Evaluation method for embedment stiffness of metal washers in bolted timber joints using torque gradient and nut factor

Assessing the residual performance of timber joints affected by decay is important for seismic diagnosis and reinforcement of timber structures. This study introduces a method to evaluate the embedment stiffness of metal washers in bolted timber joints using a torque wrench. Formulas for the embedment stiffness of metal washers using the torque gradient (the gradient of the tightening torque against the tightening rotation angle of the nut) and the nut factor are presented and the calculated values are compared with experimental values obtained by conducting tightening tests with a torque wrench and embedment tests of metal washers in bolted joints made from Japanese cedar, Hiba, and Japanese cypress. The results show that the experimental values of the embedment stiffness of metal washers in Japanese cedar and Hiba are generally within the range of the values calculated from the minimum and maximum values of the nut factor. However, for Japanese cypress, the values calculated from the maximum value of the nut factor exceed the experimental values. This was presumably due to locally large frictional forces generated on the bearing surface or threaded part.