New Materials Require New Hardware — Three Fasteners Engineered Specifically for Mass Timber Construction

Simpson Strong-Tie has always been a leader in designing innovative products for various construction markets. As an R&D engineer, I enjoy the opportunities for continuous exploring, experimenting and learning that come with my role. There’s never a dull moment working in research and development! At present, I’m thrilled to be part of a team that works on developing products for construction using cross-laminated-timber (CLT) and other forms of mass timber. So I have the honor of introducing three amazing fastener products that are load rated for use in CLT, wood, glulam and structural composite lumber (SCL) products (e.g., LVL, PSL, LSL).

The three new screws are:

  1. The Strong-Drive® SDCP Timber-CP screw – partially threaded for wood-to-wood connections
  2. The Strong-Drive® SDCF Timber-CF screw – fully threaded for wood-to-wood connections
  3. The Strong-Drive® SDHR Combo-Head screw – partially threaded for metal-to-wood connections

Strong-Drive® SDCP TIMBER-CP Screw

Strong-Drive SDCP Timber-CP screws are structural fasteners designed for use in CLT and mass timber construction, as well as sawn timber applications. They are available in several diameter and length combinations, including lengths ranging from 3 1/8″ to 11 3/4″ with a  diameter of 0.315″ and a 14″ length with a diameter of 0.394″. These screws have a yellow zinc coating and are suited for use in interior, dry-service applications. As shown in Figure 1, these screws are partially threaded with a large, flat head with underhead nibs for clean countersinking. The screws come with a six-lobe T40 drive recess for secure driving. The Type-17 point for the screws helps with fast starts and makes predrilling unnecessary in most applications. Another feature of the screws is the shank knurl that helps reduce driving torque during installation. Figure 2 illustrates various features of SDCP Timber-CP screws.

Figure 1- Strong-Drive® SDCP TIMBER-CP Screw
Figure 1- Strong-Drive® SDCP TIMBER-CP Screw
Figure 2: SDCP TIMBER-CP Screw Features
Figure 2: SDCP TIMBER-CP Screw Features

SDCP Code Report

Strong-Drive SDCP Timber-CP screws are load rated for shear, pull-through and withdrawal resistance in sawn wood, glulam, CLT and SCL products in accordance with ICC-ES Acceptance Criteria AC233 (Acceptance Criteria for Dowel-type Threaded Fasteners Used in Wood) and are described in ICC-ES ESR-3046. Refer to Table 3 of ESR-3046 for fastener specifications and strengths. Table 3A includes allowable lateral (Z) design values for wood-to-wood connections. Refer to Table 3B of the report for allowable withdrawal (W) design values, and Table 3C for allowable pull-through (WH) design values. Table 3D and Figure 4 provide connection geometry parameters for the screws.

SDCP Screw Comparison to Standard NDS Wood Screws

Standard wood screws described in NDS Appendix L comply with the dimensions of ANSI/ASME B18.6.1 (#16 screws have a minor diameter equivalent to the minor diameter of SDCP screws). SDCP screws are proprietary screws and have a minimum specified bending yield stress (Fyb) of 185 ksi for SDCP22 screws. Standard #16 screws have a Fyb of 70 ksi. Due to the aggressive proprietary thread of SDCP screws, along with a higher Fyb, SDCP screws outperform standard wood screws in installation and load performance.

Screw TypeMinor Diameter (in.)Major Diameter (in.)Min. Specified Fyb(ksi)
#16 Standard Wood Screw0.2090.26870
#18 Standard Wood Screw0.2320.29470
SDCP22 Proprietary Screw0.2080.309185

In calculating lateral loads with the yield limit equations, use the minor diameter of the screws per NDS. Graphs 1-4 compare the performance of SDCP screws and standard #16 and #18 wood screws.

Graph 1 – Allowable Lateral Loads in DF, Parallel to Grain
Graph 1 – Allowable Lateral Loads in DF, Parallel to Grain
Graph 2 – Allowable Lateral Loads in SPF, Parallel to Grain
Graph 2 – Allowable Lateral Loads in SPF, Parallel to Grain
Graph 3 – Allowable Lateral Loads in DF with Main Member Perpendicular to Grain and Side Member Parallel to Grain
Graph 3 – Allowable Lateral Loads in DF with Main Member Perpendicular to Grain and Side Member Parallel to Grain
Graph 4 – Allowable Lateral Loads in SPF with Main Member Perpendicular to Grain and Side Member Perpendicular to Grain
Graph 4 – Allowable Lateral Loads in SPF with Main Member Perpendicular to Grain and Side Member Perpendicular to Grain

Literature Available Currently for SDCP Screws

Strong-Drive® SDCP Timber-CP screw flier – this flier lists the features as well as the lengths and diameters available for this screw.

Strong-Drive SDCP Timber-CP screw engineering letter – this engineering letter has information on shear, withdrawal and pull-through capacities and allowable load values for SDCP Timber-CP screws.

Connectors and Fasteners for Mass Timber Construction catalog, C-C-MASSTIMBER20 – the Mass Timber catalog covers all Simpson Strong-Tie® products currently available for mass timber construction.

Common Applications for SDCP TIMBER-CP Screws 

CLT wall-to-wall, floor-to-floor, panel-to-beam, panel-to-wall, surface spline, lap joint and ledger connections, general timber framing connections

Strong-Drive® SDCF TIMBER-CF Screw

Strong-Drive SDCF Timber-CF screws are self-drilling structural fasteners that are designed for use in CLT and mass timber construction, as well as sawn timber applications. They are available in lengths from 4 3/4″ to 23 5/8″. These screws have a yellow zinc coating and are suited for use in interior, dry-service applications. As shown in Figure 3, these screws are fully threaded and feature a large flat head with underhead nibs for clean countersinking. The screws come with a six-lobe drive recess for secure driving. The proprietary point of the screw helps with fast starts, making predrilling unnecessary in most applications. Figure 4 captures various features of SDCF Timber-CF screws.

 Figure 1: Strong-Drive® SDCF TIMBER-CF Screw (SDCF22614 shown)
Figure 3: Strong-Drive® SDCF TIMBER-CF Screw (SDCF22614 shown)
Figure 4: SDCF Timber-CF Screw Features
Figure 4: SDCF Timber-CF Screw Features

SDCF Code Report

Strong-Drive® SDCF Timber-CF screws are load rated for shear, pull-through and withdrawal resistance in sawn wood, glulam, CLT and SCL products (e.g. LVL, PSL, LSL) in accordance with ICC-ES Acceptance Criteria AC233 (Acceptance Criteria for Dowel-type Threaded Fasteners used in Wood) and are described in ICC-ES ESR-3046. Refer to Table 4 of ESR-3046 for fastener specifications and strengths. Table 4A includes allowable lateral (Z) design values for wood-to-wood connections. Refer to Table 4B of the report for allowable withdrawal (W) design values, and Table 4C for allowable pull-through (WH) design values. Table 4D and Figure 6 provide connection geometry parameters for the screws.

SDCF Screw Comparison to Standard NDS Screws

Standard wood screws described in NDS Appendix L meet the dimensions of ANSI/ASME B18.6.1 (#16 screw has a minor diameter equivalent to the minor diameter of SDCF screw). SDCF screws are proprietary screws and have a minimum specified Fyb of 180 ksi for SDCF22 screws and Fyb of 200 ksi for the SDCF31 screws. Standard #16 screws have a Fyb of 70 ksi. Due to the aggressive thread pattern and a fully threaded shank, along with a higher Fyb, SDCF screws outperform standard wood screws.

Screw TypeMinor Diameter (in.)Major Diameter (in.)Min. Specified Fyb(ksi)
#16 Standard Wood Screw0.2090.26870
#18 Standard Wood Screw0.2320.29470
SDCF22 Proprietary Screw0.2050.315180

Following the NDS, use the minor diameter of the screws in the yield limit equations when calculating lateral loads. Graphs 5-9 illustrate the differences in allowable load capacities between standard wood screws and the SDCF screws in selected wood species and with different member orientations.

Graph 5 – Allowable Lateral Loads in DF, Parallel to Grain
Graph 5 – Allowable Lateral Loads in DF, Parallel to Grain
Graph 6 – Allowable Lateral Loads in SPF, Parallel to Grain
Graph 6 – Allowable Lateral Loads in SPF, Parallel to Grain
Graph 7 - Allowable Lateral Loads in DF with Main Member Perpendicular to Grain
Graph 7 – Allowable Lateral Loads in DF with Main Member Perpendicular to Grain
Graph 8 – Allowable Lateral Loads in SPF with Main Member Perpendicular to Grain
Graph 8 – Allowable Lateral Loads in SPF with Main Member Perpendicular to Grain

Literature Available Currently for SDCF Screws

Strong-Drive SDCF Timber-CF screw flier – this flier lists the features as well as the lengths and diameters available for this screw.

Strong-Drive SDCF Timber-CF screw engineering letter – this engineering letter has information on shear, withdrawal and pull-through capacities and allowable load values for SDCF Timber-CF screws.

Connectors and Fasteners for Mass Timber Construction catalog, C-C-MASSTIMBER20 – the Mass Timber catalog covers all Simpson Strong-Tie® products currently available for mass timber construction.

Common Applications for SDCF Timber-CF Screws

Beam reinforcement connections, CLT wall-to-wall, floor-to-floor connections, support beams, butt joint connections; also with angled washers

Strong-Drive® SDHR COMBO-HEAD Screws

Strong-Drive® SDHR Combo-Head screws are available in four sizes – two lengths, each in two separate diameters. They have a large hex head and a six-lobe drive recess. The SDHR27 screws have a 15 mm hex drive with T40 drive recess, and the SDHR31 screws have a 17 mm hex head with T40 drive recess. The screws have blue zinc coating or zinc-nickel coating and are designed for general interior, dry-service applications. They are best suited for steel-to-wood applications. The steel needs to be predrilled to fit the shoulder diameter of the screws. The predrilled hole sizes for SDHR27 and SDHR31 screws are 7/16″ and 1/2″ respectively.

figure 5 - Strong-Drive® SDHR Combo-Head Screw (SDHR27400 shown)
Figure 5 – Strong-Drive® SDHR Combo-Head Screw (SDHR27400 shown)
Figure 6: SDHR COMBO-HEAD Screw Features
Figure 6: SDHR COMBO-HEAD Screw Features

SDHR Code Report (coming soon!) 

Strong-Drive® SDHR Combo-Head screws are load rated for shear, pull-over and withdrawal resistance in sawn wood, glulam, CLT and SCL products (e.g., LVL, PSL, LSL) in accordance with ICC-ES Acceptance Criteria AC233 (Acceptance Criteria for Dowel-type Threaded Fasteners used in Wood) and are in the process of being submitted for code listing.

Literature Available Currently for SDHR Screws

Strong-Drive SDHR COMBO-HEAD screw flier – Coming soon!

Strong-Drive SDHR COMBO-HEAD Engineering Letter –This Engineering letter has information on shear, withdrawal and pull-over capacities and allowable load values for the SDHR screw.

Connectors and Fasteners for Mass Timber Construction catalog, C-C-MASSTIMBER20 – The Mass Timber catalog covers all Simpson Strong-Tie® products currently available for mass timber construction.

Common Applications for SDHR Combo-Head Screws

Metal-to-wood connections.

Now that you’ve learned about the SDCP, SDCF and SDHR screws, design away with these new screws on your next projects and drop us a line to let us know your experiences!

Neelima Tapata

Author: Neelima Tapata

Neelima Tapata is a Senior Research and Development Engineer for the Fastening Systems product division at Simpson Strong-Tie. She works on the development, testing and code approval of fasteners. She joined Simpson Strong-Tie in 2011, bringing 10 years of design experience in multi- and single-family residential structures in cold-formed steel and wood, curtain-wall framing design, steel structures and concrete design. Neelima earned her bachelor’s degree in civil engineering from J.N.T.U in India and her M.S. in civil engineering with a focus on structural engineering from Lamar University. She is a registered Professional Engineer in the State of California.