There are ASTM standards used to “generically” describe screws. For drywall applications, ASTM C 954 and ASTM C 1002 are prescriptive standards for self-drilling and self-piercing (sharp point) drywall screws, respectively. ASTM C 1513 is a prescriptive standard covering “generic” self-drilling and self-piercing screws for most other cold-form steel applications. These standards are referenced in the IBC and IRC. Screws meeting these standards can be used prescriptively for the code-listed applications.

The greatest advantage of screw fasteners is speed of installation.

In addition, the International Code Council – Evaluation Service (ICC-ES) has developed acceptance criteria for tapping screw fasteners (AC118) to evaluate IBC/IRC compliance. Test data is evaluated by ICC-ES and presented in ICC-ES Evaluation Service Reports (ESRs).

The load testing required by AC118 depends on the type of screw fastener being evaluated. All screw fasteners undergo tension and shear testing per AISI S904, which consists of fracturing the screw in tension or shear. The resulting load values are compared to connection strength, which can be determined by calculations or testing.

ASTM C 1513 or ASTM C 954 screw connection strengths can be calculated using AISI S100-2007 (NASPEC) Section E4. This provides equations for pullover and pullout limit states in tension connections, and tilting and bearing limit states for shear connections, as well as equations for combined shear and tension interaction. These values, while generally considered conservative, generate usable load capacities with minimal testing, based on historical research.

Proprietary fastener connection strengths must be developed through testing per AISI S905, which provides test methods for pullover, pullout, and shear bearing load capacities. Note that non-proprietary screw fasteners also may be tested per AISI S905, if desired.

Screw fastener design and specification
Load capacity is only part of the necessary information to properly design cold-form steel connections. Other topics that must be considered are:

Code recognition: ICC-ES is the evaluation service branch of the International Code Council (ICC) that authors the IBC and IRC. Through ICC-ES-issued ESRs, manufacturers obtain recognition for many different products, including screw fasteners. An ESR provides independent verification that the product meets the code listed in the ESR, and the product is generally appropriate for the application(s) and condition(s) specified. Specifying and/or requiring an ESR that verifies compliance with the relevant code for the project is recommended because it reduces the need for the specifier and local building official to perform their own code-compliance evaluation.

Corrosion concerns: As with most connections, the designer should be aware of possible environmental concerns. The table on page 42 provides an overview of environmental conditions, connection types, and possible screw fastener solutions. Note, however, that small changes in site conditions can significantly affect the environmental corrosivity, and the required life expectancy of the connection is an obvious factor, so a thorough analysis should be performed before final selection.

One important issue associated with self-drilling and self-tapping screws is hydrogen-assisted stress corrosion cracking (HASCC), often incorrectly referred to as hydrogen embrittlement. HASCC involves a hardened steel fastener (>Rc34), loaded in a service environment that chemically generates hydrogen (a corrosion byproduct). Hydrogen embrittlement, on the other hand, refers to a similar type of failure, but where hydrogen is generated during the fastener manufacturing process. Common environments that can cause HASCC are dissimilar metal connections (e.g. aluminum to steel) or connections involving chemically treated wood, when moisture is present. The potential for HASCC also is related to high fastener hardness. Unfortunately, these fasteners must be hard enough to drill and tap into steels. However, there are new screw fasteners manufactured with two-step heat treating, providing a hardened drill point and first few threads (tapping threads), while the remainder of the screw fastener is ductile, making the load-bearing portion of the screw virtually immune to HASCC.

Figure 1: “Bi-metal” technology was developed to fight the issues of corrosion and hardening.

Another corrosion concern is rusting (note that HASCC and rusting must be evaluated separately; a corrosion-resistant coating to inhibit rusting generally will not protect against HASCC). While improved coatings have been developed, they all have a limit to their corrosion resistance, which in highly corrosive environments may necessitate use of stainless steel screws. However, 300-series stainless steel is difficult to harden sufficiently to perform self-drilling and tapping functions, so historically, pre-drilling and tapping were required. And while 400-series stainless steel can be hardened, it generally is considered susceptible to HASCC. To address these issues, “bi-metal” technology has emerged in recent years. Essentially, the drill point and first threads of the fastener are manufactured from hardened carbon steel, which is fused to the load bearing part of the fastener made from 300-series stainless steel (see Figure 1). This provides the self-drilling and tapping performance of a hardened carbon steel screw, with the corrosion protection of a 300-series stainless steel fastener.

Drilling capacity: The type and thickness of the materials must be considered to ensure the selected screw will properly attach the two (or more) materials. The main criteria for determining drilling capacity are the drill point size and screw diameter. For example, if attaching to 3/8-inch-thick steel, a #12 diameter screw with a #5 drill point would commonly be used. However, that screw would not be recommended in 20 gauge (33 mils) base steel because the steel could be damaged, reducing performance. In addition, the drill tip must be long enough to penetrate the base steel before the threads engage and begin the tapping process. Therefore, it is necessary to consider drilling capacities when specifying screw fasteners. This should be available from most manufacturers in ESRs or technical manuals.

Self-drilling screws generally are required for 20 gauge (33 mils) and thicker steel. However, Hilti’s recent development in thread technology (Hyper Thread) makes it possible to use sharp point screws for up to 18 gauge (43 mils) steel thicknesses. The advantages are two-fold. First, a step in the setting process is eliminated, increasing installation speed. Second, virtually no steel material is removed during the setting process, increasing connection capacities considerably.

Other developments on the horizon include drill points which drill faster, and thicker steel capacity beyond the 1/2-inch limitation currently found with most self-drilling screw fasteners.

Head type: Certain head types may be required to properly attach certain fastened materials (e.g., lathing) or obtain a desired finish (e.g., flush setting). Several head types are covered by ASTM C 1513 and ASTM C 954. The most common are Hex Washer Head (HWH) and Phillips Pan Head (PPH), for typical framing applications, and Phillips Bugle Head (PBH), for drywall applications.

Tool specification: It is increasingly common to install screw fasteners with impact wrenches. However, hardened screw fasteners are not designed to be installed with these tools because impacting can crack the screw. Therefore, the project specification should state that these fasteners be installed with a non-impact, maximum 2,500 rpm screw drive tool; and if the application is “torque-critical,” the tool also should be equipped with a depth gauge or torque clutch.

Be specific: “A screw is not a screw is not a screw.” Specification of the proper screw should be as the word implies: specific. First, “screw” and “TEK screw” are not interchangeable. “TEK” is a proprietary name from a specific manufacturer, not a term for a “generic” screw. Second, preferably specify the fastener’s manufacturer, nomenclature, and ICC-ES ESR number (“the good”). If a manufacturer-specific specification is not feasible and a performance specification is needed, take care to identify and specify all factors relevant to the application (e.g., screw diameter, head type, drill point size, screw material, et cetera) and require an ICC-ES report recognizing the fastener’s compliance with the applicable code (potentially “the bad” if a relevant factor is missed). Keep in mind that simply specifying a “generic” screw and leaving selection up to others can lead to “the ugly” on your project.

Conclusion
Screw fasteners are an excellent option for many types of connections and provide many advantages compared with other fasteners. The greatest advantage is speed of installation as, in most cases, the fasteners install without a pre-drilling step. Fortunately, there are manufacturers developing new fastening technologies, making screw fasteners viable for more applications in today’s building construction market. And when in doubt, contact the manufacturer for assistance.

Andrew Liechti, P.E., is technical services manager with Hilti, Inc., and specializes in screw fastener qualification testing and evaluation. He is a member of the ASTM committee covering ASTM C 1513 and ASTM C 954. He can be contacted at drew.liechti@hilti.com.