Experimental assessment of the effect of length on the tensile strength of structural fingerjoined lumber

The effect of specimen length on the strength of fingerjoined lumber is incompletely documented in the literature. The objective of this study was to experimentally assess the effect of length on tensile strength parallel to the grain in fingerjoined structural lumber. The effect of length as observed for 38- by 89-mm (2- by 4-in.) fingerjoined lumber members made of short kiln-dried black spruce lumber segments of 1.32 and 1.83 in (4.3 and 6 ft.) in length complying with the requirements of the 1650f-1.5E and the 2100f-1.8E machine stress-rated (MSR) lumber grades. Tests were performed on specimens made of 4 and 6 segments fingerjoined together producing members of 6.10 and 8.53 m (20 and 28 ft.) in length, which were tested over gage lengths of 4.88 and 7.32 in (16 and 24 ft.), respectively. For each grade (1650f-1.5E and 2100f-1.8E), an additional group from a previous study of specimens made of two segments producing members of 3.66 in (12 ft.) in length and tested over a gauge length of 2.44 in (8 ft.) was used in the assessment of the effect of length. The length effect factors (gL) obtained for both MSR grades of fingerjoined lumber appear to be considerably higher than the gL values for unjoined visually graded lumber reported in the literature. The length effect factor at the 5th percentile was found to be more significant for the 2100f-1.8E grade (gL = 0.33) than the 1650f-1.5E grade (gL = 0.30) because a better fit on the regression line was obtained. The average tensile strength for the 1650f-1.5E tends to converge to the value of 24 MPa at longer lengths. This suggests that the linear logarithmic theory may not be the more efficient method for the evaluation of the effect of length in structural fingerjoined lumber.

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