ASSY Self-Tapping Screws Geometry Requirements, Part 2

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 2 will cover geometry considerations for screw connections in CLT and Douglas fir along with some special detailing for connections with axially loaded inclined self-tapping screws.

Previously, Part 1 of this blog series covered 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 screw types.

Approximately 7-minute read.

 

Geometry Requirements in Cross Laminated Timber

Cross-Laminated Timber (CLT) panel is an engineered multi-layer mass timber product consisting of several layers of lumber boards oriented in alternating directions, bonded with structural adhesives, and pressed into very stiff and strong panels capable of handling large structural loads. The cross-laminated structure of several wood grains stacked in an alternating fashion means that a fastener installed in CLT can apply forces along either the strong or weak axes of multiple lumber boards in combination, effectively reducing the wood’s natural tendency to split. This is reflected in the MTC Solutions’ Structural Screw Design Guide under CLT Requirements for non-pre-drilled self-tapping fasteners.

Figure 1 – Cross-Laminated Timber

 

Spacing, end and edge distance requirements for CLT are derived according to the methods described in the European Technical Approval ETA-11/0190 for self-tapping wood screws.

While North American design codes do not specifically outline or demand design calculations and checks for wood-related failures in CLT, MTC Solutions’ testing indicates that wood-related resistances should be checked when designing in CLT material.

 

Geometry Requirements in Douglas Fir

Figure 2 - chop

Douglas fir has specific geometry requirements that are significantly larger than other timber species with similar densities. This is reflected in the MTC Solutions’ design guides.

In addition to the geometry requirements set in the Eurocode 5, Table 8.2, an additional detail described in the European Technical Approval ETA-11/0190 for ASSY self-tapping wood screws is applied to the geometry requirements for usage of screws in Douglas fir. The ETA prescribes an additional 50% increase to be applied to spacings and distances parallel to grain (end distance and spacing of fasteners in a row).

The wood fibers of Douglas fir trees grow very quickly and very straight, meaning cracks can propagate further along the grain of the timber member, making the species ultimately more prone to splitting than other softwood lumber species. Therefore, larger spacing requirements must be used when installing screw fasteners into Douglas fir.

 

Geometry Requirements with Inclined Screw Connections

When detailing spacings between screws installed at 90 degrees to the surface of the timber element, it is relatively simple to measure between screw heads to meet the geometry requirements outlined in the MTC Solutions’ design guides. However, geometry requirements for screws installed at an incline between 45 degrees and 30 degrees are considered differently. When installed at an incline to the shear plane, these screws are loaded axially and geometry requirements corresponding to axial loading must be respected.

Figure 2 Axially Loaded Inclined Screw Connection<
Figure 2 Axially Loaded Inclined Screw Connection

 

Further, spacing dimensions for these connections require slightly different detailing to connections with purely axial or 90-degree shear fasteners. The first difference with inclined screw connections is that the spacing between fasteners in a row, Sp axial, is measured perpendicular to the axis of the screw as shown in the Figure 2, projecting a larger spacing measurement on the surface of the element.

The second difference is that the end distance, aaxial, is taken from the center of gravity of the effective screw length in each member. This is calculated as the mid-point of the penetrating screw length, ignoring the effective tip and head lengths in each member. Spacing requirements in the direction across the grain (edge distance and spacing between rows of fasteners) are measured in the same way as in regular 90-degree installations.

MTc Figure 3 Plan View - Axially Loaded Inclined Screw Connection
Figure 3 Plan View – Axially Loaded Inclined Screw Connection

 

Inclined self-tapping screws can be installed in conjunction with either partially threaded or fully threaded screws installed at a different inclination to the shear plane. When detailing the spacing between these different screw types or differently oriented screws, the most conservative spacings are to be respected.

 

 

This has been Part 2 of the technical blog series aimed at explaining and answering frequently asked questions surrounding the Spacing, End & Edge Distance Geometry Requirements for ASSY self-tapping screws. This second part provided special considerations for detailing in CLT and Douglas fir, along with some special considerations for connections with axially loaded inclined self-tapping screws.

This first part provided background information on the spacing values and details on the differences between fully threaded and partially threaded fasteners.

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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