Introducing The Next-Generation Strong-Drive® SDHR Combo-Head Screws: Steel-to-Wood Structural Fastening Solution with Enhanced Durability and Hydrogen Embrittlement Resistance

Now with Black E-Coat Finish — Compliant with CSA O86:24 Hydrogen Embrittlement Requirements

In response to the hydrogen embrittlement requirements defined in CSA O86:24 and the increasing structural demands of modern heavy timber and mass timber construction, Simpson Strong-Tie® has developed the next-generation Strong-Drive® SDHR Combo-Head Screw with a black e-coat finish. This updated version is designed to meet the latest durability and hardness requirements while delivering the same high-capacity structural performance that engineers have come to rely on in steel-to-wood applications. The new SDHR is listed in the ICC-ES ESR-3046 Evaluation Report.

 

The New SDHR Black E-coat Screw
The New SDHR Black E-coat Screw

Hydrogen Embrittlement Resistance and Durability

The most significant update to the SDHR screw is the introduction of a controlled core hardness profile with a maximum core hardness of 36 HRC, aligned with the requirements of CSA O86:24. This hardness limit directly addresses two critical failure mechanisms that can affect high-strength steel fasteners in sustained load applications:

  • Environmental Hydrogen Embrittlement (EHE); which can occur when atomic hydrogen is absorbed into the steel microstructure under sustained tensile stress, reducing ductility and potentially causing brittle fracture.
  • Stress-Corrosion Cracking (SCC); a time-dependent failure mode that develops in high-strength steels exposed to both tensile stress and corrosive environments, including moisture that may be present during construction.

By limiting core hardness to ≤ 36 HRC, the SDHR black e-coat screw significantly reduces susceptibility to both EHE and SCC, improving the long-term reliability of structural connections. This is especially important in long-duration mass timber and heavy timber projects where fasteners may experience sustained tension loads while the structure is exposed to ambient moisture during construction.

The CSA O86:24 standard provides specific guidance on hydrogen embrittlement mitigation for screws used in wood construction. The updated SDHR screw is fully aligned with these provisions, making it suitable for use on projects where CSA O86 compliance is required or where enhanced durability is a design priority to avoid both EHE and SCC.

Black E-Coat Finish

The black e-coat finish replaces the legacy blue zinc coating and provides corrosion resistance suitable for dry interior service conditions. The black e-coat is approved in accordance with ICC-ES AC257 for Exposure Condition 3, permitting the screws to be exposed to wet service conditions during the construction phase.

The SDHR screws are intended for dry, interior service conditions consistent with interior structural framing, heavy timber connections, and mass timber assemblies protected from prolonged moisture exposure. These screws are not intended for exterior exposure or continuously wet service conditions.

Same Trusted Performance — Now with Greater Durability

Engineers who have specified the legacy blue zinc SDHR screw can transition to the new black e-coat version with confidence: the structural load capacities have not changed. Reference design values for shear, withdrawal, and tension remain consistent between the legacy blue zinc and the new black e-coat SDHR screws of equivalent sizes. The new black e-coat screw is a direct one-to-one replacement for the legacy blue zinc SDHR in equivalent sizes, requiring no recalculation of connection designs. Additionally, the pull-over published load values have been expanded to cover more steel thickness.

The primary difference between the two versions is the improved hardness profile and updated coating system of the new screw, which together provide the enhanced durability and hydrogen embrittlement resistance described above. Structural performance, installation characteristics, and connection geometry remain unchanged.

Available Models

The SDHR black e-coat screws use metric nomenclature. For example, for an SDHR10X100 screw, 10 represents the major diameter of 10 mm and 100 represents the screw length of 100 mm (4.00″). The table below lists the available screw models along with their equivalent legacy blue zinc designations.

Table listing the available screw models and their equivalent legacy blue zinc designations
Table listing the available screw models and their equivalent legacy blue zinc designations

Combo Head Design for Installation Flexibility

The SDHR screw features a combo head that combines the benefits of a hex head with a deep internal drive recess. This dual-drive design provides:

  • Improved torque transfer during installation
  • Reduced cam-out compared to conventional recesses
  • Installation flexibility using either internal drive bits or external hex sockets

These features help ensure consistent installation quality in the field, even for longer or larger-diameter screws.

Optimized Tip and Thread Design

The SDHR screw incorporates a Type-17 style tip, allowing for fast starts and reduced driving torque, particularly in dense wood species. It also features a robust mid-shank knurl. This optimized thread profile, combined with the new Black E-coat finish, reduces installation torque and time while minimizing splitting, reducing the need for predrilling into the wood member in many applications (subject to project requirements and ESR limitations). Note that the hole in the steel side member for the SDHR screw must be predrilled or pre-punched and must be a standard round hole no greater than 0.4375 inch (11.11 mm) in diameter for the SDHR10 and SDHR27 screws, and no greater than 0.500 inch (12.7 mm) in diameter for the SDHR12 and SDHR31 screws.

Design Considerations for Engineers

When specifying SDHR screws, engineers should note the following design considerations:

  • Reference design values are provided for shear, withdrawal, and pull-over, depending on connection geometry and installation conditions.
  • Installation angle, embedment depth, and wood species can influence performance and must comply with ESR-3046 requirements.

These considerations allow SDHR screws to be used confidently within established design methodologies, including ASD and LRFD, where permitted.

Applications

Strong-Drive® SDHR screws are commonly used in:

  • Heavy timber and mass timber connections
  • Connecting steel beams, girders, and framing members to CLT or glulam
  • Steel ledger angle connections to CLT wall and floor panels
  • Hybrid steel-to-timber connections in CLT, glulam, LVL, PSL, and LSL construction
  • Transfer of diaphragm, collector, and drag forces between steel components and timber panels
  • Prefabricated mass timber assemblies where rapid installation and reduced field drilling improve construction efficiency

Learn More

For additional technical details, reference design values, and installation requirements:

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Author: Ayman Abdelaal

Dr. Ayman Abdelaal, Ph.D., P.E., is a Fastener Product Engineer at Simpson Strong-Tie, specializing in structural and mass timber fasteners. He joined Simpson in 2019 as a Branch Engineer supporting the Southwestern United States and has worked extensively with structural screw, mass timber, anchor tie-down, and Strong wall product lines. Prior to joining Simpson Strong-Tie, Dr. Abdelaal accumulated extensive experience in structural engineering, construction project management, forensic investigations and technical design of commercial, mixed-use and multi-story buildings. He earned his Ph.D. in Structural Engineering from the University of California, Irvine in 2014. At Simpson Strong-Tie, he works with engineers, designers, code officials and builders to develop innovative fastening solutions and advance safer, stronger and more efficient structural systems.