ASSY Self-Tapping Screws Geometry Requirements, Part 1

MTC Solutions’ design guides contain all the requirements for designers to properly detail connections, ensuring that stresses are safely transmitted without causing wood splitting. This two-part technical blog series, “Self-Tapping Screws, Geometry Requirements”, will answer some of the most frequently asked questions about connection geometry requirements when designing with ASSY self-tapping screws.

Part 1 will cover the basics of the Spacing, End & Edge Distance Geometry Requirements, exploring what the requirements represent, load directions, and the wedge effect, along with the differences in connection detailing when using fully threaded and partially threaded screws types.

Part 2 of this blog series is covering geometry considerations for screw connections in CLT and Douglas fir along with some special detailing for connections with axially loaded inclined self-tapping screws.

Approximately 7-minute read.

 

Spacing, End & Edge Distance Geometry Requirements

Geometry requirements are essential to transfer forces safely and efficiently between materials and the ASSY self-tapping screws. These requirements were developed through a tested understanding of wood materials and are dictated by ICC code-approval in the United States and CCMC code-approval in Canada. These approvals generally reflect the geometry requirements set in the Eurocode 5 for self-tapping screws with no pre-drilling.

The Spacing, End & Edge Distance Geometry Requirements represent the minimum spacing requirements of the self-tapping screws supplied by MTC Solutions. These requirements include:

  • end distance (a) – the minimum distance a fastener can be placed from the end of a wood member.
  • edge distance (e) – the minimum distance a fastener can be placed from the edge of a wood member.
  • spacing between fasteners in a row (SP) – the minimum spacing between fasteners in a row parallel to the grain of a wood member.
  • spacing between rows of fasteners (SQ) – the minimum spacing perpendicular to the grain between rows of fasteners along a wood member.


Figure 1 – Non-Pre-drilled Geometry Requirements

The geometry requirements vary depending on the angle at which force is applied relative to the direction of wood grain and are tabulated as linear functions of the fastener diameter. These requirements all work together to ensure that forces are safely transferred, and that wood splitting is avoided. See the MTC Solutions Structural Screw Design Guide for more information.

 

Basics of Wood Material Structure

Figure 2 - Wood Material Grain Directions

To understand the geometry requirements presented in our literature, it is necessary to understand how wood materials are structured. As a natural material, wood grows by continuously adding concentric layers or rings of material to its cross-section, giving wood its unique structure and anisotropic physical properties.

Because of these properties, it is necessary to consider the effects of loads applied at different angles to the grain. Wood is very strong when forces are applied in the direction of the grain, but weaker bonds between the linear fibers themselves mean that wood material can be split relatively easily by forces applied perpendicular to the grain.

Figure 2 - chop

Because wood is more prone to splitting along the grain, geometry requirements corresponding to this direction (end distance and spacing of fasteners in a row) are generally larger than required spacings and distances across the grain (edge distance and spacing between rows of fasteners). This anisotropic physical property of wood is also the reason the load direction impacts spacing requirements.

When designing screw connections, the number of fasteners used is only one detail a designer must consider. Because of the structure of the wood material and its limitations caused by its tendency to split, it is generally preferable to spread loads over larger volumes of material. This means that the layout of the connection is just as important as the number of screws used. The Spacing, End & Edge Distance Geometry Requirements help designers efficiently accommodate their design to the characteristics of wood members.

 

Self-Tapping Screws and the Wedge Effect

The type of fastener used, and its specific design features play a large part in determining screw geometry requirements. ASSY screws are equipped with a self-tapping tip enabling fast and simple installation. This is a major advantage when using self-tapping screws, reducing preparation and installation time, but can introduce a phenomenon called the wedge effect if connections are not properly detailed.

When installing a self-tapping screw, wood material is not removed, rather material is compressed and densified around the threads as the screw is installed. This localized densification of the wood fibers is referred to as the wedge effect and can introduce additional stresses during installation that can contribute to splitting. These additional stresses are considered within the geometry requirements summarized in our design guides.

MTC Solutions offers two different screw designs; designers familiar with these screws will notice that requirements for partially threaded screws are larger than requirements for fully threaded screws. The key difference affecting these requirements is each screw’s tip design and its impact on the wedge effect.

Figure 1 - Non-Pre-drilled Geometry Requirements
Figure 2 – ASSY Self-tapping Screw Tips

The partially threaded screw has a counter thread tip that is specially designed to bite into the wood and engage material quickly, while the fully threaded screw has a more specialized drilling tip that mimics a drill bit. While material is not removed when a fully threaded screw is installed, the drilling tip design does reduce the overall severity of the wedge effect, leading to smaller spacing requirements.

To further reduce spacing requirements, pre-drilling can be done, removing material, reducing these local stresses around the screw threads. For more information on the differences between MTC Solutions fasteners, read our blogpost Partially Threaded Versus Fully Threaded Screws.

 

 

This first part provided background information on the spacing values and details on the differences between fully threaded and partially threaded fasteners. Part 2 of this series will cover geometry requirement considerations for CLT and Douglas fir, along with some special considerations for connections with axially loaded inclined self-tapping screws.

For more information, download our Structural Screw Design Guide. This guide provides detailed instructions and considerations when designing connections with ASSY self-tapping screws. MTC Solutions technical support team is always available to answer your questions and assist you during your project design phase.

 

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