Abstract

The purpose of this paper is to present a rational method of design of wooden structural beams containing checks or fissures in the vertical faces. A safe design assumes that such checks are present. The method proposed will lead to a considerable saving of the material required by over-conservative methods of design now in common use. It is well known that the usual method of calculating the longitudinal shearing stress in the neutral plane of such beams is in error, as it predicts in certain cases stresses two or three times the ultimate shearing stress of the material in beams which are carrying their loads without failure. In the present study, made at the United States Forest Products Laboratory Madison, Wis., the elastic behavior of a loaded checked beam is examined in order to explain the discrepancy existing between the facts of experience and the predictions of the usual methods of calculating shear. The essential features of this explanation were established by an approximate mathematical analysis of the problem and a series of about 200 tests. It is found that the upper and lower halves of such a beam act to some extent as independent beams, thereby relieving the shearing stress in the neutral plane. This independent or “two-beam” action increases rapidly as the point of application of a concentrated load approaches a support. As a consequence, the point of application of the minimum concentrated load to produce failure by shear in a checked beam is not just inside a support, as is commonly assumed as a result of the usual simple beam theory, but is at some distance from a support. Accordingly, recommendations for calculating the strength of a timber in shear are made that allow a considerable saving of material.

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