A COMPARISON OF TEST METHODS FOR EVALUATING SHEAR-STRENGTH OF STRUCTURAL LUMBER

Four different test methods that used full-size specimens to determine the parallel-to-grain shear strength of Douglas-fir structural lumber were compared: three-point bending, four-point bending, five-point be nding, and torsion. A small, clear specimen test was also used to dete rmine the parallel-to-grain shear strength of wood. There were 380 mat ched specimens for the five test methods (76 specimens for each test m ethod). Analysis of variance indicated that at least one of the averag e shear strengths from the five different test methods was significant ly different from the other (p-value = 0.00) at a 5 percent significan ce level. The Duncan multiple-comparison test showed that shear streng ths from all test methods were significantly different from each other . The torsion test produced the highest shear strength and appeared to be the best test method for determining shear strength of structural lumber because this test is able to produce pure shear stress in the s pecimen. The three-point bending test was a good test method for deter mining shear strength of structural lumber because this test uses a si mple set-up that approximates real life applications of lumber, and it produced the highest percentage of shear failure among the bending te st methods.