Abstract
To investigate the accuracy of present design formulas for mechanical joints of glued laminated timbers (GLTs), dowel-bearing tests with a drift-pin were conducted in this study. GLTs with four kinds of moduli of elasticity (5.9, 7.9, 9.8, and 11.8GPa) were prepared for the tests. These GLTs were composed of mechanically graded Japanese cedar and Japanese larch laminae with uniform modulus of elasticity. Four types of specimens were cut from these GLTs; “Parallel type” in which load was applied parallel to the grain, and “Perpendicular type” in which load was applied perpendicular to the grain. Five kinds of drift-pins with the diameter of 4, 8, 12, 16 and 20mm were embedded into GLTs. Bearing stress was calculated by the ratio of a bearing load to the diameter and the length of a drift-pin.
We investigated the relation of the diameter of a drift-pin to “five percent offset values” specified in ASTM-D5764, “initial stiffness” calculated by the ratio of a bearing stress to a unit bearing deformation, and “effective elastic foundation depth” (the ratio of modulus of elasticity of wood to the initial stiffness) based on the theory of a beam on elastic foundation.
The results obtained were as follows.
(1) Five percent offset values were constant regardless of the diameter for the “Parallel type” whereas they showed a declining tendency with increasing of diameters for “Perpendicular type”.
(2) Initial stiffness decreased with increasing of diameters for the “Perpendicular type”, on the other hand, this tendency was not clear for the “Parallel type”.
(3) Effective elastic foundation depth increased with increasing of diameters for both types. Dr. Hirai's formula commonly used to estimate the relationship between the depth and diameter of fasteners was applied to these results, however the conformability was low. This discrepancy resulted from the fact that the formula was derived from experiments in which fasteners of small diameters were used.
