It would be difficult for most engineers or contractors to argue against the importance of connections in a completed structure. Whether we are considering buildings or bridges, or screws or bolts, fasteners allow connections of independent components to create complete and safe structural systems. The importance of connections is accompanied by design difficulties in fastener selection and design. Some of these factors include stress concentrations, geometric discontinuities or eccentricities and random installation errors. Many structural failures occur at connection points due to these aggravating factors and they serve as reminders of the significance of structural connections.
Considering the unquestionable importance of connections, it may be surprising that the structural performance of fasteners can be misunderstood. For commodity fasteners, such as sheet metal screws, design is relatively well documented and covered by the building code and reference standards. However, manufacturers are constantly looking to create proprietary, differentiated fasteners with enhanced performance or lower installation cost. While this can drive innovation in the industry and improve construction practice, the wide range of components can be difficult to understand and regulate.
In addition, engineers are facing more budget constraints just as contractors and owners are looking to save construction costs. Where a contractor would not have questioned a welded connection in the past, in today’s building environment contractors and suppliers will price out several alternative fastening systems to provide the lowest installed cost available. Eventually, the lowest cost substitution will be submitted to the engineer of record for their review and approval. It is important to understand the applicable design standards and how to differentiate between products in terms of quality and performance. Let’s try to help understand the process.
Structural systems of wood and cold-formed steel (CFS) utilize fasteners as the primary means for achieving inter-member connections. For wood, fastener types include nails and screws, while self-drilling tapping screws are the primary fastener type used in cold-formed steel-framed construction (see Figure 1). The use of cold-formed steel in the U.S. could be categorized as components for: 1) cold-formed steel framing; and 2) corrugated-steel decking. The first is an industry that has achieved measured growth as an alternative to conventional light wood framing for residential and certain commercial developments. Corrugated-steel decking, on the other hand, is the established and preferred roof framing system for commercial and industrial construction. Some overlap exists between the organizations that provide oversight and define best practices for design and construction utilizing CFS systems. Let our explanation focus on CFS decks given their predominance.
As a starting point, let’s try to clarify the information that defines the standard of care as it relates to CFS connectors. The International Building Code (IBC), chapter 22, Section 2210.1.1 establishes ANSI/SDI RD1.0-Standard for Steel Roof Deck and ANSI/SDI NC1.0-Standard for Non-Composite Deck as the code documents for design of steel deck components. Section 3.2 A of this document identifies three fastening types for structural connection of steel decks: welds, screws and pins. Interestingly, the code document provides specific reference for quality requirements of welds, which shall be based on American Welding Society D1.3-Sheet Steel, but not for pins and screws. So, what about screw fasteners? How do we know what screws to specify?
Two organizations are involved in the development of specifications for design of cold-formed steel structural members and their connections: The American Iron and Steel Institute (AISI) as the organization that provides oversight for cold-formed steel design and the Steel Deck Institute (SDI), with a more specialized focus on diaphragm design utilizing corrugated-steel decking. Since ANSI/SDI RD1.0 specifies that diaphragm design capacity shall be determined by testing or in accordance with the SDI Diaphragm Design Manual (DDM), let’s focus on it first. The third and current edition of the SDI DDM does not stipulate specific references to fastener quality standards for connections. It does, however, reference screw size limits and includes diaphragm load tables for some specific screw models. In the absence of a reference to specific quality requirements in the SDI DDM, it makes sense to turn to the other code document that defines cold-formed steel design parameters. Section 2210.1 of the IBC requires design of cold-formed steel members to be in accordance with AISI S100-North American Specification for the Design of Cold-Formed Steel Structural Members. AISI S100 does not include any specific reference for quality standards for screws, but section D1.1 of AISI S200 North American Standard for Cold-Formed Steel Framing-General Provisions, a guide document, establishes compliance with American Society for Testing and Materials (ASTM) standard C1513 as the basis for the quality of screws. Finally, there’s a specific reference to screw quality. What does it mean to be in compliance with this standard?
ASTM C1513 mainly establishes dimensions, material composition and test methods to verify drilling capacity. ASTM C1513 relies on the Society of Automotive Engineers (SAE) standard J78 for many of the dimensional and performance requirements. The stated objective of these standards is to establish mechanical and performance requirements to ensure that certified fasteners can drill and form or cut mating threads in the base materials into which they are driven, without deforming their own threads or breaking. Compliance with ASTM C1513 does not establish load ratings for either the individual screws or the connected system composed of screws and steel sheets. Most of the performance parameters included in SAE J78 and ASTM C1513 are summarized below:
- Screws must be manufactured from steel wire, specification A510-minimum grade 1018. This requirement relates directly to strength and stiffness of screws and helps ensure uniformity.
- Tables 1 and 2 of SAE J78 specify dimensional ranges for screw shafts, heads and threads as a function of nominal screw size.
- Performance Requirements:
- Self-drilling tapping screws shall have surface hardness greater than Rockwell C50, core hardness greater than Rockwell C32, tested in accordance with SAE J78. The hardness requirements help ensure the toughness.
- Ductility shall be sufficient to withstand five degree bend without fracture, with testing in accordance with SAE J78.
- Torsional strength shall be determined by applying moments listed in Table 2 of ASTM C1513. The specified moments are a function of screw size. Similar information is included in Table 5 of SAE J78.
- The ability of the screw to drill through metal shall be determined by subjecting screws to the drill-drive test requirements noted on Table 4 of ASTM C1513 as a function of nominal screw size.
It is clear from the requirements listed above that the referenced standards do not provide means for determining the load capacity of individual screws or the screws attached to cold-formed steel. Based on AISI S100 code requirements, determination of the load capacity of screws must be determined from testing in accordance with AISI test standards S904 (individual screws) and S905 (screws connected to cold-formed steel).
One point to mention, our only reference to ASTM C1513 (from AISI Standard S200) also provides an alternate to direct compliance with the standard. The language in the standard indicates "…ASTM C1513 or an approved design or design standard." Approved in this context refers to approval provided by the authority having jurisdiction, which in most cases relies on recognized evaluation services for assessment of relevant performance parameters. In the end, most reputable fastener manufacturers provide self-drilling tapping screws that are in compliance with ASTM C1513 and that have been tested in accordance with AISI standards S904 and S905. This establishes the highest level of quality for steel deck screw fasteners.
Tomas Montemayor, P.E., S.E., is a structural engineer for Simpson Strong-Tie and is based in Jacksonville, Fla. He can be reached at email@example.com.