214 



MEMOIR OF EATON HODGKINSON. 



but wide flanjres, but Tinited by a narrow rib. That which constitutes the 

 strena^th of tlie beam being the resistance of its material to compression on the 

 one side of its neutral axis and its resistance to extension on the other side, it is 



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evidently a second condition of the strongest form of any 

 given section, that "phen the beam is about to break across 

 that section by extension on the one side, it may be about to 

 break by compression on the other. So long, therefore, as 

 the distribution of the material is not such as that the com- 

 pressed and extended sides would yield together, the strongest 

 form of section is not attained. Hence it is apparent that the 

 strongest form of the section collects the greater quantity of 

 __ .. the material on the compressed or the extended side of the 



= ^ ^/ ^^4 beam, according as the resistance of the material to compres- 

 sion or to extension is the less. Where the material of the beam is cast iron, 

 whose resistance to extension is greatly less than its resistance to compression, 

 it is evident that the greater portion of the material must be collected on the 

 external side. 



" Thus it follows, from the preceding condition and this, that the strongest form of 

 section in a cast-iron beam is that by which the material is collected into two unequal 

 flanges joined by a rib, the greater flange beiug on the extended side, and the 

 proportion of this inequality of the flanges being just such as to make up for the 

 inequality of the resistances of the material to rupture by extension and compres- 

 sion respectively. Mr. Hodgkinson, to whom this suggestion is due, has directed 

 a series of experiments to the determination of that proportion of the flanges by 

 which the strongest form of section is obtained. 



" The details of these experiments are found in the following table : 



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" In the first five experiments each beam broke by tearing asunder of the 

 lower flange. The distribution by which both were about to yield together — 

 that is, the strongest distribution — was not, therefore, up to that period reached. 

 At length, however, in the last experiment, the beam yielded by the compression 



of the upper flange. In this experiment, 

 therefore, the upper flange was the weakest ; 

 in the one before it the lower flange was the 

 weakest. For a form between the two, there- 

 fore, tlie flanges \Cere of equal strength, to 

 resist extension and compression respectively, 

 and this was the strongest form of section. 

 In this strongest form the lower flange had six 

 times the material of the upper. It is repre- 

 sented in the accompanying figure. In the best 

 form of cast-iron beam or girder used before 

 these experiments, there was never attained 

 a strength of more than 2,885 pounds per 

 square inch of section. There was, there- 

 fore, by this fonn a gain of 1,190 pounds per square inch of the section, or of 

 two-fifths the strength of the beam." (See Moseley's " Engineering and Ai'chi- 

 tecture," art. 411.) 



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