Monotonic and Reverse-Cyclic Loading of Lag Screw Connections for Log Shear Wall Construction

The log building industry favors simple to install mechanical connections between log layers, also referred to as log courses, to facilitate fast and efficient construction. In active seismic regions, lag screws are commonly used: however, research is lacking on how these connections between log courses perform in monotonic and reverse-cyclic loading scenarios. For the current study, 406-mm-long logs were used to fabricate connection test specimens in a single-shear configuration to mimic a typical lag screw layout used in log shear wall construction. The intent of this study was to develop baseline data on these connections to relate log connection performance to full-scale log shear wall performance. A preliminary study of four commonly used lag screws of different sizes was conducted to assess the monotonic performance within log connections. Predictions of design capacities were calculated using the National Design Specification for Wood Construction which is based on derived equations from the European yield model. Predicted NDS design capacities were compared to experimental data. Design recommendations for deriving appropriate fastener spacing were provided along with fabrication parameters that were based on observed failure mechanisms. Tested connections exhibited high ductility, which is considered a positive attribute for seismic resistance, although reverse-cyclic tests revealed failures due to low-cycle fatigue of the lag screws, leading to prevalent cyclic stiffness degradation as displacements were increased.