VIII. Experiments to determine the effect of impact, vibratory action, and long-continued changes of load on wrought-iron girders

Abstract

A question of great importance to science and the security of life and property has been left in abeyance for a number of years,—namely, to determine by direct experiment to what extent vibratory action, accompanied by alternate severe strains, affects the cohesive force of bodies. It is immaterial whether the body be crystalline, homogeneous, or elongated into fibre, such as cast or wrought iron; the question to be solved is, how long will a body of this description sustain a series of strains produced by impact (or the repeated application of a given force) before it breaks? In the case of bridges and girders, this is a subject on which no reliable information has yet been given which may be considered as a safe measure of strength for the guidance of the architect and engineer. It is true that regulations have been established by the Lords Commissioners for Trade; but they appear to have had their origin on limited data, and in cases where the material and workmanship are good they may be relied upon as sufficient for the public safety. What, however, is wanted is experimental data to enable the engineer to comply satisfactorily with the conditions of the Board of Trade, and cordially to unite with the Government in affording ample security to constructions in cases where the lives of the public are at stake. To remove all doubts on this question, I have been enabled, through the liberality and at the request of the Board of Trade, to undertake a series of experiments to determine, or to endeavour to ascertain, whether a continuous change of load, and the strains produced by those changes, have any effect (and to what extent) upon the ultimate strength of the structure,—or, in other words, to ascertain the rate of endurance the material is able to sustain under these trials.