Do Group Effects Apply to Fasteners Loaded in Withdrawal?

NDS 2015 and CSA O86 2016 do not require the use of group action reduction factors for fasteners loaded in withdrawal. European standards for timber design, however, include a reduction factor when groups of screws are loaded by a force component parallel to the screw axes.

Since structural self-tapping screws are capable of transferring high axial forces, and ductility is limited for screws loaded in withdrawal, the use of a reduction factor is recommended. A practical method based on the European model can be used to calculate the effective number of screws (nF) for groups of fasteners loaded in withdrawal.

Where n is the number of fasteners acting together in a connection, the effective number of fasteners, nF = n0.9. Inclined screw connections have characteristics of connections where the fastener is loaded by a force component both perpendicular and parallel to the axis. Since inclined screw connections exhibit limited ductility, group action must be accounted for. For inclined screw connections, the effective number of fasteners, nF = max [n0.9; 0.9 ∙ n].