

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Metal fastening is designed per the requirements of the American Institute of Steel Construction (AISC): Specification for Structural Steel Buildings in conjunction with theBoltDepot Type Chart American Architectural Manufacturers Association (AAMA): TIR-A9-14 Design Guide for Metal Cladding Fasteners.


The two most common types of fastener threads used in construction are Unified Coarse Thread (UNC) more commonly known as “machine” and ‘Spaced Thread’ more commonly known as “sheet metal”. UNC fasteners are typically used in tapped holes or in clear holes with mating nuts. Spaced thread fasteners are typically self-tapping which means they can tap their own hole as they’re driven into a material.

You can see the types of fasteners from the image to the right.  Click for the full explanation of fastener types compliments of our friends at the Bolt Depot

Some spaced thread fasteners are also self-drilling which means they have a drill-like head used to drill their own hole and subsequently tap it. UNC fasteners provide more threads per inch which will in many cases have higher pull-out resistance however this is not always true for thin materials.



Allowable tensile capacities for fasteners are calculated based upon the governing limit state among pull-out, pull-over, and tensile rupture. Pull-out failure occurs when the fastener pulls out of the material(s) its fastened to. Pull-over failure occurs when the material in contact with the fastener head locally deforms and pulls over the fastener head. Washers can be utilized to combat the effects of pull-over failure. Allowable shear capacities for fasteners are calculated based upon the governing limit state among bearing capacity and shear rupture.

Material properties of all affected elements, load forces & directions including dead loads are among some of the elements needed to design connections.

AAMA TIR A9 illustrates the many equations & configurations needed to establish these capacities.

Here is a link to purchase TIR-A9-14:


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Last Update: February 19, 2022  

April 15, 2020  Codes & Standards, Engineering  
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