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Effects of screw size, pilot-hole size, and test speed on direct-withdrawal resistance of screws from eleven western woods were studied to supplement limited data now available on screw-holding ability of western woods and to encourage establishment of standard test procedures.
A total of 1350 tests were made on five softwoods and six hardwoods, using three sizes of 1-in. screws in combination with three sizes of pilot hole and three test speeds. The woods had about 7 per cent moisture content at test.
Among several conclusions drawn from the results of the study were the following: 1. Maximum withdrawal loads increased in proportion to size of screw, in all species except redwood. 2. A pilot hole of 70 per cent root diameter of the screw gave good withdrawal loads for all eleven species. 3. Screw-holding tests for comparative purposes should be made at the same testing speed. 4. The general equation given in the Wood Handbook for predicting direct-withdrawal resistance of screws was reasonably applicable to data obtained from the eleven western woods tested in the present study.
Forest Research Center, Corvallis, Ore.