Grips for Tension Tests of Structural-Size Lumber

Standard methods have been established to test the tensile strength parallel to grain of small clear specimens of wood and of slightly larger specimens of glued end-jointed material. A limitation of this method is that since natural strength reducing features such as knots cannot be included in small specimens and their effect must be estimated. To study the effect of knots on tensile strength of structural lumber, special grips had to be designed for holding the specimens during testing with a 120,000 pound testing machine. Openings in crossheads of testing machines in most laboratories are not large enough to permit insertion of full-size lumber. The contact area between the specimen and the grips must be large enough to prevent crushing of the specimen. Paddle-type tension grips made of tempered steel with inner faces grooved to provide a good surface for gripping 24 inches of each end of the board were secured to the crossheads of a testing machine with a pin and bearing block assembly. Boards were inserted between the pairs of grips and secured by tightening machine bolts with an electrically operated impact wrench. Spaced holes and slots in the grips permitted testing boards up to 2 by 10 inches in cross section. Load is transmitted to the board through friction and shear, but the board is not crushed because the frictional force is spread over a large area. Wedge-type grips may crush the specimen and concentrate stress close to the tips of the grips, causing failure in this area. The height of the columns of the testing machine limits the length of the boards and the number of defects which can be tested in a single board. The capacity of the testing machine places further restrictions on the sizes of boards tested. To overcome these limitations, an apparatus using grips to test boards horizontally is being developed. In tension tests of nominal 2 by 8 inch Douglas-fir boards, 78 inches long, kiln-dried to 12 percent moisture content, with knots of 1-1/4 to 2-1/4 inches in diameter, the grips performed well. Preliminary analyses of 102 boards indicated that both size and location of knots affect tensile strength. The average tensile strength for boards containing knots 1-1/4 inches in diameter was 4,800 psi, and for boards containing knots 2-1/4 inches in diameter it was 3,100 psi. Knots at the edges of the boards lowered tensile strength more than knots at the centers of the boards.

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