Can We Add up Force Contributions from Inclined Screws and 90° Shear Screws?

When installing inclined fasteners, fasteners driven in at 90° to the shear plane are sometimes installed prior to inclined fasteners to ensure tight connection and precise installation (these are also known as “assembly screws). Designers should not, however, sum the connection resistance from screws installed at different angles for design purposes.
Inclined axially loaded screws and 90° shear screws exhibit different load-displacement behaviour. Testing of connections combining inclined screws and 90° shear screws shows that the behavior can be thought of as a superposition of both types only across the load deformation curves (like a parallel-spring system) (Tomassi, Piazza, Angeli, & Mores, 2006). Designers cannot assume, however, that inclined screws and shear screws will reach their design load levels at the same degree of displacement. 

Inclined screws are much stiffer connection elements. Consequently, they will contribute the greater share of connection resistance at design load levels. Connection design resistance, therefore, must be restricted to the resistance of the inclined screws, disregarding the contribution of 90° shear screws. 

Nevertheless, combinations of both types of screw-in angles can improve connection behaviour in ultimate loading scenarios for special applications such as seismic design, with inclined fasteners providing high initial stiffness and design strength. In case of overloading, a brittle failure is avoided through shear fasteners, which provide necessary ductility to the system. For these applications, testing of specific screw arrangements is necessary to understand the combined behavior over large displacements.

Tomasi, R., Piazza, M., Angeli, A., & Mores, M. (2006). A new ductile approach design of joints assembled with screw connectors. Unpublished Technical Report.

(image below is an approximation of proprietary test results reported by Hossain, 2016)

Normal force contribution 2

Relevant Technical Resources

Gallery canadian screw design guide cover Canadian Structural Screw Design Guide
Gallery dg   assy fastener catalog cover American Structural Screw Design Guide

Products Covered in this Article

Gallery assy eco explosion resized ASSY Eco Countersunk Head
Gallery eco a2 ASSY Eco Stainless Steel
Gallery assy fwh explosion resized ASSY FWH Flat Washer Head
Gallery assy kombi explosion resized ASSY Kombi
Gallery kombi a2 ASSY Kombi Stainless Steel
Gallery sk a2 ASSY SK Stainless Steel
Gallery assy sk explosion resized ASSY SK Washer Head
Gallery assy vg csk explosion resized ASSY VG CSK Counter Sunk Head
Gallery assy vg cyl explosion resized ASSY VG CYL Cylinder Head
Gallery assy vg rh explosion ASSY VG RH Reverse Head

Still Have Questions? Send Us an Email and an Engineer Will Assist You

Or Call Us at