The torsional and bending behavior of wood beams



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This investigation is a report of a study of the torsional and bending behavior of wood beams. The investigation is divided in two parts. The first part considers the constitutive equations that are used as a model to study the elastic behavior of wood and which provide theoretical values of stress for comparison with experimental test results in wood. The first part also includes a study of the theory of torsion in wood. The second portion of this investigation describes an experimental program including specimen-preparation and testing. Ten circular and ten square beams were tested. Half of the specimens were Douglas Fir and the other half were Red Oak. Eight of these beams were first loaded to a specified load within the elastic region under the following loading conditions: a) pure torsion, b) pure bending, c) bending and restrained torsion, and d) bending and unrestrained torsion. Following these nondestructive tests, all twenty beams were loaded to failure under different loading conditions. These failure loading conditions included pure bending, pure torsion and combined bending and torsion. The experimental results were found to be in close agreement with the theoretical predictions. It was also found that there is no appreciable change in the magnitude of normal stresses due to restrained or unrestrained torsion when compared to the normal stresses due to flexure only. It was further found that unrestrained torsion does not cause significant warping of the cross section of the beam. Analysis of test results for the 20 specimens that were loaded to destruction, indicate that the ultimate moment and the ultimate torque are independent of each other. Thus, the presence of torsion does not appear to change the bending strength of a wood beam.