STUDY ON STRESS TRANSFER MECHANISM IN PULL-OUT OF LAGSCREWBOLT JOINT

<p> In this study, the experiments and a three-dimensional finite element analysis were conducted to verify the stress distribution in the glulam at the Lagscrewbolt (LSB) joints. In this paper, a well-formed and uniform three-dimensional finite element analysis model is used to understand the stress distribution in detail and to examine the effects of different cross-sectional sizes and LSB lengths on the stress distribution.</p><p> The experimental results (Fig. 6), using from 75-mm square to 120-mm square cross-section glulam, showed that the values of strain gages attached to the sides of the glulam gradually increased from the sides of the glulam toward the center of the cross-section, which differs from the assumption that the stress distribution within the same cross-section is constant, which is assumed by the existing theoretical equations.</p><p> Since the analysis using standard values for material constants with reference to existing literature resulted in a larger initial stiffness than the experimental results, reducing the shear modulus of elasticity in the fiber direction to 1/4 showed a good agreement with the experimental results (Fig. 9 and Fig. 10). In the analysis model with different material constants around LSB, a material constant of 1/20 times of the standard value was used, which corresponds to the experimental results (Fig. 9 and Fig. 10). However, there are still some issues that need to be considered regarding the modeling method. The shear stress distributions along the LSB showed a concave shape (Fig. 12), where the shear stress of the elements adjacent to the LSB at the end of glulam and the deepest part was larger than the theoretical equation. On the other hand, one of the outer elements showed a nearly uniform shear stress distribution in the depth direction. Stress distributions within the same cross-section at specific depths showed that shear stress decreases from the center of the glulam (LSB side) to the outermost edge of the glulam (side of the glulam) for all cross-section sizes (Fig. 14). On the other hand, normal stresses did not decrease at the outermost edge at 75- and 87.5-mm square cross-sections, while those at 100-, 120- and 200-mm square cross-sections showed a gradual decrease toward the outermost edge (Fig. 14). However, some stress loading was also observed in the elements near the outermost edge. The effective cross-sectional area borne by the stresses used in the existing theoretical equations was investigated (Fig. 17). As a result, if the effective cross-sectional area is assumed to be constant, the ratio of stress borne in the effective cross-section to the total cross-section decreases as the cross-section of the glulam increases (Fig. 19). It was found that in order for the stress borne in the effective cross-section to exceed 80% of the total, it was necessary to increase the effective cross-section as the cross-section of glulam increased. For the same cross-section of glulam, the ratio of stress borne in the effective cross-section decreased as the length of the LSB increased. Therefore, even if the load is the same, the stress is generated in the entire cross-section of the glulam as the LSB becomes longer.</p>