Evaluating the Relative Bending Strength of Crossarms

V E R a million crossarms are produced annually in the United States. In the open wire lines of the Bell System alone there are now about 20 million arms in use. It is natural, therefore, t h a t public utility engineers should have an interest in the strength of such an important item of outside plant material; and, consequently, an interest in any tool or means of evaluating the strength of such material. It is believed t h a t the moment diagram is a convenient and reasonably reliable tool for estimating the loads an arm will support, for measuring the effect of knots of various sizes and of pinhole locations on arm strength, and for answering similar questions relating to the bending strength of crossarms under vertical loads. Two moment diagrams are shown in Fig. 1 for Bell System Type A crossarms; and in the pages that follow are presented the method used in constructing the diagrams and a discussion of their use. While the calculation results apply particularly to the type and quality of arm referred to, they would also be of value as a time saving reference in f u t u r e studies t h a t m a y be proposed relating to the strength of the same or other types of arms involving different knot allowances. The resisting moment of a beam is the product of its section modulus by the unit stress on the remotest fiber of the beam. The section modulus of a beam of uniform cross-section is constant and readily determinable. T h e section modulus, however, of a beam of nonuniform cross-section, such as a crossarm, varies because of the different cross-sectional shapes and dimensions involved.