Certain scenarios for wood-to-wood (T-joints) and steel-to-wood (anchor base plates or base posts) connections may require the installation of Assy screws under end grain withdrawal. This application is generally not recommended. Withdrawal parallel to the grain is riskier due to the decreased withdrawal capacity and most importantly, the increased probability of hitting a check during installation. NDS 2018 Clause 220.127.116.11 states that “Wood screws shall not be loaded in withdrawal from the end grain of the wood”. Assy self-tapping screws can alternatively be used for an efficient and simple connection in two different ways: end grain penetration and inclined installation.
Based on the European Technical Approval (ETA) for Assy self-tapping screws, a proprietary reduction factor of 0.3 could be applied to published unit withdrawal values, given that the effective length of penetration is minimum 20D. These connections shall have a minimum of two Assy screws under this loading condition. It is worth noting that even with this factor, end grain withdrawal is not recommended for Assy screws. Furthermore, it is the responsibility of the designer to assess the applicability of this reduction factor and to provide adequate technical evidence to the approving authorities.
Preferably, Assy screws would be installed at a 30° or 45° angle with respect to the grain of the member under tension. This application greatly decreases the probability of hitting a check during installation and provides an overall stiffer connection. The use of 45° Wedge Washers is highly recommended for steel-to-wood connections. The general procedure in our Structural Screw Design Guides can be followed for connection detailing, given that the appropriate angle reduction factors are applied and the minimum spacing requirements are followed.
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