Influence Lines

Abstract

IN the analysis of beams and girders it is possible by the method of influence lines to simplify the process of determination of the stresses due to the passage of a system of rolling loads. In such cases the preparation of diagrams of maximum bending moment and shear force is usually a laborious matter; whereas it is eminently simple to construct a bending moment influence diagram which, for a particular section, will show the bending moment at that section due to a load of one ton placed at any point in the beam. For the given section the one diagram serves each and every load, but while the use of influence lines is generally referred to in structural engineering textbooks, the special technique required in their application to a given problem and their interpretation receives little or no attention. There is now available, however, a handbook (“The Application of Influence Lines to the Stress Analysis of Beams and Lattice Girders.” By R. MeCrae. Parti. Pp.42. (London: The Draughtsman Publishing Co., Ltd.) 2s.) which discusses fully their practical uses as they might be of advantage to a designer. The first part, which is now issued, deals with beams only. The author's primary object in Part 1 has been to provide the reader with a knowledge of the fundamental principles underlying the conception of influence lines. He shows how simple it is to construct the diagram for all cases and how the maximum effects due to rolling and distributed loads can be deduced. In the matter of units, it would have been an advantage to have described the ordinate of the bending moment influence line as representing ton-foot units per ton, so that when multiplied by the appropriate load the result is rational. Those who have been in the habit of using the bending moment diagram will at first find the influence line somewhat unorthodox but, once masters of the system, will doubtless agree as to its advantages, particularly in saving time and eliminating errors, and will appreciate the detailed treatment given in this publication.