Dynamic Stress Analysis of Railway Bridges

Since it is a known fact that stresses due to dynamic impact often become an important factor in the design of a railway bridge, it is the purpose of this paper to indicate a simple method by which these stresses may be determined. In order that this method may be presented in the shortest manner, it will be restricted first to the main trusses or girders of single-track spans with known static loads. It is assumed that, as a steam locomotive crosses such a span at the “critical speed,” the greatest vibrations are set up by the action of the free and unbalanced centrifugal forces of the moving parts. The various formulas deduced herein are set up in the form of nomographic charts. Seven of these are located side by side for ease of manipulation. If the damping capacity of any vibrating structure is known, the true value of the stresses due to impact can be read from the chart. One problem is worked through, showing the advantages, and also the limitations, of the method. Finally, a comparison is given with the impact percentage of standard specifications.