Relationship between clamp force and pull-out strength in lag screw timber joints

Abstract

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.

A novel method for estimating ultimate clamp force in lag screw timber joints with steel side plates

“…Figure 10 shows the results of X-ray CT observations performed on these overestimated specimens after the tests. Similar to [2], the destructive behavior differed between Table 1 Results of the tightening tests and pull-out tests. the pull-out and tightening tests: whereas the female thread formed relatively distinctly in the former, it showed signs of significant damage after the latter, suggesting that poor thread formation in the wood was a major cause of reduced pull-out strength in the tightening tests.…”

“…MATERIALS AND METHODS 3.1 Tightening tests of lag screw timber joints Figure 2 shows the experimental set-up. Lag screw timber joints with steel side plates were subjected to tightening tests according to the method in [2,8] using a bolt force tester (NST-500NM: Japan Instrumentation System Co., Ltd, Nara, Japan). Test specimens were composed of three parts: a main wooden member, a round washer, and a lag screw ( Fig.…”

“…Lag screw pull-out tests were conducted according to the method in [2] using members of the same specifications as those used in the tightening tests. Figure 5 shows the experimental set-up.…”

“…3), and la = 38.8 mm (i.e., 46.8 mm -8 mm; see Figs. [2][3]. Let μth = 0.458, the maximum thread friction coefficient observed for Cj-0 (see Table 1).…”

“…Data from tightening and pull-out tests of lag screw timber joints using several kinds of wood showed that ultimate clamp force tends to be lower than pull-out strength observed in the pull-out tests. Noting this disparity, a reduction coefficient of 0.7 was proposed to avoid pull-out damage [2]. Ultimate clamp force's smallness relative to pull-out strength is hypothesized to be the product of two forces acting on a lag screw during tightening: the axial force in the pull-out direction, and a resulting, frictional force at the wood-screw margin along the thread.…”