This study empirically examines the relationship between clamp force and pull-out strength in lag screw joints of timber members, using data obtained in tightening tests and pull-out tests. Maximum clamp force per unit screw length as determined from the tightening tests was lower than the lower bound for the 95% tolerance range for pull-out strength per unit screw length as determined from the pull-out tests. Moreover, X-ray CT (computed tomography) observations of anchor members from both tests revealed that failure behavior clearly differed between the tightening test and the pull-out test: tightening caused damage to the wooden, female thread in addition to major splitting damage in the wood perpendicular to the grain near the tip of the lag screw.

To investigate the effects of tightening speed on the torque coefficient in lag screw timber joints with steel side plates, tightening tests were conducted on main timber members made from Cryptomeria japonica, Chamaecyparis obtusa and Pseudotsuga menziesii, under four tightening speed conditions (1, 4, 10, and 20 rpm). Major stick-slip behavior was observed in C. obtusa based on the relationship of tightening angle with clamp force, tightening torque, and thread torque at tightening speeds of 1 and 4 rpm. In addition, tightening speed's effects on the torque coefficient (K) varied depending on the wood species of the main member. In P. menziesii, K was not affected by the tightening speed: the ratio of torque expended on tightening was 25% on average, and the ratio of torque expended on bearing surface friction was higher than the ratio of torque expended on thread friction.

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This paper proposes a numerical method for estimating the ultimate clamp force of lag screw timber joints with steel side plates based on the pull-out strength and thread friction coefficient of the lag screw. Lag screw tightening tests and pull-out tests were performed using timber samples from three different wood species, for three different screw insertion angles with respect to the annual ring, to evaluate how well the predictions matched the observed empirical data. Ratios of observed to estimated ultimate clamp force ranged from 0.9 to 1.3, suggesting the proposed method well approximated the empirical trend. However, it was found that the proposed method was overestimated depending on the degree of female thread formation in the wood, so it was indicated that the reduction coefficient should be taken into consideration.