Reducing Crook in Kiln-Dried Northern Aspen Studs

A method has been developed for straightening 8-feet-long 2-by-4-inch studs of northern aspen (Populus tremuloides Michx. and P. balsamifera L.) which develop excessive crook during kiln drying. Preliminary tests with small, clear specimens of each species provided constants for a model which predicts the required duration of treatment. The model was then successfully tested with single studs and finally confirmed with a test package of 20 studs. In this process, studs are first steamed for three hours to plasticize them, then a load is applied at mid-length to deflect them in the direction opposite to their crook, by an amount two to three times the amount of crook present. Stress relaxation takes place for a given length of time, then the load is removed and studs allowed to recover. Deflection decreases until, ideally, the residual deflection is equal to, but in the opposite direction to, the original crook and the studs are then straight. Equations are presented to allow calculation of the time required for stress relaxation when the initial deflection is arbitrarily selected. Straightened studs remained within limits of Stud grade, or Standard and Better Light-Framing grade, for the two- months duration of a storage period following completion of the tests.

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