An Investigation Into the Effects of Certain Variables in Scarf-Jointed Timber Laminations

A total of 950 Douglas-fir specimens were tested in tension parallel to the grain, to evaluate the strength ratio of various scarf joints, to investigate the influence of “notch effect” on the strength ratio, and to determine whether strength ratio varied with the use of more than one adhesive in bonding comparable joints. The following conclusions were drawn: 1) Change in cross-sectional area of Douglas-fir specimens has no significant effect on tension parallel to grain strength. 2) Adjustment of various moisture content to a common value does not decrease the coefficient of variation nor change strength significantly. 3) Strength ratio of single scarf-jointed laminations, when tested in tension parallel to the grain, are lower than those estimated by the provisions of the present C.S.A. specifications for glued-laminated timber construction. 4) A non-linear relationship exists between the strength ratio and the depth of the notch created by the end step of scarf joints. 5) No significant difference in strength was found between comparable joints bonded with a waterproof adhesive and casein glue in four of five shipments of scarf joints tested. 6) The strength ratio of single laminations may be increased considerably by removal of the end-step notch on the outside faces of the laminations. 7) Tensile tests of notched unjointed laminations have confirmed the trend of decrease in tensile strength with increase in notch depth of scarf-jointed laminations. 8) Slope of grain steeper than l:15 is found in Laminating Grade B lumber and is found on the wide face of flat-sawn boards; slope of grain on the edge of these boards meets the C.S.A. specifications for this grade.

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