TRANSACTIONS OF THE SECTIONS. 203 



The object of these experiments was specially directed to ascertain the moduhis 

 of elastic'ittj. They were made with the testing-machine at H.M. Dockyard at 

 Woolwdch, which machine was placed at our disposal by the Admiralty. The 

 bars were obtained, with some exceptions, in sets of six from each maker, three 

 bars of each set being used in tension and three in compression. 



Bars of iron of like dimensions were also tested in the same way, in order to 

 obtain the relative eftects in steel and iron. In these experiments sixty-seven steel 

 bars were tested whose tensile strength varied from thirty-two to fifty-three tons 

 per inch, and twenty-four iron bars varying- from twenty-two to twenty-nine tons 

 per inch. 



The amount of the extensions and compressions were ascertained by direct 

 measurement, verniers being for this purpose attached to the bar itself, 10 feet 

 apart, so that the readings gave the absolute extensions and compressions of this 

 length of the bar. 



These experiments, which were very accurately made, showed that the exten- 

 sion and compression of steel per ton per inch was a little less than wrought iron, 

 that the extension and compression were very nearly equal to each other, and that 

 the modulus of elasticity of steel may be taken at 30,000,000, which result agrees 

 with the conclusions arrived at by American engineers on this subject. 



This property of the metal is "important in two respects. First, because inas- 

 much as the extension per ton per inch is practically equal to the compression, it 

 follows that the neutral axis of a structure of steel, strained transversely, will be 

 in the centre of gravity of its section, and that the proper proportion to give to 

 the upper and lower flanges of a girder, when made of the same quality of steel 

 throughout, will be the same as in wrought iron. Secondly, because the modulus 

 of elasticitv of steel is practically equal to that of wrought iron, and the limit of 

 elasticity is greater, it follows that in a girder of the same proportions as wrought 

 iron, and strained with an equal proportion of its idtimate tensile strength, the 

 deflection will be greater in the steel than in the iron girder, in the ratio of the 

 strength of the metals ; so that if it is necessary to make a steel girder for a given 

 span deflect under its load the same amount as an iron girder of the same span, the 

 steel girder must be made of greater depth. 



The fourth series of experiments were made by the Committee on riveted steel, 

 and show clearlv that the same rules which apply to the riveting of iron apply 

 equally to steel'; that is to say, that the total shearing area of the rivets must be 

 the same, or rather must not be less, than the sectional area of the bar riveted. 



Having thus obtained a knowledge of the behaviour of steel under diflerent 

 strains, we may trace in what manner its employment would operate on the 

 weight of metal required for large engineering structures. But before doing so 

 I would call your attention to the question of the absolute tensile strength. 



Taking Mr. Kirkaldy's experiments in conjunction with those made by the 

 Committee, there is a great range of strength exhibited, commencing as low 

 as that of the best iron, and extending to about fifty-three tons per inch. 



This great range of strength is due to the different qualities and make of the 

 steels tested, and must not be mistaken for irregularity of strength in the manu- 

 facture ; on the contrary, in the experiments made by the Committee, in which 

 three bars of each make were broken, the strengths, with the exception of one set, 

 are as uniform as in the iron bars similarly tested. 



It is also to be observed that in applying steel to engineering structures we may 

 dismiss from consideration those superior qualities which are of hi>gh price and 

 made in comparatively small quantities. I propose therefore to confine my obser- 

 vations to the mild steels, such as are made by the " Bessemer," the " Siemens- 

 Martin," and other processes, having a tensile strength varying from thirty-three 

 to thirty-six tons per inch, a material which is made in large quantities and at 

 moderate cost. 



Following the same rule as is adopted for wrought iron (namely, that the maxi- 

 mum strain on the metal shall not exceed one fourth of the breaking weight), we 

 may consider steel of this quality capable of bearing at least eight tons pSr inch, 

 instead of the five tons per inch estimated for like purposes in iron. 



We know from established mechanical laws that the limiting spans of structures 



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